GMSYS-3D 3D Gravity and Magnetic Modeling for Oasis montaj
USER GUIDE
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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: 6/22/2009. Written by, Tom Popowski, Gerry Connard, and Rowland French. Please send comments or questions to
[email protected] Copyright © 2009 Geosoft Inc. All rights reserved. Geosoft is a registered trademark, and Oasis montaj is a registered trademark of Geosoft Inc. Other brand and product names mentioned herein are properties of their respective trademark owners. 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. 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. GMS3D.ug.2009.06
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GMSYS-3D User’s Guide Release 1.3 March 20, 2006
TABLE OF CONTENTS INTRODUCTION TO GMSYS-3D ..............................................................................3 RELEASE 1.3 .................................................................................................................4 INSTALLATION ............................................................................................................5 3D MODELS ...................................................................................................................6 BUILDING MODELS ....................................................................................................7 1. Consistancy with the “index” grid 7 2. Grid expansion and “fill” 7 3. Periodic Grids and Edge Effects 8 4. Observed and Calculated Data Plane 8 5. Regional Removal and DC Offset 8 6. Magnetic properties require micro-cgs units 9 7. Inversion constraints 9 GMSYS-3D MENU .......................................................................................................10 Model Definition submenu 10 Grid Editing submenu 12 Display submenu 14 2D Cross Sections submenu 16 Grid Preparation 16 Manual FFT Preparation submenu 17 Calculate submenu 18 GMSYS-3D TOOLBAR ...............................................................................................22 DEMO MODELS ..........................................................................................................23
Salt Death Valley
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REFERENCES ..............................................................................................................25 APPENDIX A MODEL FILE (*.g3d) FORMAT ........................................................1 APPENDIX B NEW FEATURES IN VERSION 1.3...................................................1
INTRODUCTION TO GMSYS-3D GMSYS-3D is a 3D gravity and magnetic modeling package for surface-oriented models. A model is defined by a number of stacked surface grids with density-, susceptibility-, and remanent magnetization-distributions specified for the layer below each surface. Calculations are performed in the wave number domain and are based on Dr. Bill Pearson’s implementation of R. L. Parker’s algorithm (1972). The program is designed to run within Oasis montaj, making available all of the familiar grid manipulation and display routines in addition to application-specific routines. The GMSYS-3D menu, gmsys-3d.omn, provides to access GMSYS-3D functions. A GMSYS-3D Toolbar also allows quick access to some of the more commonly used functions. Models are defined by specifying the grids that define the surfaces, and the layer properties. “Model definitions” are saved in a *.g3d file. The model definition can be edited through GMS3MDEF.GX, which is accessed through the Edit Model menu item. Grid editing functions, grid preparation routines, model display routines, and forward and inverse calculations also are provided through the menu structure. You can also make use of many additional Oasis GX routines for grid manipulation and grid display. GMSYS-3D requires Geosoft's MAGMAP 2D-FFT system to operate. GMSYS-3D utilizes some of the newest functionality in Oasis montaj, particularly in the 3D visualization routines, so some features will not work correctly if you are running an older version of Oasis montaj. For more information on the 3D display operations, Geosoft has a technical note, Working with 3D Views in Oasis montaj, (http://www.geosoft.com/support/technote/pdf/3DViewsOasismontaj.pdf) which is useful for understanding the 3D visualization tools. This User’s Guide provides an overview of the GMSYS-3D approach to 3D gravity and magnetic modeling and the GMSYS and the menu structure. Help files for all the dialogs provide detailed information on the parameters used for each procedure. Companion Tutorials step the user through most of the functions of GMSYS-3D. New users are strongly encouraged to work through the Tutorials.
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RELEASE 1.3 Release 1.3 adds the capability to convert a GMSYS-3D model to a Geosoft 3D voxel grid and the capability to calculate the gravity response from a 3D voxel grid. The new “Model Explorer” tool is a “tree-structure” summary of the current model that is similar to the Oasis “Project Explorer” tool. It remains visible for reference while using all of the other GX’s. If any of the model parameters are changed by other GX’s, the Model Explorer is automatically updated. The Time-to-Depth conversion allows the user to build a model in time and then use velocities to convert to a depth model. A new backup scheme has been incorporated into the inversion process. Users may choose between the old method (a single step undo), an automatic naming scheme to save each step in the inversion process, or explicitly choose a name for each backup grid. Users may also “checkpoint” (i.e., make a complete copy of the model in a separate directory) a model with one click from the inversion dialogs. A new function allows users to resample a model to half-size (i.e., double the grid cell size) and save it to a new model name. Calculations with the half-size model will be four times as fast as with the original model. A similar new function will create an “expanded” model that is fully “prepped” for the FFT. An expanded model speeds up the calculation process. The GMSYS-3D help is now integrated with the Oasis montaj help system. This integration allows hyperlinks with other help topics. A new “Auto DC-shift” button will automatically adjust the DC-shift of the calculated gravity and magnetics grids to force the mean error to zero. All of the text strings now support UNICODE The Release Notes and APPENDIX B list the new features added since release 1.2.
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INSTALLATION GMSYS-3D is distributed and installed with Oasis montaj™. To run GMSYS-3D, open Oasis and load the menu gmsys-3d.omn (GX | Load menu…). If you received updated software directly from Northwest Geophysical Associates, follow the instructions provided with that release. GMSYS-3D requires the following Oasis montaj licenses: 10105 montaj™ MAGMAP Filtering 10523 montaj plus™ Modeling 3D You can view your licensed products using File / License / View License… GMSYS-3D version 1.3 requires Oasis montaj™ version 6.3 or above. The GMSYS-3D toolbar, providing some of the more commonly used functionality, will load automatically with the menu. By default, the tool bar appears on the left edge of the Oasis window but the user may drag it to other locations. You may toggle any toolbar off or on by selecting the Tools button in the Project Explorer and right-clicking the toolbar name.
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3D MODELS 3D models are defined by a number of surface grids with a density distribution (and/or susceptibility & remanent magnetization distribution) assigned to each layer. Layer density may be specified using either a constant density, a vertical density-depth profile relative to a “reference surface”, or a laterally-varying density distribution defined by a grid. Layer susceptibility may be specified using either a constant susceptibility and remanent magnetization, or a laterally-varying susceptibility distribution. Figure 1 illustrates a three layer model where the parameters “DEN_#” are constant densities for each layer and “SURF_#” are filenames for grids defining those surfaces. Model parameters are edited using Model Definition | Edit Model from the GMSYS-3D menu and are saved in a model definition file (*.g3d). Figure 1: Model Example
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BUILDING MODELS As indicated above, models consist of layers defined by a stack of horizon grids and parameters assigned to each layer. The grids may be created outside of GMSYS-3D, imported from another application (e.g. seismic interpretation software), or built "from scratch" using the tools within Oasis (e.g. NGAGEDIT.GX, GMS3FGRD.GX) or some other application. Because GMSYS-3D uses frequency-domain (i.e., FFT-based) calculation routines, the grids need to be specially prepared to be compatible with FFTs.
1. Consistancy with the “index” grid All grids in the model must be consistent with respect to:
Grid size (number of rows and columns) Cell size and units No dummies in the grid Grid origin No crossing surfaces
GMSYS-3D provides a function “Model Definition | Check Model” (GMS3CHECK.GX) to check a model for all of these problems and adjust grids to eliminate crossing surfaces. Another function “Make Grids Coincident” (NGAGRDMCH.GX) is provided to force a grid to match an “index” grid. GMS3FGRD.GX may be used to build a new, flat grid that matches the parameters of the "index" grid for the currently loaded model. The standard grid preparation routines (GMS3XPND, GRIDFILL, or NGAPFILL) will fill in the dummies during normal grid preparation steps. However, users should be aware that this happens to all of the grids in the model and that unwanted artifacts may be introduced in critical areas of their models. 2. Grid expansion and “fill” All the grids must be expanded in size and “filled” so that they are “periodic” to eliminate edge effects. The amount of expansion required depends on the shape of the model horizons near the edges of the original grid. The final, expanded dimensions of the grid must also be “good FFT” dimensions. If you use %’s to specify expansion amounts, all of the routines described below and the Oasis automatic routines expand to “good FFT” dimensions. The user has two options with regard to the “expand and fill” preparation. The simplest is to use the “Setup Automatic FFT Preparation” option (GMS3XPND.GX). With this option, all grids are expanded inside the calculation routine each time it runs and then “masked” back to the original size at
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the end of the calculation. This option is easier to use but gives the user less control and requires significantly more computation time with larger grids. The second option is to prepare each grid separately before building the model using the procedures in the Manual FFT Preparation submenu. This option allows more control and saves time during the calculations. 3. Periodic Grids and Edge Effects Periodic grids “wrap” seamlessly left-to-right and top to bottom. This can also be visualized by placing copies of the grid on all 4 sides, and continuing the process, so the periodic grid of grids continues to infinity. When the grid is “filled periodically” a smooth interpolation is used to wrap the grid from one edge of the “real data”, off the edge of the expanded grid, and back from the opposite side of the expanded grid, to the “real data” on the other side. The analogous process wraps the top and bottom edges and corners of the grid. Calculations are carried out on the artificial array of adjacent, repeating grids. Hence, you must expand the grids sufficiently so that at the edge of the “real data” the gravity and/or magnetics are not unduly influenced by the geology in the adjacent, repeated grid. The grid filling is in fact a geologic interpretation. If you have a deep basin on one edge of the survey area, the gravity on the edge will be affected by the extension of the basin, beyond the survey area. The basin could truncate abruptly, or could continue for a considerable distance. You, as an interpreter, must make a judgement on how you want to continue it. To continue the basin for longer distances, increase the size (percent expansion) of the expanded grid. For smaller grids of smaller areas, expansions of 100% or more are not unreasonable. You have the option of saving intermediate expanded and filled grids and viewing them to assure that the extrapolated geology is reasonable. By “manually” expanding and filling the grids, you have much more control of these edge effects. 4. Observed and Calculated Data Plane The response of the model is calculated on a flat plane. If your observed data are not on a flat plane, you need to upward continue your observed data grids to the “ZREF” elevation. Previous versions required that ZREF be chosen to be above the top of the topography. GMSYS-3D provides a “draped to level” continuation function “Model Definition | Upward Continue Observed Grid” (GMS3VUP.GX). Version 1.3 can calculate gravity at any level within the model (as long as it is constant, i.e., flat). The model check routine (GMS3CHECK.GX) still reports ZREF below the top of the topography as error 5. Regional Removal and DC Offset In general we recommend removing any regional field before loading the observed data grid into GMSYS-3D. If the regional was removed by using an
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Oasis filter, you should include the same filter control file (*.con) as an “inline filter” in the GMSYS-3D forward calculations (GMS3GRAVF.GX and GMS3MAGF.GX). GMSYS-3D allows you to remove a “DC-shift” (bulk shift) from the calculated response in the forward calculations and inversion. The DC-shift is critical to the inversion process. 6. Magnetic properties require micro-cgs units Note that the susceptibility and remanent magnetization need to be input in micro-cgs units. In the future, input in SI and cgs units will be supported. When modeling vertical-sided simple magnetic bodies (e.g., vertical prisms), users may see small oscillations at the Nyquist frequency superimposed on the calculated magnetic anomalies in the areas with low gradients in the calculated field. Although this “noise” is an artifact of the frequency-domain calculation technique, the amplitude is relatively small (~.01 nT) and typically does not appear when modeling real-world geology. 7. Inversion constraints GMSYS-3D allows the use of “constraints” grids to constrain or restrict some of the inversion options. A “constraints” grid is a grid that matches the “index” grid for the model and contains values between 0 and 1. Where the value of a grid node in the constraints grid is set to 0, the corresponding location in the grid being inverted will remain unchanged. Where the value of a grid node in the constraints grid is set to 1, the corresponding location in the grid being inverted is totally unconstrained. If the value in the constraints grid is greater than 0 and less than 1, it acts as a weighting factor to slow down the amount of movement during inversion. There are four tools for creating a constraints grid in the “Grid Editing” menu. For example, if you are inverting on depth to basement in an area surrounded by outcropping basement, you might build a constraints grid that is set to 0 in the areas of basement outcrop and 1 in the center of the basin. The transition from 0 to 1 would determine how sharply the edges of the basin could drop off.
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GMSYS-3D MENU This section of the User’s Guide introduces the reader to each menu item available in the GMSYS-3D menu. In Oasis montaj most menu items invoke a GX, which is a routine to perform a given function. More complete descriptions of the parameters and operations of each GX are included in the Help for that GX. GXs may also be invoked from the GX menu or from the GX tool on the standard Oasis toolbar.
Model Definition submenu Menu item, “Model definition | Upward Continue Observed Grid to Flat” – GMS3VUP.GX The GMS3VUP GX upward continues a data grid from a “draped” surface to a level surface. Menu item, “Model definition | Upward Continue Observed Grid to Draped” – GMS3VUPD.GX The GMS3VUP GX upward continues a data grid from a “level” surface to a draped surface. Menu item, “Model definition | Show Current Model” – GMS3SHOW.GX The GMS3SHOW GX displays a summary of the current model in an information box. Menu item, “Model definition | Model Explorer” – GMS3XTOOL.GX The GMS3XTOOL GX loads a “tree-structure” summary of the current model that is similar to the Oasis “Project Explorer” tool. It remains visible for reference while using all of the other GX’s. If any of the model parameters are changed by other GX’s, the Model Explorer is automatically updated. Menu item, “Model definition | New Model” – GMS3NEW.GX The GMS3NEW GX clears the previous model from memory, prompts the user for a new model file (*.g3d), asks for new display parameters, and then runs GMS3MDEF (Edit Model). The grids that you plan to use for the new model should be in the working directory prior to running the New Model
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procedure. The model can be edited later using the Edit Model procedure to insert or remove surfaces. Menu item, “Model definition | Open Model” – GMS3OPEN.GX The GMS3OPEN GX clears the previous model from memory, prompts the user for an existing model file (*.g3d), retrieves that model, and then runs GMS3MDEF (Edit Model).. Menu item “Model definition | Edit Model” – GMS3MDEF.GX The GMS3MDEF GX opens a dialog box for entering the grids and properties that define a model. This “model definition” may be saved in a file (*.g3d). Whether or not the model is saved it will remain in the OASIS workspace as system parameters until a new model is opened. GMS3MDEF also reads the “index grid” parameters (grid size, cell size, origin, etc.) and stores them with the model. The index grid may be any one of the grids in the model and is used to define the grid parameters and grid location, which will be required for the other grids. You may specify gravity or magnetic DCShifts to be added to the respective model response grids. If “Auto Check Model” is selected, GMS3CHECK.GX (see below) will be run automatically each time the model editing dialog is closed. Menu item, “Model definition | Check Model” – GMS3CHECK.GX The GMS3CHECK GX checks all the grids used in the model definition for consistency with the index grid, with the option of running NGAGRDMCH.GX to correct the non-conforming grids, then rechecks the model after any adjustments have been made. Any time you change or add grids within a model, you should re-run the check. If “Auto Check Model” is selected, the check is run at the end of Edit Model. Parameters checked in GMS3CHECK are as follows:
Grid size (number of rows and columns)
Cell size and units
Grid Origin
Presence of dummies in the grid
Crossing surfaces
If GMS3CHECK detects crossed surfaces, you will be offered the option to adjust the upper or lower grid. If crossed surfaces are detected and fixed, GMS3CHECK runs again until no crossed surfaces are detected.
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Menu item, “Model definition | Close Model” – GMS3CLOSE.GX The GMS3CLOSE GX saves changes to the current model and clears the model parameters in the workspace. Menu item, “Model definition | Copy Model” – GMS3XHALF.GX The GMS3XHALF GX duplicates of all the files required for the current model at half scale (i.e., 2X the original grid spacing), preferably in a new subdirectory. This is to ensure that file names common to both models will be protected from modifications of the other model. This proves useful in creating faster-calculating versions of large models. Menu item, “Model definition | Copy Model” – GMS3MDUP.GX The GMS3MDUP GX creates a duplicate of the current model in another directory including all grids. This differs from the “Save As…” option in the Edit Model dialog, which merely saves a new .g3d file with pointers to the old grids. Menu item, “Model definition | Copy Model” – GMS32VOX.GX The GMS32VOX GX creates a 3D density or susceptibility grid (geosoft-voxel) from the current GMSYS-3D model. The resulting 3D grid can be displayed and manipulated using the standard Oasis GX’s in the “Voxel” menu. -- end of Model Definition submenu--
Menu Item, “New Constant Grid" - GMS3FGRD.GX The GMS3FGRD GX creates a new grid with a constant value (e.g., a horizontal plane or a constant density). The resulting grid is based on the index grid and is compatible with the current 3D model.
Grid Editing submenu These routines allow you to create or modify the values in an existing grid. Menu item "Grid Editing | Path Edit” – NGAGEDIT.GX The NGAGEDIT GX allows you to change the value of a grid along a digitized path. The user must specify the amount of change, the width at which the change rolls off to zero, and the roll-off method. For roll-off method, you may choose from one of six tool shapes, shown below, or select a "constant" value to fill the area.
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Gaussian
left wedge
cosine
tent
right wedge
square
You may use a polygon file to clip the effective area of the Path Edit function. Clipping options are "none," "existing polygon," or "digitize polygon." We recommend that you have the grid displayed in the current map prior to initiating this procedure. NGAGEDIT allows a single undo level; simply re-run the GX and select the "Undo" button. Menu item "Grid Editing | Polygon Area Edit” – NGAPOLYSM.GX The NGAPOLYSM GX replaces the area inside an existing polygon by calculating a 0- to 3rd-order polynomial surface, a minimum curvature surface or by applying an expression. The polynomial surface may be based on all points or edge points only. Selecting “All” will utilize all points within the polygon. The minimum curvature surface is calculated to fit to the area surrounding the polygon. The expression can be any valid Geosoft expression involving up to six grids or a Geosoft expression file. Menu item "Grid Editing | Minimum Curvature Fill” – USGS_GRIDPLUG.GX Fills dummy values in a grid by using polynomial or local median initialization followed by minimum curvature iterations. This is the GX released by the USGS in Phillips and others, 2003. Menu item "Grid Editing | Digitize Polygon” – POLYDEF.GX The standard Oasis POLYDEF GX allows you to digitize a polygon for use in the "Path Edit" or "Polygon Area Edit" functions.
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Menu item "Grid Editing | Draw Polygon Location” – POLYDRAW.GX The standard Oasis POLYDRAW GX draws an existing polygon file on the current map. Menu item "Grid Editing | Constraints from Wells” – NGAWCONS.GX The NGAWCONS GX applies the standard edit tool shapes (see NGAGEDIT.GX) to spot locations defined by a database (e.g., well locations) to edit a “constraints” grid. Menu item "Grid Editing | Constraints from Lines” – NGASCONS.GX The NGASCONS GX applies the standard edit tool shapes (see NGAGEDIT.GX) along a selected line in a database (e.g., seismic line shotpoints) to edit a “constraints” grid. -- end of Grid Editing submenu--
Display submenu There are three customized routines for display of GMSYS-3D models in the “Display” submenu. Four standard Oasis routines are also included without modification in the GMSYS-3D | Display menu: Menu item “Display | Create a Color Zone File for the Model” – GMS3ZONE.GX The GMS3ZONE GX creates a color zone file encompassing all the structure grids in the model. Menu item “Display | Display Model as 3D stack” – GMS3DISPLAY.GX The GMS3ZONE GX creates a 3D visualization of all structure grids plus an optional observed data grid in the open model on a new map. If the map is created and already open, this GX allows you to add additional grids to the 3D visualization. Close an existing map before running the GX to recreate it. Menu item “Display | Display Single Grid to 3D View” – GRIDIMG3D.GX The GRIDIMG3D GX plots a single grid as a new drawing plane in a 3D View. The grid may be placed in any orientation. Use this function to place a crosssection (e.g., seismic image, GM-SYS profile, etc.) into your 3D model. Menu item “Display | Display a Generic 3D Stack” – V3DIMG.GX The standard Oasis GX, V3DIMG, creates a 3D display of a grid on the current map, or on a new map, and opens the Geosoft 3D viewing tool. Additional grids/surfaces can be added to the 3D display.
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Menu item “Display | Display Structure Grids as 2D maps” – GMS3SMAPS.GX The GMS3SMAPS GX creates 2D displays of all structure grids in the open model on separate maps in one step. Menu item “Display | Display Mag Anomaly Grids as 2D maps” – GMS3MAGMAPS.GX The GMS3MAGMAPS GX creates 2D displays of the calculated, observed, and error magnetic anomaly grids from the open model on separate maps in one step. The maps will be created at ~1/3 the size of the OASIS window and placed along the left side of the OASIS window. Menu item “Display | Display Grav Anomaly Grids as 2D maps” – GMS3GRVMAPS.GX The GMS3GRVMAPS GX creates 2D displays of the calculated, observed, and error gravity anomaly grids from the open model on separate maps in one step. The maps will be created at ~1/3 the size of the OASIS window and placed along the left side of the OASIS window. Menu item “Display | Display a Single Grid” – GRIDIMG1.GX The standard Oasis GX, GRIDIMG1, creates a 2D display of any grid on the current map or on a new map. Menu item “Display | Register Current Map” – GMS3REGMAP.GX The GMS3REGMAP GX links a color method and map to one of the anomaly grids in the GMSYS-3D model. This GX is designed to be called from other GXs which presumably have altered a grid and want the changes to be reflected in an associated map, but may be called interactively as well. Menu item “Display | Rezone Current Map” – GMS3REZONE.GX The GMS3REZONE GX rezones a Map after an underlying grid has been altered. This GX searches the given map for an Aggregate containing the given grid, and creates a new ITR for the Aggregate based on the grid, color table and zoning method. This GX is designed to be called from other GXs which presumably have altered a grid and want the changes to be reflected in an associated map, but may be called interactively as well. Menu item “Display | Show Error Grid Statistics” – GMS3ERREDB.GX When used interactively, the GMS3ERREDB GX opens the statistics database created following the calculation process. Note that if the database has been
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minimumised it will make it the active database, but will not restore it to the window. -- end of Display submenu--
2D Cross Sections submenu The 2D cross section routines provide tools for creating and modifying cross section slices through your model. They also provide a means of extracting and editing GM-SYS Profile (2 D) models from your 3D model. Menu item “2D Cross Sections | 2D Density Cross Section Slice” – GMS3CX.GX The GMS3CX GX draws a vertical slice through a 3D model with density or susceptibility displayed as a color grid on a map. Calculated and observed anomaly profiles may be displayed over the section. Cursor position is displayed as model X, Y, and Z-coordinates at the bottom, right of the Oasis montaj window. Menu item “2D Cross Sections | Change Cross Section VE” – XYSCALE.GX The standard Oasis GX, XYSCALE, allows user to change the X and Y scales of a view on the current map. Menu item “2D Cross Sections | Extract 2D GMSYS Model” – GMS3PROF.GX The GMS3PROF GX loads the open 3D model horizon and data grids into the GMS3PROF dialog to make extracting GM-SYS 2D models from the 3D model more convenient. Currently the routine allows the extraction of 15 sub-surface horizons plus topography. Menu item “2D Cross Sections | Open Existing 2D GMSYS Model” – GMS.GX The GMS GX prompts for an existing 2D GM-SYS model and launches GM-SYS Profile with that model. -- end of 2D Modeling submenu--
Grid Preparation Proper grid preparation is necessary to get accurate results, extending to the edge of your data. Required grid preparations include:
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1. Upward (or downward) continuing the observed gravity (and/or magnetic) grid to a constant elevation 2. Expanding all grids to “good FFT” numbers 3. Filling the expanded grids to be periodic 4. Prior to the final display, the grids should be masked back to their original boundaries. If “Input Grids are Pre-Expanded?” is set to "NO" in the “Forward Calculation" dialogue box, items 2-4 above are done automatically in the calculation routine, using parameters set in the “Setup Automatic FFT Preparation” menu item. Otherwise, you must expand them manually, via the "Manual FFT Preparation" submenu. For a more complete discussion of grid expansion see the section Building Models on page 7. Menu item "Setup Automatic FFT Preparation” – The GMS3XPND GX is used to set the parameters for the GMSYS-3D’s internal FFT grid preparation. Using the automatic preparation you are allowed to change the size (percent expansion) and the shape of the expanded grid. Other fixed parameters are listed in the help for GMS3XPND. This GX is also available through the “Calculate” menu.
Manual FFT Preparation submenu Menu item “Manual FFT Preparation | Make Grids Coincident” NGAGRDMCH.GX The NGAGRDMCH GX forces an input grid to match an index grid in size, cell spacing, projection, X0, and Y0. If the grids do not match the input grid will be regridded with the proper parameters. Menu item “Manual FFT Preparation | Expand Grid…” -.GX The standard Oasis GX, GRIDXPND, will expand a grid and fill it with dummies. To be sure that you expand it to “good FFT” dimensions, use the menu item “Minimum % to Expand Grid” rather than specifying “ --X”, “…--Y”. Menu item “Manual FFT Preparation | Fill Grid…” - GRIDFILL.GX The standard Oasis GX, GRIDFILL, will fill expanded grids to be periodic. Note that the default parameters that appear in this dialog are different than the defaults used in the “Automatic FFT Preparation” (GMS3XPND.GX) described above.
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Menu item “Manual FFT Preparation | NGA Fill Grid…” - NGAPFILL.GX The NGAPFILL GX will fill holes (i.e. replace dummy values) in grids which have been previously expanded periodically. Dummies will be replaced using a function with minimum of variation in the filled area. Additional information is available in the help file for NGAPFILL. Menu item “Manual FFT Preparation | Mask Grid…” - GRIDBOOL.GX The GRIDBOOL GX can be used to mask the resultant expanded grids back to the size of the original grid. This standard Oasis routine has not been customized (yet) for more convenient use with GMSYS-3D. -- end of Manual FFT Preparation submenu--
Calculate submenu The calculation routines allow forward and inverse calculations from the currently defined model. The simplest inversion is the “structural” inversion which allows grid points on a single geologic horizon (a structure grid) to move up or down to minimize the misfit between the observed data and the calculated model response. One structural grid, the “Inversion Surface”, must be selected to be “free” for the inversion. Inversions can also be performed allowing lateral changes in the density or susceptibility within a given layer. Those routines require that the density/susceptibility for that layer be defined by a grid, although the initial starting grid may be a constant (uniform) grid. The edit model dialogs allow the insertion of density or susceptibility grids; however the grid must be created prior to entering the “Edit Model” dialog. A database (
_stats.gdb) is created and statistics of the error grid are added after each calculation. This database is automatically opened and appended with each calculation. An “inline” filter may optionally be applied to the calculated data grids. This provides a means of removing high frequency calculation artefacts near the nyqist frequency. Filter control files can be created using the FFT2CON GX from the Magmap routines. Menu item " Calculate | Setup Automatic FFT Preparation” – The GMS3XPND GX is used to set the parameters for the GMSYS-3D’s internal FFT grid preparation. Using the automatic preparation you are allowed to change the size (percent expansion) and the shape of the expanded grid. Other fixed parameters are listed in the help for GMS3XPND. This option is also available in the Main menu.
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Menu item, “Calculate | Forward Gravity Calculation” – GMS3GRAVF.GX The GMS3GRAVF GX performs a forward gravity calculation of the model currently in memory. You may specify an “inline filter” that will be applied to the calculated result. Filters can be created using the FFT2CON GX from the MAGMAP Oasis module. Menu item, “Calculate | Forward Gravity Calc. - Voxel” – NGAGLAYER.GX The NGAGLAYER GX performs a forward gravity calculation of a Geosoft 3D voxel grid. The result is calculated at a constant elevation and placed in a grid that matches the X and Y dimensions of the Geosoft voxel. Note that the calculation level can be within the voxel. Menu item, “Calculate | Gravity Structural Inversion” – GMS3SI.GX The GMS3SI GX performs structural inversion based on the gravity mis-fit on a selected horizon. Inversion only adjusts the selected horizon. Inversion updates the calculated response and error grids to reflect the structural changes. Note: inversion modifies the grid in the model but offers several options for saving each step in the inversion process. Menu item, “Calculate | Single Density Inversion” – GMS3RHOI.GX The GMS3RHOI GX calculates the constant density for a single layer that minimizes the average error. Menu item, “Calculate | Lateral Density Inversion” – GMS3DI.GX The GMS3DI GX calculates the density distribution in a single-layer lateral density grid that minimizes the average error. Note: inversion modifies the grid in the model but offers several options for saving each step in the inversion process. Menu item, “Calculate | Forward Mag Calculation” – GMS3MAGF.GX The GMS3MAGF GX performs a forward magnetic calculation of the model currently in memory. You may specify an “inline filter” that will be applied to the calculated result. Filters can be created using the FFT2CON GX from the MAGMAP Oasis module. Menu item, “Calculate | Magnetic Structural Inverse” – GMS3MAGINV.GX The GMS3MAGINV GX performs structural inversion based on the magnetic mis-fit on a selected horizon. Inversion only adjusts the selected horizon. Inversion updates the calculated response and error grids to reflect the structural changes. Note: inversion modifies the grid in the model but offers several options for saving each step in the inversion process.
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Menu item, “Calculate | Magnetic Susceptibility Inversion” – GMS3SUSCINV.GX The GMS3SUSCINV GX performs susceptibility inversion based on the magnetic mis-fit on a selected horizon. Inversion updates the calculated response and error grids to reflect the changes. GMS3SUSCINV ignores any existing lateral susceptibility grid in its calculations. Note: inversion modifies the grid in the model but offers several options for saving each step in the inversion process. Menu item, “Calculate | Magnetic 1-Step Suscept Inversion” – GMS3OSSI.GX The GMS3OSSI GX uses an alternative approach to perform susceptibility inversion. This GX applies the Oasis MAGMAP Apparent Susceptibility Filter (SUSC) to arrive at the lateral susceptibility variation, rather then utilizing the normal inversion routines of GMSYS-3D. This is a somewhat faster process than the “normal” GMSYS-3D routines which are used by the GMS3USCINV GX (Menu item, “Calculate | Magnetic Susceptibility Inversion”). The apparent susceptibility filter is in fact a compound filter that performs a reduction to the pole, downward continuation to the source depth, correction for the geometric effect of a vertical square ended prism, and division by the total magnetic field to yield susceptibility. Using this approach we assume that there is no remanent magnetization and that the magnetic response is caused by a collection of vertical, square-ended prisms of infinite depth extent. GMS3OSSI applies the apparent susceptibility filter to the magnetic error grid and adds the result to the existing lateral susceptibility distribution grid. This is a faster process than the “normal” GMSYS-3D routines and can be run multiple times to improve the fit. For more information see the Geosoft Help for FFT2CON SUSC. Note: inversion modifies the susceptibility grid in the model and does a simple one-level backup, which can be undone with the undo command. Back up grids manually before running multiple inversions if you want to retain the original horizon. Menu item “Calculate | Calculate Gravity Gradients” – GMS3GGRAD.GX The GMS3GGRAD.GX calculates any one of the six standard gravity gradient components. The user chooses the desired component from a dropdown list and supplies the name of the observed, calculated, and error grids.
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Menu item “Calculate | Show Error Grid Statistics” – GMS3ERREDB.GX The GMS3ERREDB GX writes a new entry in a database, recording the error statistics of the gravity error grid. This GX will be called following recalculation of the error grid, providing an ongoing record of changes during the modeling process. -- end of Calculate submenu--
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GMSYS-3D TOOLBAR The GMSYS-3D toolbar provides an alternate means of accessing many of the more commonly used procedures. All the procedures are available from the GMSYS-3D menu and the discussion below refers you to the appropriate pages of this User’s Guide. “Fly-over” help is available for each tools in you leave the cursor over the tool for a few seconds.
New Model (page 10) Open Model (page 11) Check Model (page 11) Edit Model (page 11) Show Model Parameters (page 10) New Constant Grid (page 12) Edit Layer; shortcut into Edit Model (page 11) Path Edit (page 12) Polygon Area Edit (page 13) Extract 2D Density Profile (page 16) Build GM-SYS (Profile) Model (page 16)
Gravity Forward Calc (page 19) Gravity Structural Inverse (page 19) Magnetic Forward Calc (page 19) Display Model as a 3D Stack (page 14)
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DEMO MODELS Two demo models are included with this distribution: Salt and Death Valley. These models are installed as subdirectories under Program Files\Geosoft\ Oasis montaj\ data\gmsys-3d. We recommend that you copy these subdirectories into a separate “working directory” prior to opening and manipulating the models. Thus, you will maintain the clean original copy.
Salt Salt is a demonstration model that represents a simple offshore salt dome. The model consists of six grids. The files are installed in the "salt_model" subdirectory. You may also find it on the CD in the "Salt_demo" directory. The Salt and Salt2 models are used in the GMSYS-3D Tutorial.
Grid Name
Contents
sealevel.grd
flat surface at elevation=0,
bathy.grd
bathymetric surface,
top_salt.grd
top of salt body,
bottom_salt.grd
base of salt body,
basement.grd
basement surface,
salt_obsgrav.grd
observed gravity.
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Death Valley Death Valley covers a small area in southern Nevada. The data were provided by the USGS and were taken from OFR-00-189. The installation puts this model into the “d_valley” subdirectory. In CD-based distributions, you may also find it in the DeathValley_demo directory. Just open the “dv_demo” workspace to see the 3D model. The grids in the initial model are: Grid Name
Contents
topo_m_1.grd
Topography
c1_m_1.grd
1st subsurface horizon
c2_m_1.grd
2nd subsurface horizon
c3_m_1.grd
3rd subsurface horizon
Basement_m_1.grd
Top of basement
gravity_m_1s.grd
Observed Bouguer gravity
dv_calc1.grd
Calculated model response
dv_err1.grd
Calculated error
The model used a Bouguer density of 2.67, ZREF= 2315 meters, and half-space bottom = -13,000 meters. The layer densities from top to bottom are 2.00, 2.10, 2.30, 2.42, and 2.65 gm/cc. After calculating the response of the model, thereby refreshing dv_calc1.grd, we performed a structural inversion on the basement horizon, using a “Regional Offset” of 0.0 mGals. (This assumes that you used a “DC-shift” of 18 in the first screen of the “Model Definition”.) Adjusting the Regional Offset moves the average level of the layer being inverted up or down. The resulting basement grid is in Basement_inv_1.grd, the resulting calculated response of the model is in dv_calc2.grd, and the resulting error is in dv_err2.grd. Note that inversion modifies the grid in the model and does a simple one-level backup, which can be undone with the undo command. Back up grids manually before running multiple inversions if you want to retain the original horizon.
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REFERENCES Blakely, R.J., 1995, Potential Theory in Gravity & Magnetic Applications: Cambridge University Press, pp.441. Blakely, R.J., Langenheim, V. E., and Ponce, D.A., 2000, Summary of Geophysical Investigations of the Death Valley Regional Water-Flow Modeling Project, Nevada and California. U.S.Geological Survey Open-File Report 00-189 Parker, R.L., 1972, The rapid calculation of potential anomalies: Geophysical Journal of the Royal Astronomical Society, v.42:315-334. Phillips, J.D., Duval, J.S., and Saltus, R.W., 2003, Geosoft eXecutables (GX’s) developed by the U.S. Geological Survey, version 1.0, with a viewgraph tutorial on GX development. U.S.Geological Survey Open-File Report 03-010
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APPENDIX A MODEL FILE (*.g3d) FORMAT
As GMSYS-3D is developed, new model parameters are added and some become obsolete. In order to “update” a model’s parameters to the current specification, you may run GMSPREEN GX immediately after opening a model with a new release. Model defining parameters for Release 1.2 include: NOTES: 1. # represents an integer number 2. [GOBS|GCALC|MISFIT|#] means a number of one of GOBS, etc 3. Indexed parameters like DDRHO[#] are zero based. However, The zero element as it shows up in the Parameter block is DDRHO, not DDRHO[0]. In the GX, you can use either form.
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NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL
CONVERT ............. Conversion factor; input unit to Km GDCSHIFT ............ Constant to add to calculated gravity response DEN_0 ................... Constant Layer Density for background (Air) DX ......................... Grid Spacing in X DY ......................... Grid Spacing in Y GRAVBG ............... Background Gravity (calculated) HUNITS . ................ Units (horizontal) KX ......................... Grid Orientation M2H ....................... Conversion factor to km (horizontal) M2V ....................... Conversion factor to km (vertical) NAME .................... Model File name NLAYERS ............. Number of Layers NUMDENS ............ Number of sublayers in DD relationship NX ......................... Number of grid columns NY ......................... Number of grid rows PREEXPANDED ... 0 if not expanded; 1 if expanded RECALC ................ 0 if not needed; 1 if needed ROT ....................... Grid Rotation VUNITS ................. Units (vertical; should be same as horizontal) X0 .......................... Grid Origin Y0 .......................... Grid Origin ZBOTTOM ............. Elevation of bottom of halfspace (Z+ up)
NGA_3MODEL
GZREF .................. Elevation of Gravity Obs/Calc. DDOFF_[#] ............ Offset for DD sublayer # DDRHO_[#] .......... Density for DD sublayer # DEN_# .................. Constant density for layer # DEN_APP_# ........ "Apparent" density for layer # ( last VDD sublayer density ) DEN_#[#] ............... ( obsolete ) DENSRCNAME_# Source of density info for layer # (text) DENSRC_# .......... Source of density info for layer # (list alias integer) SUSCSRC_# ....... Source of susc. info for layer #
NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL
MZREF .................. Elevation of Magnetics Obs/Calc. MDCSHIFT ............ Constant to add to calculated Mag response EFIELD .................. Earth's Field Strength (cgs) EINC ...................... Earth's Field Inclination EDEC .................... Earth's Field Declination BSUSC_# ............. Susceptibility of Layer # (cgs) BRMAG_# ............ Magnitude of Remanent Mag. (cgs) BINC_# .................. Inclination of Remanent Mag. BDEC_# ................ Declination of Remanent Mag.
NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL
DCSHIFT ............... ( obsolete ) DENDIFF ............... ( obsolete ) IDENSITY .............. ( obsolete ) NSURF .................. (obsolete) Number of Layers ZREF ..................... ( obsolete ) DENGRID_# ......... ( obsolete ) DENGRID# ........... ( a bug )
NGA_3MODEL NGA_3MODEL NGA_3MODEL
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NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL NGA_3MODEL
NDEN_# ............... ( obsolete ) THK_#[#] ............... ( obsolete )
EDITDA ................ Decimal Year of last gms3mdef.gx run EDITHR ................ Decimal Hour of last gms3mdef.gx run
NGA_E3GRID – filenames: NGA_E3GRID DDREFSUR .......... Reference surface for DD relationship NGA_E3GRID GOBS .................... Observed Gravity NGA_E3GRID GCALC .................. Calculated Gravity NGA_E3GRID GERR .................... Error in Gravity NGA_E3GRID MOBS .................... Observed Magnetics NGA_E3GRID MCALC .................. Calculated Magnetics NGA_E3GRID MERR .................... Error in Magnetics NGA_E3GRID IDX ........................ Index grid NGA_E3GRID MISFIT .................. (repl by GERR) NGA_E3GRID SURF_# ................ Surface grid for layer # NGA_E3GRID DENGRID_# .......... Lateral density grid for layer # NGA_E3GRID SUSCGRID_# ....... Lateral susc. grid for layer # NGA_E3GRID NGA_E3GRID NGA_E3GRID NGA_E3GRID NGA_E3GRID
DENSITY ............... ( obsolete ) IGOBS ................... ( obsolete ) PARTIAL ............... ( to be obsolete ) Z1 .......................... ( to be obsolete ) Z2 .......................... ( to be obsolete )
NGA_3DISPLAY – display parameters NGA_3DISPLAY GWSDIR ............... The Geosoft Workspace directory NGA_3DISPLAY 3DGMETHOD ....... Color method for gravity anomaly grids in 3DMAPNAME NGA_3DISPLAY 3DGTBL ................ Color table for gravity anomaly grids in 3DMAPNAME NGA_3DISPLAY 3DMAPNAME ....... Map containing 3D model NGA_3DISPLAY 3DVXG .................. VE used in 3DMAPNAME NGA_3DISPLAY 3DZONE ................ Color zone file for structure grids in 3DMAPNAME NGA_3DISPLAY CTBL_ANOM_DEF Default color table for 2D anomaly maps NGA_3DISPLAY CTBL_ERR_DEF .. Default color table for 2D error maps NGA_3DISPLAY CTBL_STRUCT_DEF Default color table for 2D structure maps NGA_3DISPLAY ZONE_ANOM_DEF Default color zone file for 2D anomaly maps NGA_3DISPLAY ZONE_ERR_DEF . Default color zone file for 2D error maps NGA_3DISPLAY ZONE_STRUCT_DEF Default color zone file for 2D structure maps NGA_3DISPLAY CTBL_[GOBS|GCALC|GERR] Color tables used for gravity anomaly maps NGA_3DISPLAY MAP_[GOBS|GCALC|GERR] Maps containing gravity anomalies NGA_3DISPLAY ZONE_[GOBS|GCALC|GERR] Color zone file for gravity anomaly maps
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NGA_3DISPLAY NGA_3DISPLAY NGA_3DISPLAY NGA_3DISPLAY NGA_3DISPLAY NGA_3DISPLAY
CTBL_[MOBS|MCALC|MERR] Color tables used for magnetic anomaly maps MAP_[MOBS|MCALC|MERR] Maps containing magnetic anomalies ZONE_[MOBS|MCALC|MERR] Color zone file for magnetic anomaly maps CTBL_# ................ Color tables used for structure maps ( not implimented ) MAP_# ................. Maps containing structure grids " ZONE_# ............... Color zone file for structure maps "
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APPENDIX B NEW FEATURES IN VERSION 1.3 Release 1.3 adds:
Capability to write a GMSYS-3D model to a Geosoft voxel. Capability to calculate the gravity response of a Geosoft density voxel. Time-to-depth conversion: build a model in time, then use velocities to convert to a depth model. “Seismic” tab for time models. “Velocity” tab for layer parameters. DDTab – rewritten ASCII file import is more robust fixed several bugs in the dialog. Backing up inversion target grid. Resample model to half-size and write to a new model name. Include up to 15 horizons from 3D model to GM-SYS Profile models. Integrated GX help with Oasis montaj. “Auto” DC-shift button added for Gravity and Magnetic calculations. Better UNICODE handling 3D Model Explorer; displays information about the current model in a modeless dialog. Upward continue the observed grid to a draped surface.
In addition there are a number of improvements to existing features:
The Density/Depth dialog has been completely rewritten. ASCII file import is more robust and several bugs were fixed in the dialog. DDTab parameters weren’t correctly written to the model file. Fixed loop that caused reasonable density values to cause errors. Error grid name is not required to run calculations. Improved “Undo” functionality for inversion. Improved “redraw” code so maps are only redrawn when changed. Dialogs don’t lose information when you switch tabs. Building a constraints grid from a well database doesn’t fail when the first well is off the map. New grids should always have their projection set to match the model.
Improved checking to prevent calculations starting when required grids were missing.
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