Fracture Detection in Alluvial Fan Deposits Using Near-Surface Seismic Reflection Techniques

In this study we document the observation of probable extensive shallow vertical fracture systems in unprocessed 2-D source gathers from near-surface seismic reflection surveys conducted over unconsolidated materials in alluvial fans environments. Mapping of fracture and fault systems within the sedimentary sections at hydrocarbon exploration scales has become common practice. This is due to the advent of post-stack attribute analysis of 3-D seismic images worldwide. However, examples of fracture detection and imaging in the near-surface are currently lacking in the literature. In addition, examples of fracture detection and mapping in the pre-stack domain are also lacking. In this study, unprocessed seismic source gathers from very high-resolution reflection surveys over alluvial fan deposits in tectonically active areas appear to display distinct patterns of amplitude drop off, geometrically similar to patterns expected for vertical fracture systems. The patterns can also be extracted by attribute analysis using techniques such as envelope and coherency analyses. Simple standard processing steps such as trace editing, muting, and bandpass filtering enhance interpretability. The patterns appear to be consistent and spatially fixed in the subsurface from source location to source location. These are observed in areas of obvious recent local large-scale fault movement. Examples are given from two areas, eastern Queen Valley in California and eastern Fish Lake Valley in Nevada. The stratigraphic and sedimentation patterns are quite complicated in both areas, and sediment characteristics vary considerably between sites. The surface sediments in the Queen Valley case are, in general, much coarser with many more boulder-sized clasts in the shallow subsurface. The seismic source consisted of a 30-06 rifle fired downhole at a depth of 0.5m. While the boulders interfered with seismic source operations, the record quality was excellent. The alluvial materials, especially those in Fish Lake Valley, are also probably unsaturated due to the desert environment and long-term, historic, upper watershed management by miners and ranchers. The unsaturated nature of the sediments probably contributes to the seismic detectability of the features. Other non-geological explanations for the observed amplitude features are possible, including aliasing effects, display artifacts, etc. However, the data are highly oversampled in both time and space, and the features appear in different types of displays. They are not observable in standard variable area/wiggle trace seismic displays traditionally used for displaying field records. Wider-scale mapping of these features would be an important contribution in studies of off-fault tectonic deformation, alluvial fan development, unsaturated flow, and near-surface hydrological systems in tectonically active areas.