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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
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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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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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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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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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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.
Representative terms from entire chapter:
coal seam
