BOREHOLE SURVEYING

摘要

A borehole survey is conducted at a drilling site (S) by a so-called Interpolated In-Field Referencing (IIFR) method in which: (a) absolute local geomagnetic field data is obtained by spot measurement of the earth's magnet ic field at a local measurement site (R) which is sufficiently close to the drilling site (S) that the measurement data is indicative of the earth's magnetic field at the drilling site (S) but which is sufficiently remote fro m the drilling site (S) that the measurement data is unaffected by magnetic interference from the drilling site and other man-made installations; (b) ti me- varying geomagnetic field data is obtained by combining the absolute local geomagnetic field data with data indicative of variation of the geomagnetic field with respect to time obtained by monitoring variation of the earth's magnetic field with respect to time at a remote monitoring site (P1, P2); (c ) downhole magnetic field data is obtained by monitoring by means of a surveyi ng instrument the magnetic field in the vicinity of the borehole at a series of locations along the borehole; and (d) the orientation of the borehole is determined from the downhole magnetic field data and the time-varying geomagnetic field data. Such a survey method takes into account short-term variations in the geomagneticfield caused by electrical currents in the ionosphere and is therefore more accurate than known survey methods.