Wall-Lock Borehole Geophones
2Pages

Wall-Lock Borehole Geophones Download a free copy of our tutorial paper “Borehole Shear-Wave Surveys for Engineering Site Investigations” at http://www.geostuff.com ■ 3-component tri-axial sensors ■ Motor-driven clamp mechanism ■ Fits in 2-inch (51-mm) boreholes ■ Automatic orientation of horizontal geophones to any azimuth (Model BHG-3) ■ Cable disconnects for convenient surface handling and extending depth ■ Works in wet or dry holes Geostuff’s BHG series, 3-component borehole geophones are designed for shallow seismic velocity measurements. Both units include a motor-driven clamp to hold the sensor in position in the borehole. These geophones are applicable to a wide variety of shallow surveys, including shothole logging, downhole shear wave measurements, static corrections for petroleum shear-wave reflection surveys, cross-hole, tomographic, seam wave, and shallow VSP surveys for coal, minerals, and rock mechanics. Model BHG-3 includes a fluxgate compass and servo mechanism which automatically orients the horizontal geophones to any magnetic azimuth selected by the operator. Thus, the longitudinal sensor can be aligned with the polarization of the shear wave source. While downhole shear wave surveys have tradi tionally been done with random orientation, being able to precisely align one of the horizontal geophones with the plank or energy source provides signifi cant advantages. Anisotrophy appears to be much more common than originally thought, and the velocities of horizontally polarized shear waves vary with azimuth. With an orientable geophone, these velocity variations may be measured or simply avoided. By maintaining orientation of the source-receiver combination all the way down the borehole, the user can maintain better control and recognition of shear wave arrivals. against the borehole wall. The tool may be used in soft-wall, uncased holes as well as cased holes. In the unlikely event of a failure to release, the tool may be dragged up the hole against the spring friction. The motor-driven spring is faster, more reliable, and less cumbersome than the common alternative using an inflated bladder. The clamping mechanism is a steel leaf spring, compressed by a motor-driven piston. When compressed, the spring expands, forcing the geophone

Control Electronics: The BHGC-1b controller directs the voltages to control the clamping mechanism and servo mechanism. A meter monitors motor current to indicate the clamping action and force. This unit is usable with either model. A rechargeable, internal. 24-volt battery is supplied, along with a 110/220 volt charger. r Removable connector: The cable is connected to the geophone by a high-pressure, underwater connector. The ability to disconnect the sensor makes it easier to handle and use. A male-female extension cable can be used to temporarily extend the length without permanently Big...