Seismic refraction survey

Figure 1. Cross sectional view of seismic refraction survey.

Figure 16. Geologic map of proposed field-application site summarizing bedrock depths determined from seismic refraction surveys and test boring data.

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. 

Stern TA (1984) A seismic refraction survey near the Dailey Islands, southwestern McMurdo Sound, Antarctica. New Zealand Dept Sclent Indust Res, Geophys Div, Rept 198 1-32 Stonehouse B (ed) (2002) Encyclopedia of Antarctica and the southern oceans. WUey, Chichester, UK Stott LD, McKelvey BC, Harwood DM, Webb P-N (1983) A revision of the ages of Cenozoic erratics at Mount Discovery and Minna Bluff, McMurdo Sound. Antarctic J US 18(5) 36-38 Stroeven AP (1997) The Sirius Group of Antarctica Age and environment. In Ricci CA (ed) The Antarctic region ... 

Where a seismic reflection survey is rather easy to perform seismic source and receiver are both towed behind the survey vessel (Figure B.4) the realization of a seismic refraction survey is a challenging operation with a much more complex set-up with several hydrophones and a multitude of data to be interpreted (Figure B.6). 

In some cases (for example to assess the sound velocity profile along an offshore pipe or cable route) a so-called static seismic refraction survey is performed. A group of hydrophones (a streamer) is installed static on the seabed by divers. A daily progress of 125 to 300 m is the upper limit for such a static seismic refraction survey. 

Fig. 19.2 The stratigraphy of the sediment and underlying granitic basement along hne A-A in Fig. 19.1 is based on seismic refraction and reflection surveys by Wong and Chiistoffel (1981), McGinnis et al. (1983), and Davy and Alder (1989) in addition to the core logs of ClROS-1 and MSSTS-1. The uppermost...

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. 

This information can be used to identify and delimit the fill deposits that may be covered, intersected and/or underlain by unsuitable materials. The number of boreholes, vibrocores and/or Cone Penetration Tests can be reduced by carrying out a geophysical survey. Seismic reflection or refraction surveys and geo-electrical surveys are the most common techniques. Reference is made to Chapter 3. 

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). 

Figure B.7 Typical plan view with results of a (Gambas) refraction seismic survey, the darker patches indicate rock outcrops. Seismic data from Banyan Basin - Singapore.

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