| 摘要 |
The drilling tool (14) is mounted on a rotating vertical shaft (16), a series of tubular sections (18) connected end for end, further sections being added at the well-head (20) as work progresses.Electrically conducting drilling mud fills the tube interiors (24) and the annular space (26) between the shaft and the wall (10) of the borehole. To prevent direct electrical connection between inner and outer mud columns, the tube interiors are treated with e.g. insulating resin or paint.- DETAILED DESCRIPTION - Control signals are transmitted from the well-head to the tool, and operational data returned, by a wave-guide link established between upper (32) and lower (30) coil antennas mounted within the end tubes. Associated electronic circuits are mounted externally (36) at the well head and (34) on the tool itself (14). In a variant, current circulated in the loop formed by end-connecting inner and outer mud columns is modulated to achieve bidirectional signal transmission. Operational commands and returning data are converted to binary digital form as a pulse train, the presence of a pulse indicating 1 and absence 0, the modulated frequency lying in the range 0.1-10 MHz. In the variant, toroidal wound ferrite cores embracing the inner column at top and bottom inject or detect pulses passed round the mud loop; frequency is below 20 kHz
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