Geology Reference
In-Depth Information
A large portion of the information contained within shallow, high-
resolution seismograms is not used or emphasized in standard two- or three-
dimensional reflection surveys. Cavity detection is a fundamentally different
problem than those addressed in standard petroleum exploration. Although the
reflected portion of the wavefield may yield the highest resolution information
about void location, other portions of the wavefield may provide key constraints
in the detection and interpretation phases of a survey. Thus, a variety of
different processing, modeling, display, and interpretation methods should be
investigated to determine whether it is possible to exploit seismic wavefields
uniquely altered by the presence of a shallow subsurface tunnel.
No seismic method appears to be uniformly applicable in the highly
variable near-surface geology of different tunneling environments. Each method
will probably be of use in some specific geological environment (Tables 5.2 and
5.3 ). Sidebars 5.1 , 5.2 , and 5.3 illustrate case histories where seismic reflection
has been used to locate mine workings. The mine workings cause a shadow
effect, which decreases signal strength for reflection from layers below the coal
( Sidebar 5.1 ). The void presence in Sidebar 5.2 causes the coal bed reflection to
disappear. At present the immense computational resources required limit the
full waveform inversion of both seismic and ground-penetrating radar data to
small data sets. When computing costs have decreased sufficiently, these
inversions may become commonplace. One caveat, however, is that the
inversion process treats noise with the same reverence that it treats data. When
noise is present in shallow seismic or ground-penetrating radar data, a data-
inversion routine may produce artifacts related to its attempt to invert the noise.
Automation could improve very near-surface geophysical methods— from
model airplanes carrying microsensing devices to robots roving the ground over
hazardous or polluted areas. Automatic emplacement of geophones (Steeples et
al., 1999) may significantly improve cost-effectiveness of near-surface seismic
surveys. All geophysical techniques could benefit from improved precision,
resolution, and bandwidth. Data processing would benefit from faster and more
robust methods, especially if the ambiguities and uncertainties that plague data
interpretation could be decreased. By combining robotics, automation, expert
systems, and miniature aircraft it may be possible to decrease costs and improve
productivity. The efficient and timely transfer of technology from developers to
users and potential beneficiaries could be enhanced through continuing
education programs.
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