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Appendix E Geophysical Techniques ELECTRICAL AND ELECTROMAGNETIC METHODS The induced polarization (IP) method includes analysis of the Earth's delayed response to an induced current; induced polarization is related to the resistivity method. The "P" in IP can be thought of as 'persistence" or the amount of time the Earth stays disturbed electrically after the removal of the electrical disturbing function. The discharge rate of a volume of the Earth is similar to that of a capacitor. The induced voltage's decay rate is dependent on the ion mobility in the charged volume. For example, the ions in clays are very mobile. Measurements can be made in the frequency domain, where the phase delays of various frequencies are analyzed, or in the time domain where voltage is measured as a function of time. Highly accurate clocks can be synchronized each day to determine the amount of delay for each frequency reaching the voltage electrodes, or the receiver and transmitter can be connected to a single clock. 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. Some recommendations for potentially fruitful areas for induced polarization research are given in Ward et al. (1995~. The spontaneous potential method employs natural voltages resulting from electrochemical activity in the Earth. The voltages usually average to zero over distances a few times larger than the spatial extent of any anomalies, and they rarely exceed 100 millivolts. Spontaneous potentials can be generated by fluid, ions, or heat moving in the Earth. The source current or configuration remains unchanged over the period of measurement. Because this is a passive technique the signals are vulnerable to "noise" from powerlines, pipelines, electrical storms, and other environmental sources. Sometimes the noise level may preclude the repeatability of the measurements, which is one of the problems with spontaneous potential methods. Spontaneous potential measurements have been used with some success in geothermal exploration and to monitor subsurface water 221
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222 COAL WASTE IMPOUNDMENTS movement (i.e., observing a moving conductor in a magnetic field). In the geothermal case, chemical reactions induced by mineralized waters may add to any voltage caused by movement of water. Another possible use of spontaneous potential surveys is mapping the concentration gradients of chemically active leachate. A spontaneous potential survey might be sensitive to water in mine works that are reacting with their surroundings. Spontaneous potential data may be interpreted from contour maps of voltages or by more quantitative model calculations employing geometrical shapes similar to those used in magnetic and gravity studies. The fundamentals of near-surface spontaneous potential applications can be found in Corwin (1990~. 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. The field detected by the receiving coil varies in intensity, phase, and direction from the primary field, which reveals information about subsurface electrical conductivity. Electro- n~agnetic methods have an advantage over DC resistivity because they do not require inserting electrodes into the ground. Low-flying, small (maximum dimension 3 to 6 feet) unmanned aircraft have been employed to conduct some surveys to provide access to polluted? dangerous, or inaccessible areas. Such airborne surveys have disadvantages, including small separation between the source and receiver coils and a higher noise level caused by the relatively high velocity movement of the coils through the Earth's magnetic field. The practical background and the theoretical basis for electromagnetic methods are presented in McNeill (1990~. ~ contrast to active electromagnetic surveying discussed above, passive electromagnetic surveying employs Earth's natural electromagnetic fields to provide the variations in the electric field. Electric fields generated by distant lightning flashes are the source used in the audio-frequency magnetic field technique. The passive very-low radio frequency method relies upon the 15 to 25 kilohertz electric field from distant, powerfi~1 radio transmitters used to communicate with submarines. These passive techniques may be useful for regional studies, but they do not have the resolution to find underground mine works.
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APPENDIXE 223 IN-SEAM SEISMIC TECHNIQUES Data Acquisition In-seam seismic surveys are typically performed in panels surrounding blocks of coal prior to long-wall mining operations. Seismic-wave transmission surveys are set up to test the transmissivity of the coal seam by deploying seismic sources along one face of a coal panel and placing geophones along the opposing face. If disturbances are inferred from the transmission experiment, a seismic reflection survey may be used to estimate their locations. Small explosive charges deployed in horizontal holes drilled about 3 feet into the face of the coal seam are used to generate elastic waves within the coal seam (Dresen and Reuter, 1994~. Geophones are routinely wedged into horizontal holes similar to the source holes. The geophones are sensitive to motion in the plane of the coal seam, and are designed to record channel waves of Love type (see below). Although coal has a relatively high rate of seismic-wave attenuation (Q factors range from 20 to 50), divergence of in-seam waves is two-dimensional; thus propagation distances as long as a 11/4 mile have been reported (Greenhalgh et al., 1986~. Channel Waves A coal seam is a low-velocity channel for elastic waves. If a seismic source is triggered in the middle of the coal seem, elastic waves propagate in all directions throughout the coal. Wave motion encountering the coal-rock interface along the top and bottom of the coal seam is constructively reflected back into the seam at various angles and at different phase velocities. 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. Seismic-wave phases created at various angles of reflection at the coal-bedrock interfaces cause dispersion of the channel waves, which means that the channel waves propagate at frequency- dependent speeds. Hence, at longer travel distances the wave phases get separated and are recorded as a time- elongated arrival in the seismogram. =
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224 COAL WASTEIMPOUNDMENTS Data Processing Analysis of in-seam wave dispersion can help determine whether a propagation path of a transmitted wave is disturbed by geologic or old mine features. The seismogram is transformed into a velocity-frequency diagram, where the dispersed channel waves appear as curved-amplitude plots. 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. However, if the channel waves are reflected from an obstruction back to the panel containing the seismic source, the match may be poor. If a disturbance is inferred, a seismic reflection survey conducted with both the seismic source and geophones at the same seam face may help to estimate the location of the disturbance. Because the seismic-wave velocity in the coal is known, the two-way travel time of any waves reflected from the disturbance back to the geophones can be transformed into distance to indicate the location of the reflector. NUCLEAR MAGNETIC RESONANCE Nuclear magnetic resonance measurements were initially performed by physicists investigating molecular-scale phenomena. A radio-frequency pulse excites nuclei to a higher energy state and their return to the original state is monitored, modeled as a sum of exponential decays, and recorded as two relaxation-time constants, 1~, associated with the longitudinal component of the magnetization, and T2 with the transverse component. Nuclear magnetic resonance can be used to study any nuclei that have an intrinsic magnetic moment, such as hydrogen or carbon-13. (See McMurray's 1984 review of nuclear magnetic resonance theory.) Surface geophysical nuclear magnetic resonance was pioneered by a Russian team (Semenov et al., 1988) who developed the "hydroscope" consisting of a transmitter and receiver in which antennas approach 330 feet in diameter (a lower limit on horizontal spatial resolution). The total volume of water present as a function of depth is proportional to the number of hydrogen nuclei in the sample which is proportional to the amplitude of the initial magnetization. When the transmitter is turned off, the resulting relaxation time contains information about the grain size of the water- saturated rock. If the rock or soil contains water, the relaxation time is a
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APPENDIX E 225 function of two processes: relaxation in the bulk fluid and relaxation on the solid pore surfaces. Surface relaxation is the faster of the two processes, dominating the response, and leading to a relationship between relaxation time and the ratio of the pore's surface area to volume, which is related to grain size. An empirical correlation between rock type and decay time was used by Shirov et al. (1991) to estimate grain size. Minimization of the misfit between the calculated and measured responses is via inversion is used to extract both the total amplitude and the relaxation-time constants. Paramagnetic species (such as Fe3+) 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. (2000) and Knight et al. (1999~. For example, two sands whose pore size and distribution and grain size are identical could appear to have different nermeabilities if one had a high Fe3+ content and the other did not. The variation in the content of Fe3+ and other paramagnetic species could complicate or negate permeability estimates based on . . ~ ~ . ~ NMK data tor near- surface applications. T2 would be affected, but to a different extent, depending on the specific location of the Fe3+ (i. e., in pore water, adsorbed to the solid phase, or in a solid mineral grain). BOREHOLE GEOPHYSICS The vast majority of surface geophysical techniques can be modified for application in borehole environments. This includes resistivity, electro- magnetic, gravity, radar, radiometric, and seismic methods (e.g., Daniels and Keys, 1990; Howard, 1990~. In some cases, borehole geophysical measurements are made to help tie the borehole samples to surface geophysical data. In other cases, logs are used to help interpret the samples themselves. For example, in the petroleum industry, to calculate the hydrocarbon concentration, the resistivity log is used to infer the percentage of saturation with hydrocarbons, where the salinity of the formation water is known. Sometimes a particular log will be diagnostic in a particular environ- ment, and other times the geology will defy rational analysis by even the most sophisticated suites of logs. On balance, however, it is remarkable how much geologic information can be derived from simple suites of logs, given the gross physical assumptions that are made in logging. Properties that can often be directly or indirectly determined from borehole geophysics include, but are not limited to, the following:
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226 COAL WASTEIMPOUNDMENTS 1. Lithology 2. Bed thickness 3. Porosity 4. Fluid type and amount 5. Fluid flow vector 6. Permeability 7. Trace element chemistry 8. Fracture orientation 9. Rock strength Discussed below are logging techniques to infer these properties. Problems in geophysical logging of oil wells include the presence of mud cake and of formation disturbance by drilling fluids. The presence of these materials disturbs the geophysical measurements. Density Logs The density log employs a cesium source of gamma rays shot into the formation at 45° away from the hole axis. The receiver is a scintillation counter collimated at 45° to the hole also but at a right angle to the source direction. The method uses Compton scattering and assumes a direct relationship between electron density and bulk density. This is actually surprisingly accurate assumption because the ratio is very close to 2:1 for mass compared to number of electrons. 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. The neutron source for logging is americium or beryllium, which produces a constant population of high-energy neutrons. Hydrogen selectively absorb these neutrons, and as the neutrons are absorbed, gamma rays are emitted. The more hydrogen in the rock, the faster neutrons are absorbed and the more gamma rays are emitted. Water or oil absorbs neutrons; therefore, porosity is generally proportional to hydrogen content as indicated by the neutron log. The neutron log is a porosity determination tool.
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APPENDIXE 227 Resistivity Logs The resistivity log is analogous to electrical resistivity measurements made at the Earth's surface. It measures the electrical resistance of material between electrodes placed on the sonde in the borehole. This log is especially sensitive to the electrical properties of fluids contained in under- ground formations. The resistivity log cannot generally be made through casing, although research is now being done to develop this capability. 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. They can be used in cased holes, so they are also run on casing collar logs to tie the exact location of casing to geologic rock units. This is essential if we are to perforate the casing at exactly the right spot to test the oil or gas (or fresh water) zones. Newer techniques include gamma-ray spectral logging to look at clays, based on ratios of gamma rays of known energy from uranium, thorium, and potassium. Spontaneous Potential Logs The spontaneous potential logs measure voltage between formations by attaching one voltmeter electrode on the logging sonde and the other at the Earth's surface (see above). It does not work if the drill is using salt-based mud. Sonic Logs Sonic logs are essentially a borehole seismic refraction survey. Sonic logs use a 20-kilohertz transducer and two sensors. The method makes use of the Wyllie equation which assumes that transit time is a function of the mineralogy, the percentage of pore space, and the P-wave velocity of the fluid within the pores. The equation works surprisingly well, but it is sometimes treacherous to extrapolate the velocity measured by sonic tools to =
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228 COAL WASTEIMPOUNDMENTS velocity measured by seismic waves that have wavelengths 1,000 times as long. The analyst measures the transit time in microseconds per foot to determine rock type and porosity. Temperature Logs The temperature log is useful for measuring temperature gradients, Ethology changes, and water flow in the vertical direction. The logging can be done either from top down or from bottom up, and it is common to log in both directions. However, for the greatest precision, logging from the top down is preferred because the water has not been disturbed by the passing of the tool. Caliper Logs The caliper log is used to measure borehole diameter as a function of depth. It shows the boundaries between soft shales and hard limestones very clearly and with better depth. precision than other logging tools. 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. The casing collar log is used in conjunction with the natural gamma log to provide locations for casing perforations or for hydrologic measurements such as packer tests and drill stem tests. (Packers are plugs used to isolate fluid under pressure in a specific segment of pipe in a hole.) Dip-Meter Logs The dip-meter log is obtained from three resistivity tools placed at different azimuths around the sonde. This log of measures local dip of geologic layers within a borehole. -
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APPENDIX E 229 Cement Bond Logs The cement bond log is obtained with a sonic tool to determine how good the cement seal is on the outside of the casing. The highest amplitude is related to poor or nonexistent bonds between the casing and the cement. Low amplitude indicates good bonding, which allows the sound energy to penetrate away from the casing. A good bond ensures against leaks of water or pollutants from one rock layer to another. 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. The fracture direc- tions are needed when designing horizontal drilling programs where the bit must cross fractures to drain oil reservoirs efficiently. It can be used in drilling mud. Also, this tool can be used in conjunction with hydraulic fracturing to determine in situ stress orientation of the principal tectonic stresses. The hole is first surveyed with a televiewer. Then packers are set at the top and bottom of the interval of interest and the formation is hydraulically fractured by injecting water into the interval between the packers. Finally, the packers are removed and the hole is inspected again by televiewer to determine the direction the fracture orientations. Fracturing occurs perpendicular to the direction of least principal stress. Borehole Television Camera The borehole television camera is used to develop a visual image of the borehole walls. It is used for many of the same things as the acoustic televiewer, but the water in the hole must be relatively clear for it to work. When it works, it can provide a more detailed image of fractures and even of Ethology than the acoustic televiewer. Drill Penetration Logs The rate of drill penetration into the ground is measured with a drill penetration log. It is used in conjunction with logs of mud-pump pressure, rate of spin of the bit, and weight on the bit. In areas where previous =
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230 COAL WASTEIMPOUNDMENTS experience is available, these logs can be extremely useful in knowing where the bit is geologically. This log is a measurement-while-drilling log. 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) ahead of the bit.
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