Downhole seismic exploration device and apparatus

Abstract

A downhole seismic exploration device comprises a seismic detector such as a geophone and a magnetic clamp, supported by O-rings in an open cradle carrier which can be secured to a cable linking several such devices into an array. The array is lowered into a cased borehole and the magnetic clamps are operated to clamp the devices temporarily to the casing. Seismic signals detected by the geophones are transmitted to the surface via the cable, the O-ring supports acting to reduce or eliminate transmission of vibrations from the borehole fluid and the cable via the carrier to the geophone. The array is then unclamped and moved to another location in the borehole and the operation repeated. The magnetic clamp comprises a rotatable permanent magnet with radial poles disposed between a pair of parallel pole piece plates protruding on each side of the device and separated by non-magnetic blocks on eahc side of the magnet. When the magnet poles are aligned parallel to the pole piece plates the magnetic flux extends along closed paths entirely within the pole pieces and there is no clamping action. When the magnet is rotated 90.degree. by a small motor the magnetic flux extends along the pole pieces and out of the device, clamping the device by magnetic attraction to the casing. A piezo bimorph element can be operated to vibrate the device to test the quality of the clamping before the acquisition of seismic data.