Seismic refracted waves, time

Figure 2. Time vs. distance plot of "first arrivals" for seismic refracted waves at corresponding geophones.

Seismic refraction techniques can measure the density, thickness, and depth of geologic layers using sound (acoustic) waves transmitted Into the subsurface. These sound waves travel at different velocities In various soils and rock and are also refracted (or bent) at the Interface between layers, thereby affecting their path of travel. The time required for the wave to complete this path Is measured, permitting determination of the number of layers at the site as well as the sound velocity and depth of each layer. The wave velocity In each layer Is related to layer properties such as density and hardness. 

Travel times of refracted waves are identified on the seismic record as "first arrivals" at each geophone, and they are dependent upon the depth to which the refracted wave traveled, and the seismic velocity of the wave (3). 

By contrast, the refraction acoustical technique involves the recording of refracted sound waves from the seabed and subbottom. Compared with the reflection technique, the refraction technique requires stronger energy sources and takes more time. In addition, the source and detectors must be spaced further apart. However, the refraction method provides deeper subbottom penetration. It is not commonly used in offshore engineering work. A typical arrangement for a seismic refraction survey that shows the required energy source and receiver close to or in contact with the seafloor is shown in Figure 3.7. 

When seismic waves pass from one layer to another in the ground, some energy is reflected back towards the surface while the remainder is refracted. Thus, two methods of seismic surveying can be distinguished, that is, seismic reflection and seismic refraction. Measurement of the time taken from the generation of the shock waves until they are recorded by detector arrays forms the basis of the two methods. 

While for the seismic reflection method, the reflected signal of an acoustic wave is studied, the seismic refraction method interprets the arrival time of the refracted wave. By putting several hydrophones in a known configuration on the seabed, it is possible to calculate the velocity of an acoustic wave front through different layers of the subsoil. Sediment has sound velocity values, which typically range from ... 

Fig. 2 Propagation and refraction of seismic waves, and time-distance diagram...

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