44 Seismic Imaging for acoustic logging was introduced in the 1980s (Lebreton and Morlier, 1983) and then extended to near surface refraction data, as shown in the hydrogeology field case. 2.3 Recommendations for refraction surveying For refraction surveying, the most favorable conditions are: • Layer velocity must increase with depth, • Limited number of markers, • Weak dips, • Mostly homogeneous overburden, • Limited lateral velocity variation. The choice of implementation parameters (minimum offset, distance between traces, and length of receiver spread) is made by recording refraction shots in the field with a large number of traces covering a wide range of offsets. Analysis of the refraction shot T-X curves enables the definition of the implementation parameters to track a refractor and to predict the modifications of the refraction spread as a function of refractor depth. The targeted depth defines the minimum offset, it also conditions the receiver spread length. The refraction method is only usable if the refractor velocity (VR) is clearly discernable from the velocity of the overlying or surrounding layer (VR > VM). For an accurate measurement of the velocity of the first layer, the distance between the geophones can be reduced (by 1 or 2m) close to the shot point. To obtain an accurate T-X curve, the refractor must be tracked over a sufficiently large range of offsets. In practice, with a targeted depth h, the total spread length must be 3 to 5 times the depth h. However, if the refractor is a thin bed, the portion of the T-X curve associated with it will not be detected. The seismic refraction method is then “blind”. Another problem that limits the use of refraction seismic surveying is the presence of velocity inversions. For example, for a four-layer model with velocity distributions of: V1<V2>V3<V4, the V3 layer would not appear on the T-X curve. The result of this phenomenon would be an overestimation of depths for the top of layer V4. In the case of lateral velocity variations, the T-X curve can be very complex and difficult to interpret. For the surface detection of a buried structure, the a/h ratio between the size and the depth of the buried structure must be between 2 and 3. Seismic lines must be laid out in order to facilitate the interpretation of the T-X curves as much as possible. Refraction lines must be rectilinear to avoid errors in the interpretation of the T-X curves. If possible, the receiver spread must be laid out on a plane surface to avoid topographic effects (Figure 2.2).
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