83 2. Surface geophysical methods A seismic spread is composed of a source spread and a receiver spread. In 2D seismic survey, the sources and the receivers are located on the same line which defines a 2D seismic profile. In 3D seismic survey, the sources and the receivers are usually located on 2 orthogonal lines: a line of sources and the line of receivers. A seismic record is a set of seismic traces recorded at different receiver positions. The seismic trace represents the vibrations of the ground due to wave propagation generated by a seismic source. On a field record, the geophysicist can identify the different seismic waves (Figure 2.21). Figure 2.21 shows examples of 2D and 3D records. Figure 2.21 Examples of 2D (a) and 3D (b) seismic records. After Mari and Mendes (2019). In addition to body waves (P- and S-waves) which propagate within the subsurface, a surface seismic source generates surface waves (Love and Rayleigh waves). These waves are used in civil engineering to determine the mechanical parameters (shear velocity and shear modulus) of the first tens of meter below the ground surface. The seismic method based on the analysis of surface waves is called MASW (Multiple Analysis of Surface Waves). For more complete information on wave propagation, the reader is invited to consult other works such as Achenbach (1973), Lavergne (1986), Dobrin and Savit (1988), Quiblier (1997), Mari and Mendes (2019). Today, seismic acquisitions are done with recording systems which can simultaneously record seismic vibrations on several hundreds of sensors. The distance between two adjacent sensors must be chosen small enough to have correctly sampled data and avoid any phenomenon of spatial aliasing. Consequently, the different types of waves are correctly recorded, and the same seismic record can be used whatever the seismic method. (a) (b)
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