Seismic methods refraction

The seismic refraction method Is based on several Important assumptions (1) layer acoustic velocities Increase with depth (2) sufficient velocity contrast exists between layers to discriminate between different strata of interests (3) and layers must be thick enough to permit detection. 

Several different types of sound energy (waves) are propagated through the earth. Seismic refraction methods are concerned primarily with the compresslonal wave energy, commonly called primary wave or P-wave. Primary waves move through... 

Limitations to consider when evaluating the suitability of the seismic refraction method for a given site include the following ... 

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 ... 

The major advantage of the seismic refraction method is that the derived sound velocity of an individual layer correlates very well to the compactness of the sediment or to the Unconfined Compressive Strength rock strength (UCS). However, whereas the UCS value is a rock strength property the seismic sound velocity is more a rock mass characteristic that may strongly be affected by joints, fractures and other discontinuities. These properties in combination with the UCS determine the dredgeability of rock. 

A disadvantage of the seismic refraction method is that during processing of the data one has to assume that the rock layers show an increasing sound velocity with depth. Therefore, a hard caprock layer overlaying a softer rock cannot correctly be detected. 

About 70 % of the world is covered by oceans. Because of the difficulty of accessing the ocean floor, most of the seafloor and the crust below was unexplored for a long time. In the early 1930s, the seismic refraction method was developed and geoscientists tried to develop techniques to use this method offshore. They experimented with cabled sources and geophones but also with free-fall instruments. The first layout of a standalone ocean-bottom seismometer (OBS) was published in 1938 (Ewing and Vine 1938) and tested in the years 1939-1940. This OBS used a gasoline-filled rubber balloon for buoyancy, which floats approx. 3 m above the seafloor. 

The seismic prospectors first utilized the refraction method, and it had spectacular success in locating salt domes along the Gulf Coast during the period 1923 to 1928. Since 1936, the reflection method has dominated seismic prospecting (pp353—54)... 

Reflection and Refraction Methods. See under Seismic Prospecting in this Vol... 

Currently available geophysical methods most applicable in hazardous waste site investigations include metal detectors, magnetometers, ground-penetrating radar (GPR), electromagnetic induction (EM), resistivity, and seismic refraction. These methods should be regarded as complementary, since no one method... 

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. 

The seismic reflection method is the most extensively used of all geophysical techniques, its principal employment being in the oil industry. In this technique, the depth of investigation is large compared with the distance from the shot to detector array. This is to exciude refraction waves. Indeed, the method is able to record information from a large number of horizons down to depths of several thousands of metres. 

The most common method for determining rippability is by seismic refraction. The seismic velocity of the rock mass concerned then can be compared with a chart of ripper performance based on ripping operations in a wide variety of rocks (Fig. 9.3). Kirsten (1988), however, argued that seismic velocity could only provide a provisional indication of the way in which rock masses could be excavated. Previously, Weaver (1975) had proposed the use of a modified form of the geomechanics classification as a rating system for the assessment of rock mass rippability (Table 9.1). 

The reflection seismic method, that would eventually supplant the refraction technique, was patented in 1914 by R. Fessenden in the USA. Use of this method for oil exploration was proposed by J.C. Kar-cher in 1917. In 1921 the first field tests were carried out in the USA, but it took until 1929 before the first successes were achieved (Forbes and O Beime, 1957). Soon afterwards the method was already used in several countries often together with the refraction technique. A good example of an early reflection seismogram that resulted in the discovery of a field is shown in Fig. 8. The Tucupita field in eastern Venezuela was discovered on the basis of seismic surveys carried out in the period 1939-1941 (LeRoy, 1951). 

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