METHOD AND APPARATUS FOR LOCATING A BURIED METALLIC LINE EMPLOYING MAGNETIC FIELD GRADIENT MEASUREMENTS

• US 3,617,865 A
• Filed: 05/22/1969
• Issued: 11/02/1971
• Est. Priority Date: 05/25/1968
• Status: Expired due to Term

• Current Assignee: Goroku Hakata
• Original Assignee: Goroku Hakata
• Inventors: Hakata, Goroku
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US04/826,898
• Time in Patent Office: 894 Days
• Field of Search: 324/52,67,3 336/117
• US Class Current: 324/326
• CPC Class Codes: G01V 3/104 using several coupled or uncoupled coils G01V3/101 takes precedence

Abstract

A method and apparatus for locating a buried metallic line such as, for instance, a power line, telephone cable, water pipe, gas pipe, etc. without digging. An AC current is placed through the buried metallic line by direct connection or electromagnetic induction at a frequency of, for example, 100 kc., and the gradient of the magnetic field generated by the AC current is measured on a first and a second level above the ground surface. The lateral position on the ground surface where the metallic line is buried, is determined by the maximum reading of the magnetic field gradient when it is measured along the first level, while the depth of the buried metallic line may be obtained by comparing the magnetic field gradients measured on the first and second levels, respectively, at the detected position. The apparatus essentially comprises an AC signal generator to be connnected or coupled to the buried metallic line, at least one pair of differentially connected parallel coils, and means for measuring the voltage inducted in the coil pair. Since the field gradient is measured rather than the magnetic field, itself, any uniform magnetic field such as the field directly generated by the AC signal generator will not affect the positional information derived from the measurement, and thus the accuracy of the location of the buried metallic line is not adversely affected.

Claims

1. A method for locating a buried metallic line, comprising the steps of:

inductively coupling an AC current to said line for generating a magnetic field about said line;

measuring the analog field gradient amplitude of the magnetic field generated by said AC current on a first level above the ground surface in a plane transverse to said line, including measuring the field gradient in a direction transverse to the line for detecting a position along said first level where a maximum value of said measured magnetic field gradient is obtained for laterally locating said line;

measuring the analog field gradient amplitude of the magnetic field generated by the AC current at said detected position on a second level above the ground surface; and

comparing the measured magnetic field gradient amplitudes on said first and second levels at said detected position with each other to obtain a measure of the depth of the buried metallic Line under the ground surface.

2. The method of claim 1, wherein the steps of measuring the magnetic field gradient on said first and second levels, respectively, include measuring the AC voltage induced in a pair of differentially connected parallel coils spaced at a distance along the direction of the desired gradient and having their axes directed in the direction of the field component whose gradient is to be measured.

3. The method of claim 2, wherein the step of measuring the magnetic field gradient on said second level includes the step of displacing the pair of differentially connected parallel coils from said first level to said second level at said detected position.

4. The method of claim 2, wherein the step of measuring the magnetic field gradient on said second level includes the steps of disconnecting the pair of differentially connected parallel coils disposed on said first level from means for measuring the AC voltage induced across said parallel coil pair, and connecting said AC voltage measurement means to another pair of differentially connected parallel coils disposed on said second level.

5. The method of claim 2, wherein the steps of measuring the magnetic field gradient at the second level and comparing the magnetic field gradients on said first and second levels with each other jointly include differentially connecting the pair of differentially connected parallel coils disposed on said first level with another pair of differentially connected parallel coils disposed on said second level, and varying the mutual positional relation of the component coils of said two pairs of coils so as to obtain a null output the differentially the differentially connected coil pairs.

6. Apparatus for locating a buried metallic line comprising means for inductively coupling an AC current to said line for generating a magnetic field about said line, means for measuring the analog field gradient amplitude of the magnetic field generated by said AC current on a first level about the ground surface in a plane transverse to said line including means for measuring the field gradient in a direction transverse to the line by detecting a position along said first level where a maximum value of said measured magnetic field gradient is obtained for laterally locating said line, means for measuring the analog field gradient amplitude of the magnetic field generated by the AC current at said detected position on a second level above the ground surface, and means for comparing the measured magnetic field gradient amplitudes on said first and second levels at said detected position with each other for obtaining a measure of the depth of the buried metallic line under the ground surface.

7. Apparatus as in claim 6, wherein said first level and said second level analog field gradient amplitude measuring means comprises first and second differentially connected spaced parallel coils.