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.

Fig. 1. Anomaly detected in SSA, 2-D and 3-D Vs maps.
Fig. 2. Anomalies (A and B) identified from the back scattering features on field records and corresponding locations on the field map.
Fig. 3. A table showing identified anomalies with their relative confidence levels (from Park and Miller, 2005a).