Electronic Instrumentation used in Borehole Surveying

A directional survey of a borehole is based upon a series of measurements of an azimuth angle and an inclination angle made at successive stations along the borehole where the distance between the stations is accurately known. The measurements are made using a survey instrument containing two servo accelerometers, acting as inclinometers, a compass and the requisite instrumentation. The directional survey can be supplemented with temperature, diameter, pressure and other data. The electronic data acquisition package (DAP) designed and built in the University of Nebraska-Lincoln Physics Department Electronic Shop was first successively used to log the Dye 3 Greenland borehole on June 7, 1986.

Two different survey tools are described, one using an Aanderaa Instruments compass and the other, three fluxgates arranged to measure magnetic field strength in mutually orthogonal directions, thus acting as a compass. In addition, two methods of powering the instrument, by battery and from a surface power supply, are described. The DAP uses a microprocessor which performs the data acquisition and control functions. The DAP data stream can include input from hole diameter, borehole fluid pressure, temperature and other functions in addition to that from the inclination and azimuth sensors. Caliper release is an example of a control function. Examples of the data sampling techniques, filtering and averaging, calibration of the electronics and sensors, and error analysis are given in detail. A modem circuit is used to transmit data to the surface, thus requiring only a coaxial cable for use in the logging. A BASIC program and Spreadsheet program used with a portable PC computer for storing the logged data are discussed with numerical examples of the calculations required in borehole logging position determination.