47 2. Refraction surveying The objective of calculating static corrections is to adjust the times of the various traces to simulate the situation where the shots and receivers are on the same surface plane, below which the velocity is constant along the profile. This surface is called the “reference plane” or the “datum plane”, abbreviated as DP. In calculating static corrections, the assumption is that the emergence angle is very small, i.e. the raypaths in the weathered zone (and in practice above the DP) are vertical. The static correction for a given trace is then only a function of the geographical position of the source and receiver and is independent of the sourcereceiver offset. In some cases this assumption may not be correct: if the emergence angles are not very small, then offset-dependent static corrections must be applied. Various data are used for the computation of static corrections depending on the location of the line and the land acquisition technique, such as: • Up-hole or down-hole surveys (Mari and Vergniault, 2018) to obtain the velocity distribution versus depth (Figure 2.8). An up-hole survey gives the absolute static correction at the borehole location. The borehole must be deep enough to reach below the weathering layer. The distance between up-hole surveys on a profile should be less than the spread length to correctly determine long wavelength statics. In practice, close up-hole surveys (more than 2 up-hole surveys per spread length) are needed to make correct correlations between borehole locations. This method, despite the expense due to the need to drill holes, is the best method to use if all the recommendations are followed. • Weathering shots which are special recordings using a refraction spread designed to determine the thickness and velocity of the weathering layer (Figures 2.1 and 2.4). • Seismic reflection records. Depending on the geometry designed for the acquisition, the picking of first arrival times (direct and refracted arrivals) provide velocity and delay values (Figure 2.10). If the sources are buried charges, the vertical time VT, also called up-hole time, provides the travel time from the bottom of the borehole. The total static correction applied to a trace is the sum of the static correction at the source position and the static correction at the receiver position. Determination of the static correction time tSR requires that the elevation, weathered thickness, plus velocities in and below the weathering are known at every point, as illustrated in Figure 2.7. Below we briefly present the Chronos method, developed by F. Coppens (1985), which provides a set of basic static corrections using constant offset sections and automatic picking. For more details on the Chronos method, the reader should refer to the article by Coppens published in Geophysical prospecting (1985).
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