Mon Taj 256 Channel Radiometric s Processing

montaj 256-Channel 256-Channel Radiometrics Processing Radio Radio metric Processin g Extension fo r Oasis Oasis mo ntaj v6.1 v6.1

USER GUIDE and TUTORIAL

www.geosoft.com

The software described in this manual is furnished under license and may only be used or copied in accordance with the terms of the license. Manual release date: 2/2/2005. Written by, Nancy Whitehead and Chris Musselman. Please send comments or questions to [email protected] Copyright Geosoft Inc. 2005. 2005. All All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form, or by any means, electronic, mechanical, photo-copying, reading, or otherwise, without prior consent from Geosoft Inc. reserved.. Program Copyrig Copyrig ht Geosoft Inc. 2005. All 2005. All rights reserved Geosoft and Oasis montaj are registered trademarks of Geosoft Inc. GEOSOFT, GEOSOFT, Oasis are trademarks of Geosoft Inc. Windows ®, and Windows NT™ are either registered trademarks or trademarks of Microsoft Corporation.

Geosoft Incorporated 8th Floor 85 Richmond St. W. Toronto, Ontario M5H 2C9 Canada Tel: (416) 369-0111 Fax: (416) 369-9599 Web Site: www.geosoft.com E-mail: [email protected]

Contents Geosof Geosof t License Agreement

1

Finding More Help Inform ation

3

Contacting Technical Support

Chapter 1: System Overview

3

5

Ac qu ir in g A ir bo rn e Spec tr om eter Data Dat a

5

Workin g wit h 256256-Channel Channel Radiometric s Processin g

6

Correcting Instrument Deadtime/Livetime Effects

6

Applying Nonlinear and Lowpass Filters

7

Description of Nonlinear Filtering

9

Description of Lowpass Filtering

9

Correcting Radar Altimeter Data to Standard Temperature and Pressure

9

Removing Cosmic and Aircraft Background

10

Comparing Default Cosmic and Aircraft Background Parameters

Removing Radon Background Radiation

11

11

Removing Radon Background using Upward Looking Data

12

Removing Radon Background using the Background Table Method

13

Removing Aircraft, Cosmic and Radon Background Interactively

14

Evaluating Aircraft, Cosmic or Radon Background Individually

15

Stripping Spectral Overlap from Data

15

Removing the Effects of Attenuation

17

Converting Count Rates to Apparent Radioelement Concentrations

18

Complete Expressions for Apparent Radioelement Concentrations

18

Calculating the Ground Level Exposure Rate

19

Calculating U/TH, U/K and TH/K Ratios

19

Chapter 2: Importing 256 Radiometric Data

21

Chapter 3: Tutorial

23

Before you begin

23

Create a Proj ect

23

Load the RPS menu

24

Open a Database

25

Step 1: Window ing Full Spectrum Data

25

Windowing a Single Channel

26

Subset Window (Optional)

27

Step 2: Configu ring RPS Data

28

Step 3: Preparin g Data

29

Checking Preprocessed Results

30

Answers to Some Common Preparation Questions

30

Step 4: Remov Remov ing Backgr ound Radiation Answers to Common Background Removal Questions

Step 5: Correctin g Airb orne Spectrometer Data Answers to Some Common Questions about Corrections

31 37

38 39

Step 6: Calcul atin g U/TH, U/TH, U/K and TH/K Ratios

39

Answers to Some Common Ratioing Questions

40

Ap pl yi ng Bat ch Pro ces si ng to You r Datas D atas et

Glossary

41

43 43

1

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Chapter 1: System Overview 2 6.

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3

Finding More Help Information There are several other functions included in the basic Oasis montaj help system that may be useful to your work. The entire documentation for the system is available through the online help system. This electronic library of information enables us to constantly update the information and provide you with the most up-to-date information available. The best way to find information in this system is to use the Search tab to perform a full-text search of all help topics. If you still can’t find the information you’re look ing for, the Online Books help system contains complete Geosoft manuals and tutorials in Adobe PDF format.

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Europe and North Africa

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Chapter 1: System Overview 5

Chapter 1: System Overview The montaj 256-Channel Radiometrics Processing extension for Oasis montaj is designed to work with airborne spectrometer data. The application comprises a special menu and software programs called Geosoft Executables (GX’s) that provides the capability to visualize and process 256 channel spectrometer data. Airborne spectrometer surveyors collect data that permit the in-situ evaluation of the concentrations of potassium (K), Thorium (Th) and Uranium (U) in the surface in a designated survey area. The instrumentation and the amount of data collected vary from system to system, but the survey objective is the same — to extract and present K, Th and U concentrations and Total Count (TC) data for interpretation. From a data handling and processing perspective, there are three main phases in airborne spectrometer surveying - acquisition, processing and presentation. montaj 256-Channel Radiometrics Processing is designed specifically for processing raw data collected from airborne surveys. In this chapter, we review each radiometric data handling and processing phase, and the related concepts that guided the design of montaj 256 Channel Radiometrics Processing. We also describe the specific tasks you can perform with this set of GXs.

Acquiring Airborne Spec trometer Data Airborne spectrometer (raw) data are collected by flying a survey system on parallel survey lines over small to large areas. Survey systems are installed in either fixedwing or rotary-winged aircraft, depending on the terrain conditions, size of area and type of earth science application. Systems typically record multispectral gamma rays detected in 255 channels over an energy range of 0 to 3000 keV. Standard recording windows are shown in the table below. Window Name

Minimum Energy (keV)

Maximum Energy (keV)

Major Peak (keV)

Radionuclide

Potassium (K)

1370

1570

1460

K-40

Uranium (U)

1660

1860

1765

Bi-214

Thorium (TH)

2410

2810

2614

Tl-208

Total Count (TC)

410

2810

N/A

N/A

Cosmic (COS)

3000

∞

N/A

N/A

Potassium, Thorium and Uranium concentrations in the ground are derived from the K, Th and U energy windows and the total ground radioactivity is represented by the Total Count window. Radioactivity from extra-terrestrial sources is reflected in the


Cosmic window. Cosmic window data are used to evaluate and remove cosmic background radiation from other windows. Most systems separate (or bin) gamma into pre-determined energy windows, and record the summed count rate (in counts per second) for each window. The nominal sample rate is one sample per second. In addition, modern systems record the full 256 channel spectrum each second. This data can be used for: •

Quality control checks on spectrometer energy calibration

•

Adjusting or re-calibrating the standard windows if necessary

•

Extracting other (new) window data in order to apply a different analysis method

Working with 256-Channel Radiometrics Processing To process airborne spectrometer data, you initially require a set of unprocessed (raw), survey measurements (plus field notes) and airborne system calibration information (from the survey contractor or extracted from test, calibration and survey data). The objective is to process the raw data to obtain a set of corrected or final data that you can interpret (qualitatively and quantitatively) and present in one or more graphical formats. When working with large volumes of data, one strategy is to divide processing tasks into logical steps. This enables you to view interim results after each step, and to monitor data quality and the effectiveness of selected processing parameters. montaj 256-Channel Radiometrics Processing organizes tasks as follows: • Import raw 256 channel data into an array channel. •

Window full spectrum data to create channels for K, U, TH, and TC.

•

Correct raw data for instrument deadtime, apply filters to average data and improve statistics, and correct survey altitude to Standard Temperature and Pressure (STP).

•

Remove background contributions from aircraft radioactivity, cosmic radiation and atmospheric radon.

•

Correcting data by removing spectral overlapping (stripping) and height attenuation effects, and converting data to apparent radioelement concentrations.

•

Calculating radioelement concentration ratios.

Correcting Instr ument Deadtim e/Livetime Effects Spectrometer data are typically acquired in units measured in counts per second. The instrumentation may require some time each second to process the incoming data during this time period no counts are made. This time is referred to as “equipment deadtime”. Alternatively, some systems record the time d uring which the crystal is actually "on", in which case, the resulting value is referred to as the livetime (sample period minus the deadtime).


256-Channel Radiometrics Processing enables you to apply either deadtime or livetime corrections to your TC, K, Th, U and Cosmic data.

Some older systems use a measured "average deadtime per pulse" to estimate total deadtime (proportional to the Total Count channel). If you select "deadtime" and specify a value (in microseconds), the GX applies the following: N = n / (1 - TC * t * 10-6)

Where: N = corrected count in each second n = raw recorded count in each second TC= raw total count data value t = equipment dead time (deadtime per pulse in microseconds). This value is typically supplied by the survey contractor.

If you do not want to apply the deadtime correction (because you have livetime data or you want to see the data without the correction), set the deadtime value to zero (0). When you specify a livetime channel name, the system applies a livetime correction: N = n * 10-3 / lt

Where: N = corrected count in each second n = raw recorded count in each second lt = equipment livetime (in milliseconds) Note:

It is also possible to use a deadtime channel in montaj 256-Channel Radiometrics Processing by converting it to a livetime channel. The fastest way to do this is to create a new channel, for example livetime, and apply a math expression. Then enter the new channel name as the livetime channel. To apply the required expression in Oasis montaj, first select the entire livetime channel (click three times on the channel header) and press the equal sign (=). The system displays the math expression window. Type 1000 - dt (where dt is the name of your deadtime channel in microseconds) in the window and press the [Enter] key. The system applies the expression and updates the channel with the new values.

Applying Nonli near and Lowp ass Filters montaj 256-Channel Radiometrics Processing enables you to implement both nonlinear and lowpass filters at various stages of the data processing cycle. Note that you can turn any filter off simply by specifying zero (0) for the width (nonlinear) or cutoff (lowpass) parameters.

Nonlinear filtering functionality is provided to assist in:


•

Removing spike-like noise in spectrometry data related to instrumentation effects

•

Compensating for sudden shifts in radar altimeter data.

Lowpass filtering is provided to help you: •

Reduce statistical noise in cosmic data and improve the statistics of radiometric window data

•

Smooth the estimated atmospheric radon value calculated from the upwardlooking window. A heavy filter (>100 seconds) is required to improved the statistical reliability of this process

•

Smooth any remaining noise in corrected potassium, uranium and thorium channels prior to calculating ratios

In general, radiometric window data should rarely require spike removal; however, it can benefit from filtering to help improve the statistical reliability of each data point. This can be done at an early stage (after livetime correction) but may also be done later since the radiometric reduction procedures are linear. The degree and type of filtering selected will depend somewhat on the survey parameters — detector volume, ground clearance, aircraft speed, radioactivity of the area, etc. Since the "sensing footprint" of the airborne gamma spectrometer has a diameter of approximately four times the ground clearance, there is a considerable "overlap" in information between adjacent samples (assuming reasonable aircraft speed). It is therefore, possible to use a lowpass (or other suitable) filter to improve the statistical reliability without significantly degrading the "spatial resolution" (ability to detect significant changes in radioelement content from data point to data point or to resolve narrow peaks). Filter widths between three and seven data points (assuming one second data) are appropriate for most surveys. It may be necessary to apply a stronger filter to the U channel due to lower count rates. To determine whether you need to use filtering, we suggest you display or plot your data, set filter parameters and run the corresponding GX. Repeat this process until you are comfortable with the resulting data. Generally, for production surveys, the parameters remain the same from survey to survey. However, you should still look at both raw and filtered data to validate your parameter selections. Note:

It is possible that you may want to implement another type of filter, such as a standard deviation or variable width filter. The standard deviation filter is sometimes used in place of a nonlinear filter to eliminate signal that is outside of a specified statistical range. Alternatively, the variable width filter may be used to improve counting statistics by comparing a data value with a certain statistical degree of confidence and sum adjacent data points until the confidence is met.


If you are interested in applying additional filters, we suggest that you implement this step prior to running 256-Channel Radiometrics Processing. When you run the system, you can then turn off any of the built-in filters that you do not require. D ESCRIPTION OF N ONLINEAR F ILTERING

A nonlinear filter is highly effective for removing high amplitude and short wavelength noise from data. It is often thought of as a noise spike-rejection filter, but it can also be effective for removing short wavelength geological features, such as the signal due to near surface features. A nonlinear filter can be followed by a linear lowpass filter to smooth any low-amplitude noise that may remain. Geosoft's nonlinear filter uses a method similar to that described by Naudy and Dreyer, 1968, to locate and remove data that is recognized as noise. The algorithm is 'nonlinear' because it looks at each data point and decides if that data is noise or valid signal. If the point is noise, it is removed and replaced by an estimate based on surrounding data points. Parts of the data that are not considered noise are not modified. Linear filters lack such a decision capability and therefore modify all data. The decision algorithm is based on the width of features in the data and the amplitude of those features relative to a local background. In order to be considered noise, a feature must be narrower than a specified width (in data points) and of greater amplitude than a specified amplitude tolerance (in data units). For example, single spikes in the data will have a width of 3 points. D ESCRIPTION OF L OWPASS F ILTERING

A lowpass filter will smooth the input data b y the application of a low-pass convolution filter to the data. The filter is called 'lowpass' because it allows low frequencies (long wave numbers) to pass to the output channel. All wavenumbers above the cutoff wavenumber (in cycles/metre) are removed. The convolution filter is designed using the Fraser method (1966). The default length is as long as the cutoff wavelength, which produces a filter that exhibits a minimum of side effects, such as Gibb's phenomena. A lowpass filter is considered linear because all data is treated by the filter equally. This can be a problem when attempting to remove short-wavelength, but high amplitude features because quite strong filters may be required. Such filters can affect parts of the data that you did not intend to disturb. An alternative is to use a nonlinear filter.

Correcting Radar Altimeter Data to Standard Temperature and Pressure In order to apply the radiometric analysis technique, it is necessary to convert actual conditions to "standard" conditions. The measured ground clearance is adjusted to standard temperature and pressure (STP). This "effective height" has the same mass


of STP air between the ground and the aircraft as that experienced during actual data acquisition. montaj 256-Channel Radiometrics Processing applies the required correction (using the RPSFILT GX) as follows: h e = (h * P * 273) / (1013 * (T + 273))

Where: h e = Effective height AGL (above ground level) at STP (metres) h = Filtered radar altitude (metres) T = Measured air temperature in C° P = Barometric pressure in kPa. If your data are not in millibar units, you must convert them before running the Preparation GX. The conversion factor is 101.325 kPa equals 1013.25 mbar.

This calculation assumes that you have radar altimeter, temperature and air pressure data. However, some systems do not provide pressure data. If this is the case, we recommend that you use a barometric altimeter or GPS Z data and calculate the barometric pressure. This can be done by creating a new channel and using the math editor to calculate: P = 101.325 exp (-H / 8581)

Where: H = barometric altitude in metres

If the value for H is supplied from a barometric altimeter set to provide "pressure altitude" (i.e. reference pressure set to 29.92 inches of mercury), the above expression is accurate. When H is supplied from GPS Z or a barometric altimeter that has been set to provide actual aircraft altitude, the above expression is somewhat in error due to the range of daily variations in atmospheric pressure from changing weather. However, this error has a relatively small impact on the radiometric results. Before converting to equivalent height (he) at STP, you may need to lightly filter your radar altimeter data to remove the effects of sudden jumps. We recommend that you examine your data initially to determine whether you need to apply a lowpass filter in the RPSFILT GX.

Removing Cosmic and Aircraft Background Cosmic radiation background is caused by high-energy cosmic ray particle interaction with the atmosphere (increasing exponentially with altitude). There is also background radiation from the radioactivity of the aircraft and its equipment. Your data acquisition contractor should provide cosmic stripping factors and aircraft backgrounds for each channel. These are obtained from a cosmic calibration flight at a range of altitudes in "radon-free" conditions.


The RPSLEVL GX calculates the cosmic and aircraft background correction for each window at each data point and subtracts the background from the data. montaj 256Channel Radiometrics Processing calculates the combined background as follows: N = a + bC

Where: N = Combined cosmic and aircraft background in each spectral window a = Aircraft background in the window b = Cosmic stripping factor for the window C = Filtered cosmic channel count C OMPARING D EFAULT C OSMIC AND A IRCRAFT B AC K GRO UND P AR AM ETE RS

You may compare your cosmic and aircraft background values with the ones below which are for spectrometers utilizing the standard energy windows. These values are for a system with five 4.2 litre NaI crystals detec tors (four down and one upward). Channel

Aircraft Backgroun d a [counts per second]

Cosmic Stripping Factor b [counts per Cosmic count]

TC

45

0.6

K

6

0.032

U

1.1

0.026

TH

0.75

0.030

UPU

0.30

0.0080

If your data were acquired with more than one package, multiply the typical values accordingly. For instance, the defaults in the GX correspond to a system with two standard detector packages (33.6 litres of NaI down and 8.4 litres upward). Aircraft contributions to background can vary considerably between different aircraft. The cosmic factors are relatively independent of the number of detector packages but can vary somewhat for different installations.

Removin g Radon Background Radiation The most difficult background radiation component to remove occurs from the decay of radon gas in the atmosphere. The daughter products of radon decay produce a spectrum virtually identical to that of the uranium decay being measured. Radon gas in air diffuses at rates dependent on factors such as air pressure, soil moisture, ground cover, wind speed and patterns, and temperature. All of these factors vary with time over the course of a survey.


As discussed in the Gamma Ray Spectrometer Surveying report published by IAEA, there are four methods available for radon background estimation: •

Flying at survey altitude over water

•

Using upward-looking detector crystals

•

Flying a survey altitude test line

•

Bi-214 peak at 609 keV (used in southern hemisphere where there is less fallout contamination. It replaces the upward crystal method although it is implemented in the same way).

The default 256-Channel Radiometrics Processing method is the upward-looking crystal method. You can also select a table-based or interactive overwater methods. The test line method is not supported because it is rarely performed in practice. The Bi-214 method is not currently supported. R EMOVING R AD ON B AC KG ROU ND USI NG U PWARD L OOKING D AT A

If the survey system measures data from upward-looking uran ium crystals, these data can be used to perform radon background corrections. The corrections are functions of measured count rates (corrected for cosmic and aircraft backgrounds) and factors as referred to in section 4 of the IAEA report. When you apply the Upward method in 256-Channel Radiometrics Processing, the system first prompts you to enter "skyshine" coefficients (A1 and A2) and atmospheric radon calibration constants (aTC, aK, aUp, aTh, bTC, bK, bUp and bTh) as follows: A 1 and A 2 = Skyshine coefficient or calibration factors. These factors are determined from the equation u g = A 1UG + A 2Tg Where: u g, Ug and Tg are the contributions in the windows that originate from the ground. Ug, Ug and Tg must be calculated independently by the user as described in IAEA Technical Report 323, and then A 1 and A 2 can be determined using a least squares method. a TC= Radio of downward TC and U windows a K = Radio of downward K and U windows aTh = Radio of downward Th and U windows aUp = Radio of upward U and downward U windows b TC = TC intercept for U = zero (0) b K = K intercept for U = zero (0) b Th= K intercept for U = zero (0) b Up = Upward U intercept for U = zero (0)

The a and b values are supplied by the survey contractor and are determined using the method described in IAEA Technical Report 323. If the cosmic/aircraft background constants have been well determined, the radon b values should be quite


small and in most cases, set to zero. This is the default for montaj 256-Channel Radiometrics Processing. You can change them as required. When the RPSLEVL GX runs, it evaluates the following expression to determine the radon component: Ur = (u Up - A 1U -A 2Th +A 2b Th -b Up ) / (aUp - A 1 -A 2aTh)

Where: Ur = Uranium radon component u Up = Upward-looking Uranium count (aircraft/cosmic background removed) U

= Uranium count (aircraft and cosmic aircraft background removed)

Th = Thorium count (aircraft and cosmic background removed)

When you use the upward method, the system prompts you to enter the channel name for your raw upward uranium data and specify filtering parameters for interim channels used to calculate the radon component. The purpose of the lowpass filters is effectively to determine a regional radon component so that we recommend that you heavily filter these channels (> 100 seconds). After evaluating the radon component, the system evaluates the following expressions (that describe the radon component in each window) and subtracts the corresponding value from each of the Total Count, Potassium, Uranium and Thorium windows: u r = aUpUr + b Up = Upward-looking Uranium radon component K r = aKUr + b K = Potassium radon component THr = aThUr + b Th= Thorium radon component TCr = aTCUr + b TC = Total Count radon component R EMOVING R AD ON B AC KG ROU ND USI NG THE B AC KG ROU ND T AB L E M ETHOD

Survey flights over water yield no response from the ground, meaning that the uncorrected data represents the combined background due to aircraft, cosmic and radon sources. montaj 256-Channel Radiometrics Processing takes advantage of this relationship to provide you with two methods for removing radon background. The first method, called the Background Table method enables you to use a table of radiometric values corresponding to locations at which the detector is over water. The second method, called the Overwater method, enables you to create a reference channel containing one (1) values at locations at which the detector is overwater. With this method, the system automatically selects the co rresponding radiometric values and performs the radon correction. Since you do not have to create a table, the Overwater method is recommended. However, we provide the Background Table method because it is useful when you have to "invent" or adjust backgrounds.


The Background Table method assumes that your survey has data flown over lakes and rivers and that the data was acquired at survey altitude (altitude must be monitored since radon is inversely proportional to height). The type of table and selection of reference points or segments of line data you include depend on the survey procedures followed in the field, the equipment used, the availability of over water measurements, the fiducial or time channels recorded, and other factors. The following example shows a sample background removal table called rpsbackg.tbl. This table is shipped with the 256-Channel Radiometrics Processing. / / / /= /= /= /= /= / /

ATMOSPHERIC RADON CORRECTION TABLE FOR BACKGROUND TABLE

METHOD

FID:REAL:I URADREF:REAL KRADREF:REAL THRADREF:REAL TCRADREF:REAL FID 28825 29382 32117 37913 39464 40207

URADREF 157.3 157.3 118.4 75.8 79.9 79.9

KRADREF 10.0 10.0 8.3 5.5 5.1 5.1

THRADREF 12.7 12.7 10.2 6.4 6.9 6.9

TCRADREF 1.3 1.3 1.2 0.8 0.7 0.7

/ / FIRST AND LAST VALUES ARE REPEATED AT BEGINNING AND END OF SURVEY FLIGHT / TO EXTRAPOLATE RADON BACKGROUNDS TO THESE POINTS.

R EMOVING A IRCRAFT, C OSMIC AND R AD ON B AC KG ROU ND I NTERACTIVELY

montaj 256-Channel Radiometrics Processing also provides a specialized, interactive method for removing the combined background from aircraft, cosmic and radon sources.

The basic approach is to interactively mark all fiducial, date or time points where you know the system is over water (i.e. where the contribution from ground sources is minimal) in a reference channel. When you run the RPSLEVL GX, the system uses this reference channel to create interim reference channels (called KRADREF, THRADREF, URADREF and UPURADREF) consisting of interpolated background values. The system then subtracts the interpolated values from each of the input radiometric channels. This method only works on individual lines. Since it is time consuming to interactively select and process line data, we suggest that you work with flight data (a single, long line of data). You can then run all GXs (as well as the Overwater method) on the flight data and reformat data in lines later.


An effective way to display your results is by creating a custom data view consisting of the UFILT, RADREF, URADREF and ULEVL channels. Reference channel points are displayed as symbols and all other channels are displayed as lines. To create a channel with symbols, select Profile|Show symbols option in Oasis montaj. We recommend displaying your data in this format routinely to assist in quickly evaluating the success of the interpolation process and background removal. When you use this method, you also have the option of applying a broad filter to the interpolated KRADREF, THRADREF, TCRADREF and URADREF channels. The objective of filtering is to improve the statistics for these channels. E VALUATING A IRCRAFT, C OSMIC OR R AD ON B AC K GRO UND I NDIVIDUALLY

As part of the quality control process, Earth Science professionals may want to evaluate the resulting data from one type of correction, such as aircraft or radon corrections, only. Effectively, this means that the user must have the ability to switch certain corrections on or off as required. To switch off aircraft and background corrections in 256-Channel Radiometrics Processing, simply set the corresponding background and stripping values to zero (0) and run the radon removal process. The resulting data in the KLEVL, THLEVL, TCLEVL and ULEVL channels will have radon corrections only. To switch off radon corrections, set your aircraft background and/or cosmic stripping values as required. Then select the Background Table radon removal method and enter (rpszero.tbl). This table contains all zero values. When the GX runs, it only performs the selected aircraft and/or cosmic removal and stores the results in the TCLEVL, KLEVL, ULEVL and THLEVL channels.

Stripping Spectral Overlap from Data Spectral stripping is a prerequisite in airborne spectrometry data processing to give the counts in the potassium, uranium and thorium windows that are uniquely from potassium, uranium and thorium. The spectral ratio refers to the counts in one window to the counts in another window for pure sources of U, K, and TH. The notation adopted for these ratios is: α = U/TH (pure source); β = K/TH (pure source); γ = K/U (pure source);

a = TH/U (pure source); b = TH/K (pure source); g = U/K (pure source);


Stripping ratios are determined experimentally by a procedure over calibration pads. The values of α, β, and γ increase with altitude above the ground - a correction is applied to these ratios based on the STP equivalent altitude according to the following theoretically and experimentally determined factors: Stripping Ratio

Increase per Metre

α

0.00049

β

0.00065

γ

0.00069

The RPSCORR GX calculates the α, β, and γ stripping ratios at STP equivalent height for each record and uses these in the stripping calculations. After you enter stripping ratios and run the GX, the system performs the following calculations: αe= α + 0.00049 * he βe= β + 0.00065 * he γe= γ + 0.00069 * he where

he = equivalent height AGL (above ground level) at STP.


The GX then applies stripping corrections to preprocessed and background-removed data (i.e. LEVL channels) as follows: nK,K = [n TH (αe γe - βe) + n U (aβe - γe) + n K (1 - aαe)] / A nU, U = n TH (gβe -αe) + n U (1 - bβe) + n K (bαe - g)] / A nTH, TH = n TH (1 - gγe) + n U (bγe -a) + n K (ag - b)] / A A =1- gγe - a (αe - gβe) - b (βe - αe γe) where nK = background corrected counts in K nU = background corrected counts in U nTH = background corrected counts in TH nK,K = stripping corrected counts in K nU,U = stripping corrected counts in U nTH,TH = stripping corrected counts in TH

Removin g the Effects of Attenuation After preprocessing, removing backgrounds and stripping, the RPSCORR GX removes the attenuation effects of the air due to the height of the sensor above the ground. It applies the following correction to each of the TC, K, U and Th channels (data in LEVL channels): Ns = Nm exp [µ(h0 - he)] where Ns = the count rate normalized to the nominal survey altitude ho Nm = the background corrected, stripped count rate at STP equivalent height he µ = the attenuation coefficient for the window

Before running the GX, you must supply the attenuation coefficients for each channel. Your contractor typically flies a trial survey at different heights over a calibration range and supplies you with these values. This procedure is described in the IAEA Technical Report 323. If you do not have values readily available or want to test the processing system, you can run the GX with the following typical values: Window

Height Attenuation Coefficient (per metre at STP)

Total Count

-0.0070

K

-0.0088

U

-0.0082

TH

-0.0070


Converting Count Rates to Apparent Radioelement Concentrations The final correction the RPSCORR GX applies is to convert count rates to apparent radioelement concentrations. The term, apparent concentrations, refers to the concentration in the ground of the elements potassium (K), uranium (U), and thorium (TH). This term also refers to the apparent Total Count at ground level, which is also calculated by the system. These estimations are useful because they yield results that are independent of survey variables such as crystal volume and survey height. Having this data may enable you to join adjacent survey data (using concentration data). In the RPSCORR GX, the system calculates the K, U, and TH apparent concentrations as follows: C = Ns /S

Where: C = the apparent concentration of the element (K percentage, eU ppm, eTH ppm) Ns = the count rate for each window (from the attenuation section) S = the broad source sensitivity for the window (see IAEA for determination)

If you do not have sensitivities or want to test the system, you can use the following values: Window

Sensitivities

TC

23 [(c/sec) / (ng/h)]

K

75 [(c/sec) / %K]

U

7.5 [(c/sec) / eppm ]

TH

4.5 [(c/sec) / eppm ]

Complete Expressions for Apparent Radioelement Concentrations To assist you in determining how the system calculates final concentrations, we have included complete expressions for Potassium and Total Count channels. The complete expression (stripped, height corrected result) for the apparent Potassium concentration is: CK = [n TH (

e

e -

e)

+ n U (a e -

e)

+ n K (1 - a

e)]

/ A * exp [ (h 0 - h e)] / SK

In this expression, the nK , nU and nTH values are the background-corrected data (KLEVL, ULEVL and THLEVL) created when you ran the Background GX.


The complete expression (height-corrected result) for the apparent Total Count concentration is: CTC = n TC * exp [ (h 0 - h e)] / STC

Similarly, the nTC value is the background-corrected result (TCLEVL) created when you ran the Background GX.

Calculating the Ground Level Exposure Rate If you are performing a spectrometer survey for environmental or human health purposes, you may be interested in the ground level exposure rate. montaj 256Channel Radiometrics Processing calculates this value as a function of the K, U, and TH apparent concentrations: E =1.505K + 0.653eU + 0.287eTH

Where: E = ground level exposure rate [ R/h]

The system stores the result in a separate channel ( TCEXP). This calculation only includes the gamma ray exposure from radioactive sources in the ground.

Calcu lating U/TH, U/K and TH/K Ratio s In calculating ratios, there are two main challenges to consider, namely: •

How do you process data to avoid zero values in the ratio denominator?

•

How do you extract meaningful ratios from areas that have low count rates?

montaj 256-Channel Radiometrics Processing has two options for dealing with the question of the zero denominator. The first is to insert placeholder (dummy) values that tell the system not to process certain data. The second option is to set values below a certain threshold to a specified minimum concentration. The advantage of using the DUMMY method is that 256-Channel Radiometrics Processing handles placeholders elegantly — simply interpolating based on the adjacent values. montaj 256-Channel Radiometrics Processing deals with the question of low count rate data through a filtering process. Since regions that have low count rate have higher errors, lowpass filtering is implemented to smooth the data. This is a simple technique based on the concept that smoother input data produces smoother output.

Another way of dealing with the regions that have low count rates is to apply a variable width filter. Variable width filters compare the data at a single point with a statistical benchmark value. If the data falls below this value, the filter adds adjacent data and compares this summed value. This process continues until the counting statistics meet the benchmark value. The result is that areas with low count rates and high count rates have a predefined statistical confidence level.


Although 256-Channel Radiometrics Processing does not currently implement a variable width filter, it is possible for you to intercept (export) the data before you run the RPSRATIO GX, apply a variable width filter and import it back into Oasis montaj for final ratioing. If you are interested in this approach, we suggest that you first copy the corrected K, U and TH data in the KCORR, UCORR and THCORR channels to backup channels in Oasis montaj. Then export this data, apply the filter and import it back into Oasis montaj into the KCORR, UCORR and THCORR channels. You can now run the RPSRATIO GX.

Chapter 2: Import ing 256 Radiometri c Data 21

Chapter 2: Importing 256 Radiometric Data To process 256-channel radiometric data, you must import the data into an array channel of an Oasis montaj database. An array channel stores multiple values (or measurements) for a single location as a profile line in a single cell of the database. This is different from a regular channel that contains a single number value in a cell. The array channel format is used for 256-channel data because of the normally large volume of data. Since data volumes are large, we recommend creating a template file that describes the data format. Once a template is created, it can be reused for importing data in the future. The following table describes various strategies available for importing and working with 256-channel data depending on the data source. Note that 256 channel data must be stored in a channel of Array type. The import templates each support the creation of Array channels in an Oasis montaj database. Data Source

Import description

Picodas PDAS data file

256-channel Picodas PDAS can be imported using the SCANPICO and PICODAS GXs. To access these GX’s, on the Data menu, click Import , then click PDAS , then click scan data… The SCANPICO GX will scan an existing PDAS file and create a template to import all data contained in the file. Normally, this template should be edited to remove any data components that are not required for your application. Refer to the on-line help for these GX’s for more information on working with PDAS files.

RMS data file

An RMS acquisition system will store 256-channel radiometric data in a blocked binary format which should be documented by the survey company that has collected the data. The IMPORTBB GX can be used to import such data. To access this GX, on the Data menu, click Import, then click RMS or Blocked Binary . This GX requires a template that accurately describes the contents of the RMS file. Refer to the on-line documentation of the IMPORTBB for information on ho w to create a template.

Binary survey file

The data may be stored in an existing documented binary data file. The IMPORTBB GX can be used to import such data. To access this GX, on the Data menu, click Import, then click Blocked Binary . This GX requires a template that accurately describes the contents of the Blocked Binary file. Refer to the on-line documentation of the IMPORTBB for information on how to create a template.

ASCII flat archive

If the data is stored in ASCII flat archive format, use the IMPARC GX to import the data. To access this GX, on the Data menu, click Import, then click Flat archive . A Flat archive file contains the data organized in rows and columns. The IMPARC GX supports importing of both fixed-data and free-formatted (comma or space delimited) data. IMPARC requires a template to describe the data to be imported. Refer to the on-line documentation of the IMPARC GX for information on how to create an ASCII file template for your data.

Third party system

If the data is contained in a third-party software system, you must export the data to one of the formats above to be imported into Oasis montaj. If your processing operations will require importing such data on a regular basis, Geosoft Technical Solutions can develop an automated import procedure for your data. Contact your local Geosoft office for more information on this service.

Note: If you are having difficulty please contact your local Technical Support office.

Chapter 3: Tutori al 23

Chapter 3: Tutorial This tutorial will show you how to extract information from 256 channel data and create the following channels: K, U, Th, TC, and Cosmic. Please note that this tutorial includes optional procedures that may be useful in the future, but are not necessary to complete the tutorial. These optional procedures are included in the tutorial for information, and to show you when you would perform these tasks if your data required them.

Before you begin This tutorial uses sample data provided on the Oasis montaj v 6 CD-ROM. This data can be found in your ‘C:\Program Files\Geosoft\Oasis montaj\data\rps’ directory or downloaded from the Geosoft web site (www.geosoft.com/downloads/). The data used in this tutorial is provided in a Geosoft database file (256_spec.gdb) and two table files (rpsbackg.tbl and rpszero.tbl). You do not have to import any data for this tutorial. Create a working directory for this tutorial on your computer called D:\Tutorial. Copy the data files into this working directory.

Create a Project In order to access the Radiometric Processing System menus in Oasis montaj, you must have an open Project. An Oasis montaj "Project" encompasses every item in your working project; from the data files in your project (databases, maps, and grids), to the tools used (including auxiliary tools such as histograms, scatter plots etc.), to the project setup including the menus you have displayed and whether you are working on a map or profile and the state in which you left it the last time you used it. The project also controls your working directory. Projects are saved as (*.gpf) files. If you open an existing project from a directory, the system assumes that all your project files are located in the same directory. To streamline your work, as well as keep it organized, you may wish to make sure that your project file is in the same directory as the other files you want to use. We recommend that each project you work on have its own project (*.gpf) file. If you use a number of applications or addon tools in Oasis montaj that have different menus, you can use the project to display only the menus you require. The Project Explorer tool enables you to browse as well as open any project item. The Project Explorer has two tab windows, the Data window that includes all data files included in the project and the Tools window that organizes and maintains the project tools. To access the Tools window click the Tools bar on the bottom of the Project Explorer. To return to the Data window, click the Data bar on the top the Project Explorer.

24 Chapter 3: Tutori al

Important Note: Workspace files (*.gws) used in Oasis montaj prior to version 6.0 can be easily converted to Project files (*.gpf) simply by opening them in Oasis montaj 6.0. On the Open Project dialog (File|Project|Open) select File of Type as "Workspaces (*.gws)" and when asked if you want to convert the old workspace into a new Oasis montaj project file, select "Yes". The workspace file will be converted to a project file and all associated workspace information will be transferred to the new project file. In addition, the workspace file will remain untouched so that it can be opened in previous versions. T O C REATE A P ROJECT : 1.

Start Oasis montaj.

2.

On the File menu click Project and then click New. The New Project dialog is displayed.

3.

Specify a name and directory for the project. For example, name the project rps and specify the working directory as D:\Tutorial. Oasis montaj will automatically look for your data in the directory containing this project. Make sure you copy the sample database file (256_spec.gdb) and the two table (*.tbl) files to your project directory ( D:\Tutorial).

4.

Click the [Save] button. The system saves the project and indicates it is open by opening the Project Explorer window, enabling the buttons on the Main toolbar and adding menus to the menu bar. These are visual clues indicating that you are ready to start working with the system.

Load t he RPS menu To begin working with the RPS system you must load the RPS menu (Radiometric_Processing_System.omn) 1. 2.

On the GX menu, click Load menu. The Load Menu dialog is displayed. Select the Radiometric_Processing_System.omn file, and click [Open]. The RPS menu is displayed on the main menu bar.


Open a Database You must have a database open in your working directory to apply the RPS system to your data. 1.

On the Data menu, click Open Database . The Open Database dialog is displayed.

2.

Select the 256_spec.gdb file and click [Open].

3.

The 256_spec.gdb database will be displayed in the spreadsheet window.

Tip:

When you open the database, you will notice that the profiles in the array channels are very small. You can make these lines easier to see by increasing the spreadsheet font size. To do this, click the Edit menu, and then click Font . Choose a fixed width font such as ‘Courier New’ with a large point size (i.e. 20pt).

Step 1: Wind owi ng Full Spectrum Data To process full spectrum (256-channel) radiometric data, you will need to extract Region Of Interest (ROI) windows for Total Count (TC), Potassium (K), Thorium (Th), Uranium (U), and Cosmic radiation from spectral radiometric data stored in an array channel. Each spectral channel corresponds, in this case, to the number of gamma rays detected with energy levels falling within a pre-determined range. Historically, radiometric processing has been carried out on predefined regions of interest (ROIs), namely potassium (K), uranium (U), thorium (Th) and total count (TC), each of which span a number of spectral windows. The lower and upper limits of an ROI are defined in terms of the spectral channels. The first channel is 0 and, in the case of a 256 channel spectrometer, 255 is the last. When extracting single spectral channels, such as COSMIC, which is usually located in the last spectral channel (255), LOWER would be defined as 255.0 and UPPER as 256.0. You should consult your spectrometer specifications to determine the optimum windows for your spectrometer. The default window limits noted above are typical for a Picodas spectrometer. The procedures below assume that you have already imported your spectrum data into a database. T O WINDOW 25 6 C HANNEL D AT A : 1.

On the RPS menu, click 256 channel , and then click Window All Channels .

2.

The Window spectral radiometric data dialog box is displayed. Select TC as the spectral data channel . This is the array channel that contains the raw 256 channel data.


3.

Specify the window range for each of the elements (K, U, Th), the Cosmic ray channel and the Total Count channel. The data ranges shown above are for the Picodas spectrometer, the ranges will vary with different instruments. For more information specifying window ranges, click the [Help] button on the dialog box.

4.

Click the [OK] button. New channels are created showing the sum for each region of interest. The created channels are: K_raw, U_raw, Th_raw, TC_raw, Cosmic_raw.

Windowin g a Single Channel To window the Upward Uranium channel (Up_U_raw), follow the procedure below to extract the sum of values from a window of the Upward Total Count (U_TC) array channel to a new channel. T O W INDOW A S INGLE C HANNEL : 1.

On the RPS menu, click 256 channel , and then click Window a Single Channel .

2.

The Fraction window of an array channel dialog box is displayed. Select U_TC in the Array channel to window box. This is the array channel that contains the raw 256 channel data.


3.

Specify the output channel to use (Up_U_raw). This channel contains the simple sum of values in the array window. If any array values are dummy (i.e. missing as indicated by an asterisk), the result will be dummy.

4.

For more information click the [Help] button.

Subset Window (Option al) If you want, you can create an array channel that is a subset of another array channel. This is useful if you want to view a specific range of the spectrum, such as the range of a particular element. Note:

When you display an array channel with a smaller spectrum (range of values), the profile line in the spreadsheet cell is larger and easier to see.

1.

On the RPS menu, click 256 channel , and then click Subset window. The Create a subset array channel is displayed.

2.

In the Input array channel box, specify the array channel (TC) containing the full spectrum survey data.

3.

In the Output subset array channel box, specify the new array channel (i.e. TC_crop) you want to create to contain the subset array.

4.

In the subset Start element(>=0) box, specify the value you want the subset array to begin at (60).

5.

In the subset End element box, specify the value you want the subset array to end at (125).

6.

Click the [OK] button to create the new array channel containing the subset.


Step 2: Conf igu rin g RPS Data This step is a “bookkeeping” step to define the channel names you want to use for each type of compound and reading. Defining all the channel names will help you to keep your data organised during processing. If you want to see more information during this procedure, click the [Help] button on any the dialog boxes. T O D EFINE R AW D AT A C HANNELS (RPSCNFIG GX): 1.

On the RPS menu, click Configure . The Define Raw Data Channels dialog is displayed.

2.

Specify the names of the channels for the radiometric counts and the altimeter settings as shown below:

3.

Click the [OK] button to continue. In the Define Output Data Channels dialog box, specify the names of the channels for the corrected data, ratios and other information as shown below:


4.

Click the [OK] button to finish.

Step 3: Preparing Data You can use the Preparation menu item to perform one or more specific tasks including: •

Correcting the radiometric window data for spectrometer deadtime

•

Applying nonlinear and lowpass filters

•

Converting radar altimeter data to Standard Temperature and Pressure (STP).

This section describes how 256-Channel Radiometrics Processing helps you to remove deadtime effects, filter data and correct radar altimeter data. It also tells you how to run the Preparation GX. T O P ERFORM D EADTIME C ORRECTIONS AND A PPLY F ILTERS (RPSFILT GX): 1.

On the RPS menu, click Preparation . The Define Deadtime OR Livetime Parameters dialog is displayed .

2.

Select a method for applying deadtime corrections (Deadtime or Livetime). •

If you have a single deadtime value for the entire survey, select the Deadtime method and type the value in microseconds per pulse. The deadtime value is


preset to 0 meaning that the correction is not applied. If you want to see the effect of deadtime corrections on the dataset, try entering another value (in microseconds). •

If you have a livetime channel , select the Livetime method and type the corresponding channel name. Make sure the units for your livetime channel are in milliseconds and set the instrument deadtime to zero (0) in the dialog box.

3.

When you have selected a method, click [OK]. The Define Optional Lowpass Filter Values dialog box is displayed:

4.

If you want to apply nonlinear or lowpass filters, specify filtering parameters or accept the defaults. To turn off one or more nonlinear filters, set the filter width to zero (0). To turn off one or more lowpass filters, set the wavelength cutoff to zero (0).

5.

When you are finished setting filter parameters or turning off filters, click the [OK] button. The GX runs and stores corrected filtered data in corresponding filtering channels in your database.

C HECKING P REPROCESSED R ESULTS

After you apply filters, we recommend that you view the data to ensure your process is working and that your parameters are correct (i.e. provide sufficient filtering). If you are not satisfied with the results, simply reset your parameters and run the GX again.

Answ ers to Some Commo n Preparation Questi ons If you’re not sure how to set up this GX, look below to find answers to some more common questions: Q: What should I do if I do not have a DEADTIME channel or contractor-supplied deadtime value?


A: If you know the model of spectrometer used, the manufacturer or another user should be able to provide an estimated deadtime per pulse value. Many spectrometers used in radiometric surveying have a deadtime per pulse in the order of 10 to 12 microseconds. If you cannot find specific information for your system, use this value. Q: What are typical filter parameters for radioelement channels?

A: Typical values for filtering U, K, TH, and TC are NL Width = 2 data points, NL Tolerance = 5 counts per second and Short Wavelength Cutoff = 7 fiducials. Of course, the values chosen depend on the data sampling rate, aircraft speed, and data noise levels. If the parameters chosen do not have the desired effect, change them and try again. Q: How do I turn filters off if I do not want to use them?

A: To turn off nonlinear filters, specify zero (0) for the filter width. To turn off lowpass filters, specify zero (0) as the low cutoff frequenc y. Q: What do I do if I don't have temperature data?

A: You can estimate the temperature based on location and season. Another solution is to contact the flight services section of the airport nearest the survey area. They should be able to provide historical temperature data at the airport. You can then extrapolate this to the survey area using a standard temperature lapse rate and the mean altitude of your survey area. Q: What other filtering techniques are used for radiometric window data?

A: The Geological Survey of Canada uses a 5-point Savitsky-Golay filter to improve profile presentation. There is also a special recursive filter specifically for radiometric profile data.

Step 4: Removi ng Background Radiation After you prepare your data, the next step in the process is to remove Aircraft and Cosmic background radiation, and atmospheric radon background radiation. The Radiometric Processing System first corrects data for co smic and aircraft background radiation. It then corrects data for atmospheric radon background radiation using one of several methods (Upward-looking crystals, Overwater, or Background table methods). This section describes how to perform both Aircraft/Cosmic and Radon Background Removal. It also details how you can use the system to turn each removal process on and off. This capability can be useful if you want to evaluate aircraft, cosmic or radon components individually before applying all corrections.

32 Chapter 3: Tutori al T O R EMOVE B AC K GRO UND FRO M R AD IOM ET RIC D AT A (R PSLEVL GX)

To level your database, you must complete multiple dialog boxes. The first of these is for aircraft background and stripping corrections and the second is for selecting a radon levelling method. Depending on the method you select, the system displays either two more or one more dialog boxes. 1.

On the RPS menu, click Background . The Specify Aircraft Background & Cosmic Stripping Values dialog is displayed.

2.

Specify the aircraft background and stripping values. These values are normally supplied by your survey contractor. You can either enter new values or accept the defaults as displayed. Click [OK].

3.

The Select Radon Background Levelling Method dialog box is displayed. Select a radon background method from the dropdown list (Upward).

4.

Click the [OK] button to continue. The following section (A) describes the steps involved in the Upward method.

Note:

The procedures for the Background Table and Over Water methods are provided in sections (B) and (C).

Chapter 3: Tutori al 33 A) Upw ard Metho d

1.

The Define Up Uranium Deadtime OR Livetime Parameters dialog box is displayed.

2.

Select a method from the dropdown list (Deadtime) and specify the instrument deadtime (12).

3.

Click the [OK] button. The Specify Values for Upward Crystal Radon Levelling dialog box is displayed.

4.

Specify the "skyshine" coefficients (A1 and A2) and the atmospheric radon calibration constants. Descriptions of these coefficients are in Chapter 1, Removing Radon Background using Upward Looking Data topic.

5.

For the purpose of this tutorial we will accept the default values. The default values correspond to a system with two standard detector packages (33.6 litres of NaI down and 8.4 litres upward.

6.

Click the [OK] button, the Define Channels & Filters for Upward Crystal Radon Levelling dialog box is displayed.


7.

This dialog box is used to confirm the raw upward uranium data channel (Up_U_raw) and to specify filtering parameters for interim upward u ranium, uranium and thorium channels.

8.

You can also change the names of the interim processing channels, such as UPUTEMP, although this is not necessary.

9.

You can apply filters to any of the upward uranium, uranium and thorium channels. Since the upward uranium channel typically has a very low count rate, we suggest that you apply a heavy smoothing filter (for instance, 200 fiducials) to suppress statistical noise.

10. Click the [OK] button. The system removes the background and stores the data in

corresponding background removal channels URADREF, KRADREF, THRADREF and TCRADREF. You can display these data as profiles to see if reasonable (expected) backgrounds have been calculated. B) Background Table Method

Complete steps 1 – 4 of the “To Remove Background from Radiometric Data” procedure before continuing with the following steps in the Background Table method. 1.

If you select Backgnd_Table from the Select Radon Background Levelling Method dialog box, the Table Method – Minimum 2 points per flight dialog is displayed.


2.

3.

Select the reference channel for background removal (FID) and select the name of the background file (rpsbackg.tbl). For more information about values in the table, refer to Chapter 1, Removing Radon Background using the Background Table Method . Click the [OK] button. The system removes the background and stores the data in corresponding background removal channels in your database. The radon backgrounds are stored in channels URADREF, KRADREF, THRADREF and TCRADREF. You can display these data as profiles to see if reasonable (expected) backgrounds have been calculated. C) Overwater method

Complete steps 1 –4 of the “To Remove Background from Radiometric Data” procedure before continuing with the following steps in the Overwater method. 1.

If you select Overwater from the Select Radon Background Leveling Method dialog box, the Overwater Method dialog box is displayed.

2.

You must define an Overwater Reference Channel in Oasis montaj, and interactively view your database and mark fiducial, time or distance reference points where you know the system is over water (i.e. select the centre of each over water segment). Marking reference points consists of inserting a one (1) value in the reference channel for the fiducials that are over water. If you have not done this yet, click the [Cancel] button to exit the current dialog box.

3.

To create a radon reference channel, click the left mouse button on the header cell of an empty column in the spreadsheet. Type the name of the reference channel (overwater_ref ) and press the enter key. Click [OK] on the Create Channel dialog box.

4.

Scroll down the reference channel and type the number 1 in for the fiducials that are over water.

36 Chapter 3: Tutori al 5.

On the RPS menu, click Background . Specify the aircraft background and stripping values. Click [OK]. Select the Overwater method and click [OK].

6.

You will see a dialog box for specifying a reference channel and a lowpass filter width for the resulting interpolated channels.

7.

Select the overwater reference channel (overwater_ref ).

8.

Choose the filter width (75) to reflect the average length of the overwater segments but not so long as to contaminate the shorter segments with adjacent ground-source data.

9.

Click [OK]. The system interpolates background values between all fiducials marked by one (1) values and removes the aircraft, cosmic and radon background from the potassium, thorium, total count and uranium c hannels.

10. The radon backgrounds are stored in channels URADREF, KRADREF,

THRADREF and TCRADREF. You can display these data as profiles to see if reasonable (expected) backgrounds have been calculated.


Answ ers to Common Backgr ound Removal Questi ons If you’re not sure how to set up this GX, look here to find answers to some more common questions: Q. I have upward looking data, but I want to use the Overwater method for radon background removal. Which method is better?

A. If you have many good overwater backgrounds, you can use the overwater method. Q: What if I don't have a cosmic window channel or aircraft background values?

A: It may be possible to deal with these deficiencies if you have frequent background overwater (BOW) sections within your survey data. The aircraft background can be included in the "radon" background measured over water. The se two backgrounds are then combined and dealt with together as "radon". Use a zero value for the aircraft background prior to evaluating the BOW data with the RPSZERO table. The lack of cosmic data is more problematic, but if your survey area doesn't have a significant altitude range (say about 1000 feet or so), then you can also add the cosmic background in with the radon by setting the cosmic coefficients to zero as well. The BOW data will then contain radon and aircraft background plus a varying cosmic value. Use the Background Table method to apply these values (i.e. run the RPSLEVL GX again). The error due to varying cosmic (1000 foot altitude range) will be less than approximately +/- 0.5 c/sec for K, U and Th and approximately +/- 6 c/sec for TC (these estimates use a 33.6 litre detector system with two standard packages). Q: What length of lowpass filter should be used for the atmospheric radon (Ur) derived from the upward-looking technique?

A: The best filter length must be determined by trial and error since it depends on a number of factors. These include the volume of the upward-looking crystal, shielding geometry, atmospheric radon concentration and variability, objectives of the survey, type of terrain, etc. Try using 100, 200 and 300 second filters and evaluate the resulting uranium image (after completing the processing each time). A longer filter improves the statistical reliability and tends to reduce level busts that can occur if shorter term variations in the atmospheric radon are smeared out.


Step 5: Corr ecting Air bor ne Spectro meter Data The Corrections dialog (RPSCORR GX) performs three processing functions — spectral stripping, removing the effects of height attenuation and calculating radioelement concentrations. T O C ORRECT R AD IOM ETR IC D AT A (R PSCORR GX): 1.

2.

On the RPS menu, select Corrections . The Specify Correction Parameters 1 dialog is displayed.

You can specify new values for the stripping ratio values; however for the purpose of this tutorial we will accept the default values.

Note:

3.

The Stripping Ratio values you enter for the correction are measured at “ground level”. The GX then calculates Alpha, Beta and Gamma stripping ratios based on the STP equivalent height. The GX then applies stripping corrections to preprocess and background removed data.

Click the [OK] button; the Specify Correction Parameters 2 dialog box is displayed.


4.

In this dialog box, you can specify new values for height attenuation coefficients and sensitivities, or accept the defaults, and click [OK].

5.

The Correction dialog runs and stores corrected data in the corresponding CORR channels in your database. It also stores data in a ground level exposure rate channel.

Answ ers to Some Common Questio ns about Correcti ons If you’re not sure how to set up this GX, look here to find answers to some more common questions: Q. What should I do if the system sensitivities for the survey system are not available?

A. Use the “typical sensitivities” included in the table. Q. What should I do if the height attention values for the survey system are not available?

A. Use zero, and note this on the image products and in the report. Q. When entering the stripping ratios, what height should be entered, the nominal flight level or ground level? A. The stripping ratio values are calculated at ground level.

Step 6: Calculating U/TH, U/K and TH/K Ratios montaj 256-Channel Radiometrics Processing uses the apparent concentration data created using the Corrections option to calculate standard U/TH, U/K, and TH/K that are often used for final map presentation and for detecting subtle geologically significant features in the data.

40 Chapter 3: Tutori al T O C AL CUL AT E C ONCENTRATION R AT IOS (R PSRATIO GX): 1.

On the RPS menu, select Ratios. The Specify Radioelement Ratioing & Lowpass Filtering Values dialog is displayed.

2.

In the dialog box, specify new minimum concentration and filtering cutoff values, or accept the defaults.

3.

Using the Choose CLIP or DUMMY for minimum threshold method dropdown menu, select a method (Dummy).

Note:

4.

If you select Clip, the system changes any values below your minimum concentrations to the minimum concentration values. If you select Dummy, the system replaces these values with placeholder (dummy) values.

When you are finished, click [OK]. The RPSRATIO GX runs and stores ratios in the corresponding RAT channels in your database. It also stores data in a ground level exposure rate channel.

Answ ers to Some Common Ratio in g Questions If you’re not sure how to set up this GX, look here to find answers to some more common questions: Q. How do I determine what minimum concentration cutoff levels to choose?

A. To choose the minimum cutoffs, look at the concentration data (UCORR, KCORR, THCORR) on lines flown over water (no ground source of radiation). Ideally, over water the data would all be zero and you could choose zero as the cutoff. However, if this is not so, choose cutoffs at the lowest reliable levels for each concentration. Q. How do I determine what filter cutoffs to choose?

A. First look at the concentration data for each channel. If they look smooth, no filtering is required. If not, the filter applied is a lowpass filter. Choose a cutoff


to remove the noise visible in the profile view. If data were already filtered, filtering may not be required.

Applying Batch Processing to Your Dataset To perform batch processing in Oasis montaj, you must create a Geosoft Script (*.gs) file and then run the script file. The easiest way to create a Script file is to use the Script Recorder. Simply open your database and click the Start Script Record i ng button ( ). Then complete all dialog boxes and run all GXs you want to include. The system stores all parameters and GX commands in a Geosoft Script (*.gs) file. When you are done recording, click the End Script Recording button ( for your entire dataset.

). You can then edit this file and create a custom script

Note:

For more information about applying batch processing, please refer to the Creating Scripts in Oasis montaj technical note available in the Online Manuals, tutorials and technical notes.

Glossary 43

Glossary channels

In the Oasis montaj spreadsheet, a channel is essentially a column that contains a specific type of data.

column

In the Oasis montaj spreadsheet, a vertical line of cells that contain data.

database

See Oasis database

desktop

Background area in the Oasis montaj project. You can open and display a virtually unlimited number of Spreadsheet, Profile and Map windows in this area.

Elevation Units

The elevation units of the DEM grids (Metres or Feet).

fiducials

Points accepted as fixed bases of reference. Marks indicating the order in which each reading or sample reading was taken.

Graphical User Interface

Interactive software environment where functions are performed by selecting graphic objects.

grid

Collection of points along rows and columns that define a two-dimensional rectangular area on some plan, usually a ground plan.

Grids or Grid file

Files containing location (X and Y) and data (Z) values. Values are typically interpolated to create a regular and smoothly sampled representation of the locations and data.

groups

A set of graphics elements that make up a graphic component of the map. For example, a line path plot, a contour plot or a profile plot would all be separate graphics groups within the Data View.

GX or Geosoft eXecutable

Programmed process (identified by the *.GX file extension) used to process data in Oasis montaj.

Images or Image file

Files containing location (X and Y) and color values. The values are not interpolated. Standard PC file types created using imaging or electronic photoediting techniques.

line

Linear array of observation points.

Main window

Primary tool used to create and maintain databases, display data and process data. Oasis montaj is a Graphical User Interface (GUI) system that provides all functionality required to process and display virtually any type of Earth Science data.

Map (*.MAP)

Geosoft-developed file that integrates all graphics elements (lines, polygons and text) and layers (base maps, data, grids, plots and images) constructed in Oasis montaj.

Mon Taj 256 Channel Radiometric s Processing

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