Seismic Refraction Profiling

OUTLINE

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between sub-surface layers of different velocities. Seismic energy is provided by a source (‘shot’) located on the surface. Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.

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DETAIL

For shallow applications the technique normally involves a hammer and plate, weight drop or small explosive charge (blank shotgun cartridge). Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface using a linear array (or spread) of geophones spaced at regular intervals. Beyond a certain distance from the shot point, known as the cross-over distance, the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival). Observation of the travel-times of the direct and refracted signals provides information on the depth profile of the refractor.

Shots are deployed at and beyond either end of the geophone spread in order to acquire refracted energy as first-arrivals at each geophone position.

Data are recorded on a seismograph and later downloaded to computer for analysis of the first-arrival times to the geophones from each shot position. Travel-time versus distance graphs are then constructed and velocities calculated for the overburden and refractor layers through analysis of the direct arrival and T-minus graph gradients. Depth profiles for each refractor are produced by an analytical procedure based on consideration of shot and receiver geometry and the measured travel-times and calculated velocities. The final output comprises a depth profile of the refractor layers and a velocity model of the sub-surface.

The primary applications of seismic refraction are for determining depth to bedrock and bedrock structure. Due to the dependence of seismic velocity on the elasticity and density of the material through which the energy passes, seismic refraction surveys provide a measure of material strengths and can consequently be used as an aid in assessing rippability and rock quality. The technique has been successfully applied to mapping depth to base of backfilled quarries, depth of landfills, thickness of overburden and groundwater delineation.

RESULTS

During data acquisition individual shot records are displayed as variable area wiggle traces displaying travel time against distance. These enable an initial calculation of overburden and refractor apparent velocities and provide an important check on the quality of the data. Following acquisition wiggle traces are used to display the data during picking of the first-arrivals for each geophone position and shot.

The processed data is normally presented as a series of three plots; time-distance graphs for the picked first-arrivals on each shot, a true depth profile for the identified refractors and a velocity profile for the overburden and refractors. Any existing ground truth information such as borehole and trial pit logs, is overlain on the depth profile in order to help calibrate the seismic results and then provide an indication of the level of correlation along the survey line. The refractor depth is displayed as a series of overlapping arcs that represent the solutions for each geophone in the array. The refractor can lie anywhere on the arcs below the intersections with adjacent arcs.