Refraction Surveys

Deployment of towed system pulled along the seafloor. 

Hydrophones are simply receivers of sound energy and pick up noise and multiple reflections of bottom features that bounce back and forth between the mirror effect of the air-sea interface and a bottom reflector. By proper filtering, systems have been developed to minimize the effect of multiple reflections. 

---

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 ... 

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. 

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. 

The techniques just surveyed necessarily determine average refractive indices. And k values for atmospheric aerosols measured by these techniques suggest mixtures of vastly different kinds of particles. For we have noted that no... 

In the recent review on column LC by Majors et al. (21), a survey on the use of detector types was carried out in the same manner as that for the use of the various separation modes already mentioned. The results, shown in Table VI, were tabulated for the periods 1982-83 and 1980-81. The increased use of electrochemical and refractive index detectors is significant in these data. The authors speculated that the increased use of refractive index detectors resulted from the increased number of publications on the separation of carbohydrates. The increased use of electrochemical detection is probably a function of many different factors cell designs that are easier to use, expanding sales... 

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). 

The term controlled index means that the refractive index can be made smaller than that of the bulk precursor by controlling the microstructure via the porosity. When silica is deposited, for example, the film index n can be varied over a wide range (7) from n = 1.1 to 1.5. This process control makes sol-gel coatings interesting for many optical, electronic, and sensor applications, but the evolution of the microstructure during film formation is not well understood, in spite of efforts to survey the variables (8, 9). This chapter reviews the important factors determining the microstructure of dip-coated films and explores at length two of them, evaporation and surface tension. 

---