Using trapped waves for mapping shallow fault zones

Recent studies have shown that shallow fault zones can be identified by anomalous behaviour of the wavefield recorded by high-resolution seismic surveys. Analysis of seismic records acquired using off-line shooting geometry, where sources and receivers are located along two separate parallel lines crossing a suspected location of a fault zone, reveals prominent anomalies which may be identified with the waves trapped within the zone. On seismic records, trapped waves usually appear as regular low-frequency high-amplitude wavetrains. In order to facilitate the identification of the trapped-wave-related anomalies, we propose two procedures utilizing specific properties of the trapped waves. The first procedure is based on stacking time-scaled seismic traces in the common-shot and common-receiver domains with the subsequent application of eigenimage analysis. In the second procedure, band-limited spectral energy of time-scaled traces is represented as a map in the source–receiver coordinates. By detecting and mapping trapped-wave anomalies in the source and receiver domains, the spatial location of a fault zone can be estimated by linear interpolation between the locations of the corresponding anomalies on the source and receiver lines. Application of the above procedures is illustrated by a number of synthetic and real data examples.