| 摘要 |
A borehole guidance system for guiding a well being drilled with respect to an existing reference well includes a multi-sectioned casing in the reference well. A selected section of the casing, usually the lowest section, is electrically conductive and the next adjacent section is electrically nonconductive. A wireline connects a current source on the surface to the selected conductive section to produce a reference current in the well. The reference current is injected into the earth and dissipated so that the current produces a reference magnetic field surrounding the well which is unaffected by return currents.
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