Figure 1. The VSP tool shown above consists of 12 individual sondes linked by an electronic cable and terminated with a logging cable head. The distance between each sonde can be 10, 15 or 20 m. Each sonde contains three orthogonal geophones (two horizontal and one vertical) and a single hydrophone. A hydraulically powered locking arm (shown retracted) ensures that the geophone package is secured against the borehole wall. (Photograph furnished by CGG.)