Case studies

Geophysics Online Course
https://giladjames.com
Section: Three–Dimensional Seismic Diffraction Imaging for Detecting Near-Surface Inhomogeneities
Lesson: Case studies
Geophysics.
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Introduction
The problem of detecting localized near-surface inhomogeneities is encountered in a variety of applications, such as engineering site investigation, environmental studies, archeology and security problems. Typical examples of such sources of inhomogeneities are cavities, caves and tunnels and other natural and man-made local inhomogeneities.
In the past decades, many attempts have been made to solve the problem using various geophysical methods. The methods in use can be roughly subdivided into two groups. The first group includes low-resolution bulk techniques such as microgravity, geoelectric method and seismic surface wave method. A typical drawback of these methods is their low detectability, limiting them to relatively large sources, that is, such sources whose dimension-to-depth ratio exceeds 1/2. The second group includes the high-resolution ground – penetrating radar (GPR) method. The main drawback of this method is its high sensitivity to electrical conductivity of near-surface material which makes it limited to very shallow depths usually not exceeding a number of meters. Furthermore, our experience shows that the general problem of all the existing techniques is a high degree of ambiguity of data interpretation and, as a result, a high percentage of errors. One of the methods that may prove very effective for this purpose is active seismic, which is widely employed by seismic exploration industry in its extensive and detailed studies of the subsurface. Adaptation of these methods for the purpose of searching for local underground heterogeneities is not an easy task. The primary aim of seismic exploration is to study geological sources of large lateral extent, such as boundaries of geologic formations, oil and gas deposits, etc. To this end, seismic exploration (“reflection seismic”) utilizes seismic waves emitted from man-made (active) seismic sources and reflected from the boundaries of interest back to the surface. The methodology, the data acquisition and processing techniques employed by the reflection seismic are largely based on the model of the subsurface consisting of a number of layers divided by smooth curvilinear boundaries with gradual changes of parameters within layers. Virtually, the same model of the subsurface is behind the refraction seismic method, another technology used primarily to study the shallow part of the subsurface (first tens of meters). Clearly, such methods are not directly applicable to a search for local subsurface inhomogeneities of small lateral extent (10–100 m). The amount of energy reflected by such small sources would be very small, and it would be indistinguishable from other geological features of the subsurface. In order to be effective, the search for local inhomogeneities has been employed to image the features of seismic wavefield, which is primarily associated with the presence of local heterogeneity in the subsurface. Such features are known as scattered (diffracted) waves. In a standard seismic prospecting, these waves are usually treated as noise. But for locating local sources these waves provide the most valuable source of information .
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