Seismic mapping of subsurface cavities Page: 3 of 12
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Seismic Mapping of Subsurface Cavities
Roland Gritto and Ernest L. Majer
Center for Computational Seismology, Earth Sciences Division
Lawrence Berkeley National Laboratory (LBNL)
1 Cyclotron Rd, MS 90-1116
Berkeley, CA 94720
This numerical study investigates the possibility of inverting seismic data for
the location and volume of a cavity. The data is generated using an exact solution for
the scattering of elastic waves by a sphere, whereas the inversion is based on the low
frequency Mie approximation to the exact solution. We use correlation analysis
between these solutions to solve the inverse problem in two steps. First, the location
of the cavity is determined, before the volume is estimated in a second step. The
robustness of the results is tested by adding correlated and uncorrelated noise. We
find this method to be robust as long as the source receiver coverage is good enough
to record various scattered phases (e.g. back and side scattered) and to reduce the
level of coherent or random seismic noise. The locations of the cavities are well
determined while the estimates of the radii are more susceptible to noise and the
deviation between approximation and exact solution.
A problem in geophysical exploration is the determination of the location and
the size of cavities in the subsurface. The applications are widespread, ranging from
the detection of abandoned mine shafts over construction sites to the detection of
underground facilities for military purposes. In the past, attempts have been
undertaken to solve this problem by seismic means. Although some methods provide
an approximate location of the structure, most fail to give reliable estimates of the
actual volume. This is based on the fact that most methods rely on linearized
solutions that are not suitable for the problem, as one of their main restrictions is the
assumption of small perturbations in the elastic properties. In the case of a cavity,
however, the contrast between the surrounding material and the cavity is large, which
causes most approximations to fail. Although the strong contrast poses a severe
problem, it simultaneously provides valuable apriori information as the elastic
parameters of an air filled cavity are known. If through additional measurements (in
boreholes or at the surface) the parameters of the surrounding material can be
estimated, the experiment reduces to' a problem with two unknowns: the location of
the center and the volume of the cavity. This scenario is the basis of the current study.
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Gritto, R. & Majer, E.L. Seismic mapping of subsurface cavities, article, November 1, 1999; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc702876/m1/3/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.