Geology Reference
In-Depth Information
related to the resistivity method, except that the Earth's delayed response to an
induced current is analyzed. The spontaneous potential method measures the
natural voltage in the Earth resulting from electrochemical activity. If water in
mine workings is reacting with its surroundings, this might be detectable with a
spontaneous potential survey. Water in motion through fractured and porous
media produces a “streaming potential,” which is useful in detecting leaks in
dams. This technique might also be useful in looking for mine works in which
water is flowing.
Active electromagnetic methods have become more popular in near-
surface geophysical applications (
Appendix E
). The theoretical basis for, as
well as practical background for, electromagnetic methods is provided in
McNeill (1990). These methods have a major advantage over direct-current
resistivity because they do not require placing electrodes in the ground. Indeed,
the surveys can sometimes be conducted from low-flying aircraft. A recent
development in airborne electromagnetics offers the advantages of increased
surveying speed and access to polluted, dangerous, or inaccessible areas via
small (maximum dimension 3 to 6 feet) unmanned aircraft. However, airborne
surveys also have disadvantages, including limited separation between the
source and receiver coils and a higher noise level caused by the movement of
the coils through the Earth's magnetic field (Blakely, 1996; Nabighian, 1988,
1991).
Potential Field Methods
Buried metal objects such as steel drums are often found with
magnetometer surveys in which measurements with precision of one part in
50,000 of the Earth's total magnetic field are made. Although data precision and
collection rates continue to improve, magnetic surveying is a relatively mature
science. In the future, vector recording of the magnetic field instead of the
commonly used total field could be useful. Because coal is relatively
nonmagnetic, the removal of coal does not alter the magnetic field very much.
Consequently, magnetic surveys are not commonly useful in finding
underground mine workings. They could be useful, however, in detecting old
cased wells or mine workings that contain metal pipes, cables, rails, or
equipment.
Figure 5.1
shows the size of a magnetic anomaly that is typical at
various distances for common metallic items such as tools and vehicles.
Magnetic gradiometry consists of taking simultaneous readings from two
magnetometers spaced a few inches to several feet apart and analyzing the
difference (the magnetic gradient). Magnetic surveys are also useful in mapping
faults, locating magnetic bodies, and estimating the depth to
Search WWH ::
Custom Search