55 2. Well seismic surveying anchor arms that open and close. In geotechnics, a single hydrophone or hydrophone array (12 levels, for example) can also be used. • If the anchoring system allows the anchor arms to be opened and closed, it is recommended to make seismic measurements at several depths while descending and to make measurements at the same depths when ascending, to control depth tying. • VSP acquisition is carried out from the bottom of the well towards the surface for reasons of data acquisition convenience. Figure 2.4 shows some well probe types: a 3-component geophone, a single hydrophone, and a hydrophone array. The figure also shows an example of a VSP obtained with a hydrophone. a b c d Figure 2.4 Examples of well probes (a: 3C geophone, b: hydrophone, c: hydrophone array, d: example of VSP recorded with a hydrophone).
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