Technologies for Locating Mine Workings - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 73

the data collection process, including changing the extent and emphasis of
a survey.
The appropriate strategy for finding subsurface cavities depends on the
size of the target. That size determines the spacing of geophysical
measurements, which should be sufficient to define subsurface conditions.
Decreasing density costs less but may increase the probability of error in the
detection of voids. To achieve a suitable detection probability, any site
investigation should work from a regional to a local to a site-specific approach;
techniques should progress from large-scale and noninvasive (e.g., aerial
photography, remote sensing) to detailed site data (e.g., drilling, borehole
geophysical methods). Holes should not be drilled until the investigators know
what they are looking for, and measurements should not begin without an
adequate understanding of the surrounding geology (R.Benson, Technos,
personal communication, 2001).
The inclusion of microprocessors in modem geophysical instrumentation
has made on-screen, menu-driven geophysical data collection easy for those
who have limited geophysical training. Problems can arise when the data are
collected with inadequate spatial sampling density or with inappropriate
techniques, and these problems can be compounded when individuals with
inadequate geophysical education and experience attempt to interpret the data.
Even if the data have been collected correctly, inappropriate data processing
and interpretation can introduce many pitfalls. Furthermore, no single
noninvasive technology works everywhere all the time. Consequently, an
interdisciplinary approach is needed to interpret and to integrate multiple
geophysical data sets with local geological and engineering data.
This chapter offers an overview of the site characterization techniques that
can be utilized to locate abandoned underground mines. A more detailed
description of these techniques is available in a guide for selecting surface
geophysical methods issued by the American Society for Testing and Materials
(ASTM, 1999) and in Appendix E .
DRILLING
There are several ways to drill shallow holes into the Earth (e.g., NRC,
1994). The simplest is done using an auger that brings soil material to the
surface much like a drill bit boring in wood brings wood cuttings to the surface
(Hynes, 1995). Augers cannot penetrate solid rock; they are used primarily to
drill in soil, unconsolidated sands and clays, and soft shales. Only the
shallowest of coal beds can be reached with auger drills. Most modern drilling
at depths of more than 100 feet or so is done using rotary