Seismic reflection time

Allmendinger RW, Nelson KD, Potter CJ, Barazangi M, Brown LD, Oliver JE (1987) Deep seismic reflection characteristics of the continental crust. Geology 15 304-310 An Z, Kutzbach JE, Prell WL, Porter SC (2001) Evolution of Asian monsoons and phased uplift of the Himalaya-Tibet Plateau since Late Miocene times. Nature 411 (6833) 62-66 Avouac J-P, Burov EB (1996) Erosion as a driving mechanism of intracontinental mountain growth. J Geophys Res 101(B8) 17,747-17,769... 

Seismic reflection profile An image of the structure below the seafloor along a ship trackline that is created by using sound pulses and recording the time of return from various reflecting layers. 

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, also called sub bottom profiling, involves the recording of the travel time interval of seismic (acoustic) waves, which are emitted from the surface and reflected by the seabed and underlying soil layers. A seismic source generates a seismic (acoustic) signal with a relative high frequency. 

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 ... 

P. Steeghs, R. Baraniuk and J.E. Odegard (2003) Time-frequency analysis of seismic reflection data. Applications in Time-Frequency Signal Processing,... 

Of course the typical seismic trace has many hundreds of reflections in it, all the way down from the surface to the deepest times measured. These days, engineers and geologists prefer to see the seismic in terms of the acoustic impedance rather than reflection data and this can be obtained by inversion from the seismic volume. Aseismic volume is made up of hundreds of thousands of traces. 

Figure 14.10 illustrates the method of seismic prospecting on land by what is known as reflection shooting. A hole usually 10 to 12 cm in diameter is drilled to a depth of 15 to 30 m. The charge of explosive is likely to be 5 to 12-5 kg and the stemming used is usually water. As the explosive must fire under a depth of water which may exceed 45 m, special varieties of gelatines are employed (see p. 53). Alternatively, a powder explosive can be sealed into pressure-resistant metal containers. Special detonators are also employed, not only to withstand the possible head of water, but also to have a specially short bursting time (see p. 113). 

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. 

FIGURE 5 Seismic refiection profile near the crest of the Blake Ridge off South Carolina in the southeastern United States. Note the strong reflection marked BSR that defines the base of the gas hydrate stability zone. The vertical axis is in two-way travel time of sound, which varies with sound velocity in the medium thus, this axis is a variable scale with respect to distance. 

SRP sections were then revised (Revision 2) with the following principal areas of change Section 2.5.2, updated to reflect the current NRC staff review practice Section 3.7.1, design time history criteria Section 3.7.2, development of floor response criteria, damping values, SSI uncertainties, and combination of modal responses and Section 3.7.3, seismic analysis of above ground tanks, and Category 1 buried piping. 

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