Seismic Survey Types¶

The two main types of seismic surveys are refraction seismics and reflection seismics. Both use the same sources and receivers, but refraction seismics aims at creating a map of subsurface velocities, whereas the goal of reflection seismics is to create an image of subsurface interfaces, similar to a GPR profile (but to much greater depths).

Refraction surveys

Seismic refraction is most useful when the velocity of seismic signals in each layer increases with depth. Therefore, where higher velocity (e.g. clay) layers may overlie lower velocity (e.g. sand or gravel) layers, seismic refraction may yield incorrect results. In addition, seismic refraction requires geophone arrays with lengths of approximately 4 to 5 times the depth to the layer of interest (for example the top of bedrock). Therefore seismic refraction is commonly limited to mapping layers to depths less than 30-50 meters. Greater depths are possible, but the required array lengths may exceed site dimensions, and the shot energy required to transmit seismic arrivals for the required distances may necessitate the use of large explosive charges.

Reflection surveys

By contrast, reflection surveys are not hampered by low velocity layers but they have difficulty imaging the top 50m of the earth because reflections from such shallow depth are difficult to distinguish from direct arrivals and sound waves travelling through the air. In general, identifying a reflection event in a seismic record is more difficult than picking first arrivals for a refraction survey. Much signal processing is typically involved. In order to improve the ability to detect and image a given reflection event geophysicists typically design reflection surveys to detect a reflection from a particular point in the subsurface multiple times.