TABLE OF CONTENTS Boreholes yield much geological and engineering information, particularly when extensive lengths of core are recovered. Because coring is so expensive, holes are usually drilled with an ordinary rotary bit. Further information is gained about the penetrated formations by measuring their geophysical properties with the aid of wireline logs. Formally geophysical logs were run after drilling a section of the well and before setting casing, so several log runs are necessary during the drilling of a single well. Nowadays, however, it is possible to run some geophysical logs within the drill pipe while the well is actually being drilled. MWD (measurement while drilling) is an important advance in formation evaluation, providing early warnings of rock type, hydrocarbon saturation and potentially perilous zones of high pressure.
The equipment that measures the geophysical properties of the penetrated rocks is housed in a cylindrical sonde, which is lowered down the borehole on a multiconductor electric cable (Fig. 3.16). On reaching the bottom of the hole, the recording circuits are switched on and the various properties measured as the sonde is drawn to the surface. The measurements are transmitted up the cable and recorded on film or magnetic tape in the logging unit. In onshore operations the recorders are generally mounted on a truck (Fig. 3.17).
