Coal Waste Impoundments Risks, Responses, and Alternatives (2002) / Chapter Skim
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Appendix E: Geophysical Techniques
Appendix E: Geophysical Techniques
Pages 221-230
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From page 221... ...
The typical induced polarization frequency range is between 0.05 hertz and 1 kilohertz. Induced polarization surveys have been used in some cases for groundwater exploration and the method is frequently used in sulfide mineral exploration.
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From page 222... ...
Active electromagnetic surveying employs a primary field induced by an electrical current passing through a coil, which induces three-dimensional currents through underground conductors according to the physical laws of electromagnetic induction. This underground current induces a secondary electromagnetic field which then distorts the primary field; the resulting final field is sensed by a receiving coil.
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From page 223... ...
This constructive-interference system is a channel wave that propagates within the coal seam without radiating significant wave energy into the surrounding bedrock. The two types of seismic waves, commonly interpreted as part of in-seam seismic surveys, are Rayleigh waves comprised of body waves of the P and SV type and Love waves comprised of SH waves only.
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From page 224... ...
Because the seismic velocities and densities of both coal and bedrock can be measured easily in coal mines, accurate theoretical dispersion curves can be calculated for the rock-coal-rock geology and plotted for comparison with the seismic velocity-frequency data (Raeder et al., 1985~. If the theoretical dispersion curve and field data velocity-frequency plots match closely, an undisturbed propagation path is indicated.
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From page 225... ...
can cause dramatic changes in T2 so that the direct nuclear magnetic resonance link to the ratio of the surface area to volume breaks down. These effects, which make it much more difficult to obtain estimates of permeability, were examined by Bryar et al.
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From page 226... ...
The level of gamma radiation caused by the scattering is proportional to the total number of electrons in the formation near the sonde. Neutron Logs Nonradioactive elements emit gamma radiation if they are sprayed by a stream of neutrons.
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From page 227... ...
There are several types of resistivity logs including direct-current resistivity logs and electromagnetic induction logs. Gamma Logs Gamma logs measure natural gamma radiation, and are particularly useful for finding shales that have a high gamma output because clay collects radionuclides.
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From page 228... ...
It is useful to find evaporites and washouts of shale. Casing Collar Logs The casing collar log is used to count joints of casing to know exactly how far down the hole specific geologic layers are.
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From page 229... ...
Borehole Acoustic Televiewer Logs The borehole acoustic televiewer log is used to develop acoustical analog images of the rock face around the borehole. This is particularly useful for determining fracture patterns and directions.
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From page 230... ...
Other measurement-while-drilling tools, such as resistivity tools, are also available to help prevent blow-outs. They detect highly electrically resistive conditions (e.g., overpressured gas)
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