Seismic Refraction

	Global Geophysical

Seismic Refraction

Introduction

The seismic refraction method utilizes seismic waves travelling through different parts of the subsurface. A seismic source is used to generate compressional waves, which is measured by a seismograph and a series of evenly spaced sensors (typically 12, 24, 48 or more geophones). Typical sources include a hammer and plate (for imaging depths up to 10's of metres), as well as explosive sources such as dynamite for deeper penetration. Seismic refraction is a quantitative method as it produces depths of various geological layers, as well as the seismic velocities of these various layers. Seismic velocities can assist in the interpretation of geological layers as well as determining the rippability of bedrock.

The geophysical property that is measured in seismic refraction is seismic velocity. In seismic refraction surveys, two kinds of waves are of importance, namely the P-wave (a compressional, longitudinal wave) and the S-wave (a shear, transverse wave). P-waves propagate at the highest velocity of any seismic waves and are therefore commonly used to pick the first breaks of seismic waves that propagated through earth materials. Since travel time equations can be derived as a function of velocity, depth to a refractor such as bedrock can be determined in a seismic refraction survey.

Principle

Head waves involve energy that enters a high velocity medium (refractor) near the critical angle and travels in the high velocity medium nearly parallel to the refractor surface. Since seismic waves move faster in the high velocity medium than the upper, at some point, the wave refracted along that surface will overtake the direct wave. This refracted wave is then the first arrival at all subsequent geophones, at least until it is in turn overtaken by a deeper, faster refraction. The difference in travel time of this wave arrival between geophones depends on the velocity of the lower layer. If that layer is plane and level, the refraction arrivals form a straight line whose slope corresponds directly to that velocity. The point at which the refraction overtakes the direct arrival is known as the "critical distance", and can be used to estimate the depth to the refracting surface.

The results of a refraction survey are based on the times of arrival of the initial ground movement generated by a source recorded at a variety of distances. Later arrivals are discarded. These are then interpreted in terms of the depths to subsurface interfaces and the speeds at which motion travels through the subsurface within each layer. These speeds are controlled by a set of physical constants, called elastic parameters that describe the material.

Applications

Mapping bedrock topography
Determination of seismic velocities to determine rock rippability

Limitations

Both penetration and resolution of the refraction method is inferior to those of seismic reflection if both techniques use the same seismic source and work well in a given situation. Penetration with a hammer is usually up to 30 metres while greater penetration is achieved with explosives or weight drop. Lateral resolution provided by one spread is governed by the geophone spacing. Vertical resolution of a stratum requires that it have a thickness that is a substantial fraction of the depth to surface.
There is one significant limitation to the refraction method namely the so-called hidden layer problem. The seismic refraction method requires that seismic velocity increases with depth. It is difficult to resolve a thin, low velocity sand/gravel bed beneath a high velocity clay layer where a velocity inversion is thus present.
Another type of hidden layer is produced by media whose velocity greatly increases with a small change in depth. Unlike the previous example, head waves are produced at both interfaces just as described previously. Because the layer is thin and the velocity of the underlying medium is larger, it is overtaken by the rapidly travelling head wave coming from the bottom boundary before it can overtake the direct arrival. In both of these cases, notice that the existence of the hidden layer can not be determined from the travel-time observations.