Downhole Seismic Survey

image010OUTLINE

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).

DETAIL

Downhole seismic surveys are the simplest and cheapest method in the suite of borehole seismic techniques requiring 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.

P-wave energy is normally provided by a hammer and plate or weight drop similar to shallow seismic reflection and refraction profiling surveys. Polarised S-waves are generated using a shear wave hammer. This comprises two hammers connected to either end of a plank that is held to the ground using a vehicle or heavy weight. Collecting both positive and negative polarised (so called A and B) S-waves using the two hammers separately, enables the S-wave arrivals on the receiver shot records to be distinguished from those of P-waves and coherent noise.