90 Geophysics in Geothermal Exploration land acquisition) or an array of sensors. If the receiver is a single sensor, the interval between 2 receivers is of several meters, if it is an array, the interval is of several tens of meters. The maximum source-receiver offset to the far receiver is about the same as the maximum depth of the geological objective. The offset of the near receiver is chosen to minimize interference between ground roll (surface waves) and the reflection arrivals. Acquisition is more complex for land 3D. Source and receiver lines are laid out to provide the most homogeneous coverage. The more conventional implementation is the cross-spread design with lines of sources perpendicular to lines of receivers (Figure 2.26b). In 2D or 3D, the number of times a reflecting point in the sub surface is reached by different raypaths associated with different source-receiver pairs provide the fold of seismic coverage. Such a gathering, called Common Midpoint point (CMP), is theoretically valid for flat and horizontal geological models. In 2D, the distance between two CMP is equal to half the receiver interval. In 3D, the CMP is replaced by a cell or bin, the size of it being the product of half the source interval by half the receiver interval (Figure 2.26b). Traces contributing to the same CMP bin have irregularly distributed azimuths and offsets. Implementation is optimized to ensure the most regular azimuth and offset distribution possible. In the case of complex geological structures, the CMP is replaced by a common image gather. (a) (b) Figure 2.26 Seismic spread; (a) in 2D, (b) 3D (lines of sources are indicated by green triangle, lines of receivers are indicated by red points). After Mari and Mendes (2019). The reader will find more information about acquisition and survey design in Galbraith (2000), Lansley (2000), Mayne (1962), Meunier and Gillot (2000), Meunier (2011), Monk and Yates (2000), Musser (2000), Vermeer and Hornman (2000), Chaouch and Mari (2006), about signal processing in Mari (2011), and about seismic processing in Yilmaz (1987), Robein (2003). The classical approach to seismic processing can be summarized in two main steps.
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