Seismic refraction

In engineering geological and geotechnical investigation, seismic refraction survey is used to determine the overburden and weathered zone thickness, rippability of bedrock, to identify geological anomalies and highly cracked zone, to determine the boundary between rocks of different lithological composition. In addition, it is also used to draw the geological boundary related aquifers, especially to determine the water table, to search for building materials, to contribute to assess liquefaction and landslides of layers near the surface.

Field layout of seismic refraction

Field layout of seismic refraction

In seismic refraction survey, a seismic source in surface generates the elastic waves that travel through the soil or the rock from the source. When the seismic wave reaches the interface having higher velocity, it is refracted and travel along the refracted surface. When the the angle of incidence equals the critical angle at the interface, the refracted wave moves along the interface between two materials, transmitting energy back to the surface. This refracted angle is dependent to layer composition and structure.

When the seismic wave back to the surface, it is discovered by a series of geophones and recorded by a seismograph. Each geophone spread usually consists of 12 to 24 geophones, usually located in a line with at uniform intervals. Geophone placements are usually spaced from about 3 to 15 m depend on the desired resolution and depth of investigation. A refraction survey line may require a source-to-geophone distance of up to three to five times the required depth of investigation. To analyse or process data, we need from 5 to 7 shot-points per single geophone spead: two at the beginning and the of the spread, two forward and reverse shot-point outside the spread and shot-points inside the spread. Inside shot-points are used to determine thickness of layers, so they usually enhance the resolution and correct the depth to bedrock.

There are some choices of seismic source such as sledge hammer, electromagnetic vibrators or explosives. In the case of consolidated overburden having a thickness of less than 10 – 15 m, one can use the sledgehammer. When unconsolidated overburden having greater thickness, one can use explosives or electromagnetic vibrators instead. The interpretation of seismic refraction data may be used the intercept time method (ITM), reciprocal method (RM), generalized reciprocal method (GRM) or tomography method in Plotrefra module of SeisImager 2D software.

Processing Seismic Refraction Data

A result of proceesing seismic refraction data by SeisImager 2D software

Fieldwork, processing and interpretation of seismic refraction survey are carried out in accordance with ASTM 5777 – 00: Standard Guide for Using Seismic Refraction Method for Subsurface Investigation.