Abstract: Methods and apparatus for precisely tracking a borehole using electronically controlled DC current switching in a source loop in the vicinity of the borehole together with measurements of the apparent Earth's magnetic field are disclosed. A field averaging method optimally gives the location of points in the borehole in the presence of intense magnetic noise. The expected uncertainty of these locations can be found from a method of generating an ensemble of simulated surveys using a noise characterization parameter found from apparent Earth field measurements. Time synchronization between apparent Earth field measurements and the instants of switching to positive current flow into the loop is found using a correlation method in conjunction with different time durations for positive and negative current flow in the source loop.

Abstract: Measurement of the location and direction of a drill head in a borehole extending beneath an obstacle such as a body of water utilizes a single AC solenoid at a known location but having an unknown orientation. The measurements are used to direct the path of the borehole.

Abstract: An apparatus and method for surveying the path of a borehole includes three single-axis electromagnetic field sensors and three single axis gravity direction sensors in the borehole being surveyed. A pair of closely spaced, parallel guide wires carrying oppositely directed electric current in a reference borehole generate a known electromagnetic field, which is detected by the sensors. Mathematical analysis of electromagnetic and gravity measurements determines the lateral location and the direction of drilling in the neighboring borehole.