Seismic Techniques
Downhole Seismic Surveys
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.
