Seismic refraction remains the most utilitarian investigative technique used in the GGB, primarily due to its widespread, cost-effective applicability to transportation projects, and in advances in instrumentation and processing techniques that allow .
Seismic refraction utilizes arrays of low-frequency transducers (geophones) in conjunction with timer-recorders (seismographs) to measure the travel of sound waves in the earth. These seismic waves are generated via impact sources, such as sledge hammers, shotgun shells or small explosive charges.
The method is well-suited to delineation of bedrock surfaces and water table. Empirical estimates of ripping ability and earthwork factors may also be obtained from seismic velocities of soil and rock layers.
Many techniques are available to interpret refraction data. Standard seismic refraction data interpretation at Caltrans uses a layered approach, known as the Generalized Reciprocal Method of seismic refraction interpretation (GRM).
The GRM provides a significant improvement over simpler layer-cake interpretation models, with the advantage that refractor depths are calculated across the entire profile, instead of just at the shot points. This provides a more robust model that better represents undulating refractor surfaces and structures with steeper dips. Another advantage over simpler models is that the GRM can accommodate lateral variations in layer velocity, enabling recognition of structural or facies changes within a designated layer.
In the example at right, a profile across a debris flow shows excellent agreement with bedrock depths observed in test holes (labeled B-2 through B-14). Geologic interpretation from boreholes is shown as a dashed line, the refraction interpretation is the series of superimposed arcs (denoting the range of uncertainty in the interpretation). The discrepancy at B-12 and B-3 is due to highly-weathered (saprolitic) rock at that location. The GRM model accurately identifies the better quality rock at depth.
Where geology is complex, the GRM can yield false results. This occurs because the GRM assumes continuity of refractor surfaces across a profile. Where this assumption is not valid, refraction inversion (RI) modeling may provide a better result.
Caltrans uses a linear-optimized, quasi-tomographic inversion algorithm (SeisOpt) to determine velocities of individual 2-D blocks (pixels) within a profile. Surface and borehole arrays can be employed. Since layered strata are not required, more complex geologic problems may be imaged using this method.
In this example, acquired down an earthflow, recent movement can be clearly distinguished from an older event. A high-velocity, embedded graywacke block is evident at the right side of the profile. This feature could not be discerned using layered, refraction interpretation techniques.