|Development Geology Reference Manual|
|Series||Methods in Exploration|
|Author||Arnold S. Orange|
The electrical geophysical methods are used to determine the electrical resistivity of the earth's subsurface. Thus, electrical methods are employed for those applications in which a knowledge of resistivity or the resistivity distribution will solve or shed light on the problem at hand. The resolution, depth, and areal extent of investigation are functions of the particular electrical method employed. Once resistivity data have been acquired, the resistivity distribution of the subsurface can be interpreted in terms of soil characteristics and/or rock type and geological structure. Resistivity data are usually integrated with other geophysical results and with surface and subsurface geological data to arrive at an interpretation.
Electrical methods can be broadly classified into two groups: those using a controlled (human-generated) energy source and those using naturally occurring electrical or electromagnetic energy as a source. The controlled source methods are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivities down to depths as great as 1 to 2 km2,000 m
78,740.2 in, although greater depths of investigation are possible with some techniques and under some conditions. The natural source methods are applicable from depths of tens of meters to great depths well beyond those of interest to hydrocarbon development.
Possible applications of electrical methods for the development geologist range from the investigation of soil contaminants and the monitoring of enhanced oil recovery (EOR) projects to reservoir delineation and the evaluation of geological stratigraphy and structure. The application of electrical methods has been primarily confined to the onshore environment. The offshore use of some techniques is possible, particularly for permafrost delineation and shallow marine geotechnical investigations.
Electrical properties of materials
The application, interpretation, and understanding of electrical methods requires a familiarity with the relationship between soil and rock characteristics and the resistivities obtained from electrical data. The resistivity of subsurface rock formations is one of the physical properties determined through the process of logging that is performed on most oil and gas wells, utilizing instrumentation inserted into the wellbore. The concept of formation resistivity plays an important part in log analysis. Although there is a correlation between rock resistivities measured by well logs and those measured by electrical methods, the log is used to investigate properties only in the immediate vicinity of the wellbore while electrical methods yield information on bulk properties averaged over a considerable volume of material.
The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock (see Determination of water resistivity). This is because the dry soil or rock matrix is a virtual insulator at DCdrill collar and near DCdrill collar frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.
Faulting or fracturing of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture porosity changes the already high porosity by only a small percentage. However, in very tight rocks, such as igneous, metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see Porosity).
In summary, the factors affecting in situ average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.
Controlled source methods
Controlled source methods use generated currents or electromagnetic fields as energy sources. An advantage is the control over energy levels and the attendant positive effects on signal to noise ratio in areas of high cultural noise. A disadvantage of controlled source methods is that the complex nature of the source field geometry (the geometry of the electromagnetic field or currents induced with the earth by the transmitter) may present quantitative interpretation problems in areas of complex geology.
In the DCdrill collar method, a current (usually a very low frequency square wave and not actually direct current) is injected into the earth through a pair of current electrodes, and the resulting potential field is mapped. Various geometries of current and potential electrodes have been employed, with the choice primarily based upon the depth and geometry of the survey target. The measured surface potential field is interpreted in terms of the subsurface resistivity distribution through modeling and inversion techniques. Induced polarization (IP) and complex resistivity (CR) techniques are special cases of the DCdrill collar method in which the induced potential field is measured and interpreted in terms of mineralogy and/or soil characteristics. IP and CR have been applied with some success to hydrocarbon exploration through the measurement of geochemical alteration halos that have been found to be related to reservoirs under some conditions.
In the electromagnetic (EM) method, an electromagnetic field is produced on or above the surface of the ground. This primary EM field induces currents in subsurface conductors. The induced currents in turn reradiate secondary EM fields. These secondary fields can be detected on or above the surface as either a distortion in the primary field (frequency domain methods) or as they decay following the turning off of the primary field (time domain methods). Both loops and grounded wires are used to generate the source field. Resistivities are calculated from the observed electromagnetic field data using modeling and inversion techniques.
EM techniques have been adapted to a variety of surface and airborne configuration, with the airborne instruments generally limited in penetration to 100 to 200 m656.168 ft
7,874.02 in. Airborne electromagnetic surveys have proven very effective for mapping the shallow resistivity distribution, leading to cost-effective surveys over large areas. Surface loop or grounded wire systems are applicable to depths well in excess of 1 km1,000 m
39,370.1 in, although high power transmitters are required as depth increases. The resolution attainable is normally considered as a percentage of penetration depth, such that absolute resolution decreases with depth.
In the controlled source magnetotelluric (CSMT) method, a low frequency electromagnetic wave is generated, and the electrical and magnetic fields are measured at some distance from the transmitter. The wave impedance of the electromagnetic wave at the receiver is calculated from the electrical and magnetic field values as a function of frequency and then interpreted in terms of the subsurface resistivity distribution. Depths of penetration in excess of 1 to 2 km2,000 m
78,740.2 in are attainable under suitable conditions.
Ground probing radar (GPR) is used for detailed investigations of the shallow subsurface. An extremely short pulse is generated and transmitted into the earth and reflections are received from interfaces between materials of differing resistivity and dielectrical constant. GPR instrumentation is sophisticated but highly portable. Depth of penetration is limited from less than 0.3 m0.984 ft
11.811 in in silty soils to over 100 m328.084 ft
3,937.01 in in permafrost, freshwater-saturated sand, and some very low porosity rocks. Successful applications include the measurement of ice thickness, the location of cracks in ice, permafrost studies, the detailed mapping of the bedrock surface, the examination of soil stratification, and the mapping of contaminant plumes in the shallow subsurface. An important application of GPR is locating buried pipes, tanks, and other objects that reflect the radar pulse.
Natural source methods
Natural source methods take advantage of naturally occurring electrical potentials and electromagnetic fields as energy sources. Advantages of natural source methods are that there is no dependence on an artificial energy source and that the natural electromagnetic field is well understood. The principal disadvantages are the unpredictability and lack of control over energy levels and the attendant effects of cultural noise on the signal to noise ratio.
The self-potential (SP) method examines the slowly varying surface potential field caused by electrochemical and electrokinetic actions in near-surface materials. Potentials can form, for example, at interfaces between materials containing fluids with different ion contents, or they can be caused by moving groundwater or by differential oxidation of ore bodies. The method has been applied successfully in geothermal and mineral exploration and in the delineation of certain groundwater contaminants. Field procedures are straightforward, with the potential measured between carefully designed electrodes using what is essentially a highly sensitive DCdrill collar voltmeter. The potential field is mapped along profiles or on a grid of measurement stations. Interpretation is generally qualitative, with SP anomalies interpreted in terms of the shape and depth of the causative body or fluid flow.
Magnetotellurics (MTmiddle tubing) is an electrical method of geophysical exploration that makes use of naturally occurring electromagnetic energy propagating into the earth to determine the electrical resistivity of the subsurface. The low frequency electromagnetic field is measured, and the wave impedance is calculated and expressed in terms of the resistivity of the subsurface. The depth of investigation is a function of the frequency of the electromagnetic wave, taking advantage of the fundamental principle that the lower the frequency of a wave, the deeper the penetration into the crust. MTmiddle tubing surveys generally involve applications that range in depth from a few hundred meters to 10 km10,000 m
393,701 in or more.
The resistivity versus depth cross section developed from MTmiddle tubing data can be interpreted in terms of rock type. Spatial variations in the resistivity-depth relationship observed at closely spaced locations on the surface can be interpreted in terms of subsurface geological structure. While MTmiddle tubing cannot be used to detect oil directly, the identification of favorable rock types and the presence of geological structure capable of trapping hydrocarbons is critical to successful exploration. MTmiddle tubing data are interpreted using forward and inverse modeling techniques. Resolution is considered low when compared with exploration or exploitation seismology, but may be adequate in certain instances to provide valuable information concerning reservoir geometry, rock characteristics, and a regional geological framework. For the larger deep reservoirs, MTmiddle tubing may be considered as a possible candidate for EOR monitoring if model studies indicate that the resistivity changes over time associated with the operation are within the resolving power of the method.
Applications for development geology
The following is a brief summary of some of the many possible applications of electrical methods of interest to the development geologist:
- Evaluation of various characteristics of the shallow environment. Examples of such characteristics are the classification of unconsolidated materials based on their electrical properties, the identification of a lateral and/or vertical freshwater-saltwater boundary, the depth to bedrock, and the identification and mapping of conductive groundwater contaminants.
- Monitoring of reservoir stimulation and enhanced recovery projects, where the stimulants and propants or flood materials can be expected to modify the resistivity of the formations.
- Investigation of permafrost and ice characteristics in the Arctic.
- Investigation of stratigraphy and structure, in particular as an adjunct to seismic data and in those areas where seismic data are poor or unreliable.
- Seafloor geotechnical mapping, as an adjunct to high resolution seismic studies.
- Seismic data acquisition on land
- Seismic interpretation
- Cross-borehole tomography in development geology
- Synthetic seismograms
- Vertical and lateral seismic resolution and attenuation
- Forward modeling of seismic data
- Borehole gravity
- Displaying seismic data
- Three-dimensional seismic method
- Full waveform acoustic logging
- Seismic migration
- Basic seismic processing
- Marine seismic data acquisition
- Introduction to geophysical methods
- Seismic data - mapping with two-dimensional data
- Seismic inversion
- Checkshots and vertical seismic profiles
- Amplitude versus offset (AVO) analysis
- The gravity method
- Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.
- Zody, A. A. R., 1989, A new method for the automatic interpretation of Schlumberger and Wenner sounding curves: Geophysics, v. 54, n. 2, p. 245–253., 10., 1190/1., 1442648
- Nekut, A. G., and B. R. Spies, 1989, Petroleum exploration using controlled source electromagnetic methods: Proceedings of the IEEE, v. 77, n. 2, p. 338–362., 10., 1109/5., 18630
- Sill, W. R., 1983, Self-potential modeling from primary flows: Geophysics, v. 48, n. 1, p. 76–86., 10., 1190/1., 1441409
- Orange, A. S., 1989, Magnetotelluric exploration of hydrocarbons: Proceedings of the IEEE, v. 77, n. 2, p. 287–317., 10., 1109/5., 18627
- Vozoff, K., ed., 1986, Magnetotelluric methods: Tulsa, OK, Society of Exploration Geophysicists, Geophysics Reprint Series, 763 p.