Geology Reference
In-Depth Information
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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