| 摘要 |
An apparatus and method for orienting directional tools within a bore hole b y recognizing and compensating for field biases brought about by ferromagnetic anamolies surrounding the bore hole, ferromagnetic casing strings and electrical/electronic tool components . The present invention includes the steps of measuring the magnetic field within a bore hole before casing strings are put in place and again measuring the magnetic field after such casings are in place. The ferromagnetic formation anamolies are detected in the first step of measuring the field prior to casing placement and further bias characteristics are determined in the second magnetic field measurement step after the casings are placed. Alternately, a conventional gyroscopic survey can be carried out to establish a bias in an already cased bore hole. Once a magnetic field bias has been established for the bore hole (for a particular casing string) this fie ld bias is utilized to calculate and correct an azimuthal reading measured by electronic tools duri ng placement within a particular casing. For a given location Y within the bore hole the field biases previously determined and resulting from formational anamolies and adjacent casings are factored into an azimuthal calculation in order to provide an accurate azimuth for tool orientation.
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