Seismic Reflection and Refraction Methods

In order to establish the outer limit of the continental shelf, as defined by article 76 of the Convention (UNDOALOS, 1993), it is necessary for the coastal State to determine the foot of the slope and to know the thickness of the sediments beneath the ocean floor. Geophysical surveys, using seismic techniques, have been extensively used for mapping of subsurface geological structures. In seismic surveys, seismic waves are generated by near-surface artificial explosions at a series of sites; the resulting waves are then recorded digitally and as an analogue record. The regional geological structure and sediment thickness can then be deduced from analysis of the travel times of identifiable wave groups. This chapter briefly outlines the various seismic survey methods with special emphasis on seismic reflection and refraction surveys. It also discusses the most commonly used techniques for determining the subsurface structure, including determination of the velocities of sediments using seismic waves. Seismic reflection surveys have been extensively used for mapping structures in sedimentary sequences, especially as part of exploration programs for oil and gas. Two seismic reflection methods are widely used: singlechannel and multichannel seismic profiling systems. Although the former typically used an analogue recording system with a single receiver, digital recording is now commonly employed. The single-channel method is often employed during shallow reconnaissance exploration or in offshore engineering surveys because it is relatively cheap. But this advantage of the single-channel system is countered by the fact that the maximum depth of penetration of the single-channel system is rather shallow, and it usually does not give information on the deep geological structure or on the seismic velocity of the sedimentary layers. The multichannel method is characterized by digital recording and multiple receivers in a long multichannel streamer cable. Most marine seismic reflection profiling has now shifted from analogue recording of singlechannel data to digital recording of multichannel data, largely because digital recording and processing of large amounts of data improve the signal-to-noise ratio and provide high-quality seismic records. A data acquisition system for reflection profiling consists of three basic subsystems: the energy source, the receiving unit, and the digital recording system.