Skin depth compensation in underground boring applications

In an overall process in which a boring tool is moved through the ground within a region along an intended path while transmitting a locating signal, said region including an electrical conductivity characteristic and said locating signal exhibiting a field defined forward point which field defined forward point is vertically above an inground forward point on said intended path through which said boring tool is likely to pass with the boring tool initially located at a particular point on the intended path, said system including an above ground arrangement for tracking the position of and/or guiding the boring tool as it moves through the ground using a locating signal that is transmitted from the boring tool to said arrangement such that the electrical conductivity of said region results in a skin depth which modifies penetration of the locating signal into said region and, consequently, which produces skin depth error when magnetic readings of the locating signal are used to determine the location of the boring tool relative to said arrangement under an assumption that said region is electrically non-conductive, a method of determining the depth of said boring tool at said particular point, said method comprising the steps of:

• a) measuring the skin depth in said region in a predetermined way;
• b) identifying said field defined forward point on or above the surface of the ground and identifying an overhead point on or above the surface of the ground and directly above the boring tool at said particular point;
• c) measuring a forward distance defined at the surface of the ground between the overhead point and the forward point;
• and
• d) using the forward distance, said skin depth and certain characteristics of said locating signal at said forward point, determining the depth of said boring tool at said particular point.

In an overall process in which a boring tool is moved through the ground within a region along an intended path while transmitting a locating signal, said region including an electrical conductivity characteristic and said locating signal exhibiting a field defined forward point which field defined forward point is vertically above an inground forward point on said intended path through which said boring tool is likely to pass with the boring tool initially located at a particular point on the intended path, said system including an above ground arrangement for tracking the position of and/or guiding the boring tool as it moves through the ground using a locating signal that is transmitted from the boring tool to said arrangement such that the electrical conductivity of said region results in a skin depth which modifies penetration of the locating signal into said region and, consequently, which produces skin depth error when magnetic readings of the locating signal are used to determine the location of the boring tool relative to said arrangement under an assumption that said region is electrically non-conductive, a method of determining the depth of the boring tool from any forward point along the intended path of the boring tool, said method comprising the steps of:

• a) configuring the intended path in said region such that said forward point is at a higher elevation on the surface of the ground than said particular point;
• b) establishing the actual depth of the boring tool at said particular point;
• c) measuring a vertical elevation difference between the particular point and the forward point;
• d) sensing the locating signal at said forward point while the boring tool is at said particular point to determine an uncorrected depth of the boring tool which is subject to skin depth error;
• e) using the measured vertical elevation difference, the actual depth of the boring tool at said particular point and the uncorrected depth of the boring tool measured from the forward point, determining a forward point skin depth correction factor;
• f) after having advanced the boring tool to a subsequent particular point on said intended path associated with a subsequent forward point, determining a corrected depth of the boring tool
• i) measuring the locating signal at said subsequent forward point using said arrangement to produce electromagnetic data,
• ii) determining the uncorrected depth of the boring tool at said subsequent particular point using the electromagnetic data such that the uncorrected depth at the subsequent particular point is subject to skin depth error, and
• iii) multiplying the uncorrected depth at the subsequent particular point by said forward skin depth correction factor to determine depth of the boring tool at the subsequent particular point corrected for skin depth.

A drilling apparatus for performing underground boring in a region having an electrical conductivity which results in a skin depth that modifies penetration of electromagnetic signals into said region, said apparatus comprising:

• a) a boring tool which is configured for moving through the ground including means for emitting a locating signal using at least two frequencies each of which is subject to said skin depth;
• b) an above ground arrangement configured for receiving said locating signal at said frequencies as the boring tool moves underground to determine an intensity measurement at each of said frequencies where each intensity measurement varies according to the frequency at which that intensity measurement is made as a result of skin depth influence;
• and
• c) a processing arrangement forming part of the above ground arrangement for using the magnetic intensity measurement at each of the received frequencies to establish a corrected position of the boring tool in a way which compensates for said skin depth based, at least in part, on variation of the intensity measurement among said frequencies.