Seismic Techniques
Downhole Seismic Surveys
Outline
Downhole seismic surveys are the simplest and cheapest method in the suite of borehole seismic techniques, as they require only a single borehole. Seismic energy is generated on surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves are measured at regular intervals down the hole using a string of hydrophones or, in the case of S-wave surveys, a single clamped triaxial geophone that is gradually moved down the hole. The P- and S-wave arrival times for each receiver location are combined to produce travel-time versus depth curves for the complete hole. These are then used to produce total velocity profiles from which interval velocities and the various elastic moduli can be calculated (in conjunction with density data from geophysical logging of the borehole).
