Void (Anomaly) Detection
Although a large-size anomaly can be detected from the (1-D and 2-D) Vs profiling previously outlined, a more effective detection of such a significant anomaly as a void can
be accomplished with the side scattering analysis (SSA) previously described. The most comprehensive approach, however, is the combination of and cross checking
between different types of data with the highest priority on the SSA results. For example, if a suspicious scattering feature is identified from a SSA map, then existence of the
velocity anomaly is examined on the 2-D (or 3-D) Vs maps previously obtained for those portions near the locations identified on the SSA map (Fig. 1). If a location is close to
a 2-D survey line, then the existence of the back-scattering feature is also examined on the adjacent field records (Fig. 2). Combining all these types of information together, a
confidence level (usually in a scale from 1-10 with 10 the highest level) is assigned to the identified anomaly (Fig. 3). Field engineers use this information in conjunction with
other types of information such as local geology, results from other types of surveys (for example, a Ground-Penetrating-Radar survey), drilling, etc., to make the decision on
the next approach to be taken.
(Right) Fig. 1. Anomaly detected in
SSA, 2-D and 3-D Vs maps.
(Above) Fig. 2. Anomalies (A and B) identified from the back
scattering features on field records and corresponding
locations on the field map.
(Right) Fig. 3. A table showing identified anomalies with their
relative confidence levels (from Park and Miller, 2005a).