Locating technique and apparatus using an approximated dipole signal

A method for location determination, comprising:

• configuring a transmitter to include an elongated generally planar loop antenna defining an elongation axis;
• positioning the elongation axis of said antenna along at least a portion of a path such that different points along said elongation axis are pitched by different degrees;
• and
• distributing a plurality of pitch sensors along said elongation axis to establish the pitch at said different points.

A method for steering a drill head along a desired path in relation to a reference borehole having a centerline, said method comprising:

• configuring a transmitter to include an elongated planar loop antenna defining an elongation axis and having a length therealong and further including at least one sensor package for sensing one or more orientation parameters of the planar loop antenna including roll of the planar loop antenna about the elongation axis and thereby the roll of the transmitter;
• positioning the transmitter in the reference borehole such that the elongation axis of the planar loop antenna is generally aligned along at least a section of the centerline of the reference borehole;
• generating a magnetic field from the planar loop antenna using the transmitter, said magnetic field having flux components;
• configuring a receiver to include (i) a pair of spaced-apart sensors, each of which sensors includes at least one flux sensing device, aligned to define a receiving axis for detecting the magnetic field, (ii) at least one pitch sensor supported for detecting pitch of the receiving axis, and (iii) at least one roll sensor supported for detecting roll of the receiving axis;
• positioning the receiver in a second borehole proximate to a drill head for movement therewith, which drill head is rotatable through a number of roll positions and may be pitched in a range of pitch angles such that the receiving axis is generally aligned with at least a section of a centerline of the second borehole;
• measuring the flux components of the magnetic field using the receiver disposed in the second borehole;
• determining a roll position of the drill head using the receiver roll sensor, a pitch angle of the drill head using the pitch sensor, and a roll position of the transmitter using a transmitter roll sensor in said sensor package;
• establishing a yaw angle difference between the centerline of the reference borehole and the centerline of the second borehole;
• projecting flux components received by the receiver onto a global coordinate axis system having one axis generally aligned with the centerline of the reference borehole;
• using the projected flux components to determine a vertical offset and a horizontal offset between the receiver and the transmitter in a plane generally normal to the centerline of the reference borehole;
• and
• using the roll position of the drill head, the pitch angle of the drill head, the orientation parameters of the transmitter, the yaw angle difference between the reference borehole and the second borehole, and the vertical and horizontal offsets between the receiver and the transmitter in the plane normal to the centerline of the reference borehole, steering the drill head along the desired path.

A method for calibration of a transmitter for transmitting from a reference well having a well casing, said method comprising:

• configuring said transmitter to include an elongated generally planar loop antenna defining an elongation axis such that the planar loop antenna is characterized by a number of windings that carry a current and having a current loop area for producing an approximated dipole signal responsive to said current;
• determining a loss coefficient that is associated with using the transmitter in said well casing, which well casing attenuates said approximated dipole signal;
• with said planar loop antenna transmitting the approximated dipole signal from within the well casing responsive to said current, measuring said current flowing in said windings;
• and
• based on the loss coefficient, the number of windings and the measured current flow, determining a dipole strength of said approximated dipole signal, independent of any measured signal strength of the approximated dipole signal.

A method for calibration of a transmitter for transmitting from a first well, said method comprising:

• configuring said transmitter to include an elongated generally planar loop antenna defining an elongation axis such that the planar loop antenna is characterized by a number of windings that carry a current and having a current loop area for producing an approximated dipole signal responsive to said current;
• positioning the transmitter in said first well at a given depth;
• with said planar loop antenna transmitting the approximated dipole signal from within the first well responsive to said current and based on the given depth, measuring a total flux intensity of the approximated dipole signal at a known distance from the planar loop antenna;
• and
• based on the known distance and the total flux intensity, determining a dipole strength of the planar loop antenna.