80 Geophysics in Geothermal Exploration 2.2.4 Seismic methods Seismic prospecting consists of generating very low-amplitude artificial earthquakes at predetermined times and positions. The seismic disturbances generated by a seismic source are recorded by a seismic receiver spread. The acquisition geometry is defined by the distribution of the source spread and the receiver spread. The following elements are needed to observe the propagation of seismic, acoustic, or elastic waves: 1. A source spread. The source is a device capable of producing a deformation in a medium. In land acquisition, it can be an explosive charge (dynamite), a weight dropper or a vibrator. In marine acquisition, it can be an air gun, a sparker, or a vibrator. Seismic energy radiated by the source is split between body waves (compressional P and shear S waves) and surface waves. A P-wave has a particle motion parallel to the direction of propagation. A S-wave has a particle motion perpendicular to the direction of propagation. P and S waves propagate at VP and VS velocities respectively. When a P- or S- wave strikes an interface at some angle of incidence not equal to zero, four waves are generated: two transmitted (one P- and one S-wave) and two reflected (again one P- and one S-wave).The angular relationships between the propagation directions of each of these waves are given by Snell’s law (Figure 2.19a). When P- or S-waves strike the interface at the critical angle ic, head waves or refracted waves are generated. This only occurs when a wave perturbation passes from a medium with velocity Vi to another with velocity Vi+1 which is greater than Vi and at the critical angle given by sin(ic) = Vi/Vi+1. The critical angle ic is the criterion for differentiating the various seismic methods associated to body wave propagation (Figure 2.19b): • i < ic: the method is seismic reflection, • i = ic: the method is seismic refraction, • i > ic: the method is wide angle reflection. In wide angle reflection there is no transmitted energy, only reflected. 2. A physical medium defined by its geometric and mechanical characteristics. Here we consider the geological formations defined by the following mechanical properties: • propagation velocity of the compressional P-waves in the rock: VP (expressed in m/s), • propagation velocity of the shear S-waves in the rock: VS (expressed in m/s), • density ρ (expressed in g/cm3 or kg/m3), • quality factor Q which characterizes the ability of the rock to absorb seismic energy: a higher value indicates lower absorption of seismic energy. Sedimentary rocks have Q ranging from about 10 to several hundred. 3. An elastic deformation of the medium after the initial shaking caused by the source. A deformation is considered elastic when the medium returns to its
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