Electrical Methods
References •
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Burger, H. R., Exploration Geophysics of the Shallow Subsurface, Subsurface, Prentice Hall P T R, 1992. Robinson, Robinson, E. S., and and C. Coruh, Basic , ,
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Telford, W. M., L. P. Geldart, and R. E. Sheriff, Applied Geophysics, 2nd ed., Cambridge University Press, 1990 SEG Journals
References •
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Burger, H. R., Exploration Geophysics of the Shallow Subsurface, Subsurface, Prentice Hall P T R, 1992. Robinson, Robinson, E. S., and and C. Coruh, Basic , ,
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Telford, W. M., L. P. Geldart, and R. E. Sheriff, Applied Geophysics, 2nd ed., Cambridge University Press, 1990 SEG Journals
Geophysical Surveys: Active Versus Passive •
Passive geophysical surveys incorporat incorpo rate e measurements of naturally occurring fields or properties properties of the earth. –
Magnetic
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Electrical Self Potential (SP)
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Electr Ele ctric ical al Magneto Magnetolur luric ic (MT)
Geophysical Surveys: Active Versus Passive •
Active geophysical surveys, a signal is injected into the earth and we then measure how the earth responds to this signal. –
Seismic reflection reflection
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Electrical Electr ical (geoelect (geoelectric rical al )res )resist istivi ivity ty
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Electrical Induced Polarization (IP)
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Electrical Electromagnetic (EM)
Electrical Methods Overview •
Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. –
current flow,
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electrical potential (voltages), (voltages), and a nd
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electromagnetic fields.
Electrical Methods Overview •
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DC Resistivity - This is an active method that employs measurements of electrical potential associated with subsurface electrical current flow generated by a DC, or slowly varying AC, source. Induced Polarization (IP) - This is an active method that is commonly done in conjunction with DC Resistivity. It employs measurements of the transient (short-term) variations in potential as the current is initially applied or removed from the ground.
Electrical Methods Overview •
Self Potential (SP) - This is a passive method that employs measurements of naturally occurring electrical potentials commonly associated with the weathering of sulfide ore bodies. Measurable r p n s v s n s rv n association with ground-water flow and certain biologic processes. The only equipment needed for conducting an SP survey is a high-impedence voltmeter and some means of making good electrical contact with the ground.
Electrical Methods Overview •
Magnetotelluric (MT) - This is a passive method that employs measurements of naturally occurring electrical currents, or telluric currents, generated by magnetic induction of electrical currents in the ionos here. This method can be used to determine electrical properties of materials at relatively great depths (down to and including the mantle) inside the Earth. In this technique, a time variation in electrical potential is measured at a base station and at survey stations. Differences in the recorded signal are used to estimate subsurface distribution of electrical resistivity.
Electrical Methods Overview •
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Electromagnetic (EM) - This is an active method that employs measurements of a time-varying magnetic field generated by induction through current flow within the earth. => GPR Ground Penetrating Radar (GPR) The GPR method uses high-frequency electromagnetic waves to provide detailed subsurface cross sections. Microwave energy reflected back to the surface from different materials produces various electrical results , , GPR can also identify excavations, for example where tanks were removed, and can delineate the boundaries of landfills and buried lagoons higher-frequency antennas (typically 900 or 1,500 MHz) are utilized for surveys on concrete slabs, to locate rebar, electrical conduits, or other pipes prior to core drilling or saw cutting. Suitable signal processing can reduce background noise and identify weaker reflections (such as from fiberglass USTs, or subtle stratigraphic variations) that would remain undetected using analog GPR instrumentation.
SP (Self Potential)
SP (Self Potential or spontaneous potential) •
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Self-potential (or spontaneous potential, SP) measurements belong to the earliest methods used in applied geophysics. Originally applied in mining to ore body exploration, SP became a standard tool with borehole logging and is now of increasing interest in environmental geophysics, geothermal application, and hydrogeology. the earth. There are four principal mechanisms producing natural potential: – – – –
Electrokinetic potential (streaming potential) Liquid-junction (diffusion) potential Shale (nernst) potential Mineralization potential
Theoretical Background •
Electrokinetic potential (streaming potential) –
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Observed when a solution of electrical resistivity ρ and viscosity η is forced through a capillarity or porous medium. The resultant potential difference between the ends of the
Ek = (β∆Pkρ)/(4πη) β = adsorption (zeta) potential ∆P = pressure difference k = solution dielectric constant
Streaming current in a capillary.
Theoretical Background
Liquid-junction (diffusion) potential This is due to the difference in nobilities of various ions in solutions of different concentrations –
R is the constant (8.31 J/Celcius), F is faraday constant (9.65 x 104 C/mol), θ is absolute temperature, n is the valence, Ia and Ic are mobilities of anion and cation. C1 and C2 are the solution concentrations.
Theoretical Background
Shale (Nernst) potential –
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When two identical metal electrodes are immersed in a homogeneous solution, there is no potential difference between them. The combined diffusion and nernst potentials are known as electrochemical, or static self-potential.
Theoretical Background
Mineralization potential –
When two dissimilar metal electrodes are immersed in a homogeneous solution, a potential difference exists between the electrodes.
Field Procedure •
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The voltage between two unpolarizable electrodes, a fixed base station and a moving probe, is measured by a highimpedance voltmeter and a cable layout SP stations are arranged in a grid or - in the case of twodimensional geologic structures - on profiles perpendicular . Station spacing depend on the project conditions and may be of the order of meters or tens of meters. In large survey areas, a gradient layout may preferentially be used. Modern equipments use multichannel digital voltmeters or large PC-controlled electrode arrays which allow also a monitoring of SP fields.
Fixed base - moving electrode method
ra en me o
SP noise •
SP noise sources may considerably affect a survey making reliable data acquisition a difficult task. – – – – –
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Telluric currents, Industrial currents, , Electrode drifts, and Inhomogeneous soils High-amplitude bioelectric potentials from vegetation can seriously overprint the geologic SP signature, and time variations of SP fields are a matter of further complexity
Conceptual schematic models
Qualitative prediction of SP due to rainfall on a horizontal homogeneous earth.
Conceptual schematic models
SP due to horizontal groundwater flow. .
Conceptual schematic models
SP due to groundwater flow with a cavity and a low permeability material. .
Conceptual schematic models
SP due to rainfall with a cavity and low permeability material..
Conceptual schematic models
Combined effects of groundwater flow and rainfall on SP.
Conceptual schematic models
Combined effects of groundwater flow and rainfall on SP.
Case Study •
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The reference electrode is moved to the position of the measuring electrode and the measuring electrode is moved to the next station down the line. A station spacing is 7.5 m. was used to make the measurements. The polarization between the two electrodes was checked before, during, and after data acquisition. No telluric current correction was made as the total measurement time is quite short (~30 minutes).
Case Study
Integral plot of SP under dry conditions.
Case Study
Integral plot of SP under dry conditions minus trend line.
Case Study
Integral plot of SP under wet conditions.
Case Study
Integral plot of SP under wet conditions minus trend line. .
Case Study
Correlation plot of wet and dry residuals..
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The self-potential geophysics can indeed be used to detect cavities. Although theoretically rainfall should intensify SP measurements by increasing flow, in reality it enhances the in homogeneity of the subsurface which generates cave-like anomalies that are misleading. These anomalies are not present in dry conditions because there is essentially no flow in the near-surface vadose . The SP can be a tool to determine direction of groundwater flow and provide a relative measure of hydraulic gradients as observed in the slope difference between the wet and dry surveys. In conclusion, SP is a useful geophysical method to detect caves given the following conditions: (1) the target cave is a local sink for water flow and electrically resistive and (2) SP measurements are taken when the near surface is dry.
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Case study 2 •
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A good correlation between the two methods used to identify geological structures or areas of possible accumulation of groundwater. Resistivity imaging and self potential methods groundwater accumulations in the crystalline rock areas of the dry zone. These methods used with aerial photographs and geological maps will improve the accuracy of locating groundwater wells which are both productive and sustainable.
we survey
Spontaneous Potential (Self Potential) Log Basic Principle •
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The log works by comparing the voltage on the wall of
The change in voltage through the well bore is caused by a build up of charge on the well bore walls. Clays and shales will generate one charge and porous sands will generate an opposite one
Spontaneous Potential (SP Log) • Detects porosity filled with saline formation water. • Useful for differentiating between sandstone and shale, and evaluating the amount of clay in sandstones. Not very useful in carbonates. • Sandstones cause negative readings, shale gives positive readings. • Recorded in left-track with scale in millivolts • Maximum deflection to the right (+) gives shale base line • Maximum deflection to the left (- ) gives clean sand line
Uses •
Identify permeable bed boundaries
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Stratigraphic correlations
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Estimation of shale content Vsh for shaly sand formations. Determination of formation water resistivity (in order to do formation evaluation), R w
Spontaneous Potential (SP Log) ( IDEAL ) (-)
SP
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7700 7750 7800
--------------------------- - - -Shale ------------------------------------------------------------……………………………… ------------……………………………… --……………………………… --……………………………… ……………………………… ……………………………… Sand ……………………………… ……………………………… ……………………………… ……………………………… ……… -------------……… -------------- - - - -Shale --------------------------------------------------------------------------------
Spontaneous Potential (SP Log) ( Typical ) (-)
SP
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Clean Sand
Shale Line
Shaly Sand
Identification of boundaries (Helander, 1983)
Geologic Environments and SP Curve •
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Different types of lithologies in a formation will produce different patterns of SP curves. A rapid change of the energy distribution in the time of deposition is represented by a clean break between shale and sand. The rapid transitions can be said to be produced in the areas with a slight difference in the water depth or energy distribution and led to large variations in the de ositional environment. A broad transition on the SP curve would mean that the unit was under a more stable depositional conditions and little variations in its sedimentary process. Some tooth- like shapes on the SP curve is common in most of the areas, which suggests a rapid transition and non- equilibrium conditions. When the SP is moving towards the shale line that means there was a decreasing rate of sedimentation and a total energy decrease, like in a fluvial environment.
SP pattern for distributary channel (from Merkel, 1981)
SP pattern for channel (from Merkel, 1981).
