83 3. Seismic tomography For the purposes of this study, four cased 7-inch diameter boreholes were available (Figure 3.16) and a prototype weight drop downhole source was tested, designed at the Institut Français du Pétrole (IFP). A diagram showing the principle and mechanism of this downhole weight-drop source, which generates P and S-waves, is shown in Figure 3.17. Figure 3.17 Principle and mechanism of the downhole weight-drop source developed at the Institut Français du Pétrole (IFP). Adapted from Beydoun et al. (1989). The seismic source was deployed with drill strings in borehole 162 and wall-clamped at depth 455 m, the firing position, with a packer (Brown 7 inches, type MI) which locked and unlocked to the hole through the tool’s rotation. The loading of the source (lifting of the mass) is carried out with the drill strings, the weight is then dropped automatically, hitting an anvil bound to the packer. The seismic data were recorded simultaneously in borehole 164 with a vertical hydrophone streamer and in borehole 161 with a three-component geophone tool. The basic acquisition tools were as follows: – Source in borehole 162: • weight-drop, generating mainly S-waves perpendicular to the borehole; • 3,000 joules/shot; • 2 shots/minute capability; • depth = 455 m; – Receivers in borehole 164: • vertical hydrophone streamer not anchored, thus highly sensitive to tube waves; • band range 10-5,000 Hz;
RkJQdWJsaXNoZXIy NjA3NzQ=