Seismic refraction introduction

Seismic methods involve generating acoustic energy in the form of pressure or shear waves that propagate into the ground. Sensors (geophones) are installed in lines on the surface, and they record the faint ground motions caused by the acoustic energy (seismic waves) that returns from boundaries where the elastic properties of subsurface materials change. At such boundaries there are two fundamental physical effects. "Reflection" means some of the energy impinging on the interface is returned or "bounced" back towards the surface - light reflecting from a semi-transparent mirror is a good analogy. "Refraction" is less intuitive, but the concept is recognizable in images of things under water. When light arrives at a boundary, the portion that passes through is bent so it's direction of propagation changes. In fact, seismic refraction involves detecting energy that has been bent so that it travels along the interface between two subsurface materials, before returning to the surface where it's effects can be measured. The phenomenon is not intuitive, but it is explained in the "Ray paths" section of this chapter.

There are far more professional geophysicists employed in the seismic industry than any other branch of applied geophysics. Most are working at seismic reflection in it's many forms for the oil/gas exploration industries. Some mineral exploration companies have recently made 3D seismic reflection surveying common practice for helping delineate known ore bodies, but it is not universally used. The geotechnical and environmental industries use seismic refraction regularly for efficient assessment of thickness and mechanical properties in the top few 10s of metres. Seismic reflection has more recently become economical for shallow work (from roughly 5 metres depth, down to 100's of metres).

A small refraction survey on mine tailings. There are 12 geophones, a hammer is used on the side
of a metal block (the operator is standing on it) to produce shear waves.
The recording device, it's computer and battery are just left of centre.

In this chapter we will discuss only one form of seismic surveying - general purpose seismic refraction. Seismic refraction 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 metres. 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.