Apparatus and method employing extraneous field compensation for locating current-carrying objects

• US 4,639,674 A
• Filed: 04/11/1983
• Issued: 01/27/1987
• Est. Priority Date: 04/11/1983
• Status: Expired due to Fees

• Current Assignee: Schonstedt Instrument Company
• Original Assignee: Schonstedt Instrument Company
• Inventors: Rippingale, John B.
• Primary Examiner(s): Karlsen, Ernest F.
• Assistant Examiner(s): Snow, Walter E.
• Application Number:
  US06/483,613
• Time in Patent Office: 1,387 Days
• Field of Search: 324/66, 324/67, 324/326, 324/327, 324/328, 324/329, 324/244
• US Class Current: 324/326
• CPC Class Codes: G01R 33/025 Compensating stray fields G01R33/0017 takes precedence
• CPC Class Codes: G01V 3/081 the magnetic field is produced by the objects or geological structures characterised by the method of magnetic field measurement G01R33/00

Abstract

In locating current-carrying objects, such as buried pipes in the presence of extraneous magnetic fields by detecting the magnetic field produced by the current, first and second spaced primary sensors measure the magnetic field at first and second locations, and a compensating sensor measures the extraneous field at a location midway between the first and second locations. The signal from the compensating sensor is combined with the signals from both of the primary sensors so as to compensate for the extraneous magnetic field at the primary sensors. The invention compensates for ambient magnetic fields associated with propagating electromagnetic energy that induces current in an object, and compensates for extraneous magnetic fields produced by current flow in an adjacent object.

Claims

1. Apparatus for locating an obscured elongated object carrying current induced therein by electromagnetic energy propagated from a remote source, the current producing a circumferential magnetic field, comprising first and second sensor means arranged to measure at first and second locations, respectively, a component of said magnetic field and for producing corresponding first and second signals, respectively, portions of said signals being due to measuring by said first and second sensor means of a magnetic field component of said propagated electromagnetic energy, third sensor means for measuring at a third location between the first and second locations said magnetic field component of said propagated electromagnetic energy and for producing a corresponding third signal, said first, second, and third sensor means being positioned at the vertices of an isosceles triangle, with the first and second sensor means located at the base of the triangle, the sensor means having a fixed positional relationship with respect to one another and being movable as a group relative to said object and relative to said remote source, means for combining the signals from the sensor means so as to produce a first resultant signal representative of subtraction of the magnetic field component measured at the third location from the magnetic field components measured at the first location and so as to produce a second resultant signal representative of subtraction of the magnetic field component measured at the third location from the magnetic field components measured at the second location, means for processing the resultant signals, and means responsive to the processing means for indicating the location of the object.

2. The apparatus of claim 1, wherein the sensor means are arranged to measure vertical magnetic field components.

3. The apparatus of claim 1, wherein each of the sensor means comprises a coil wound upon a magnetic core oriented to have a vertical magnetic axis.

4. The apparatus of claim 3, wherein the coil of the third sensor means has one-half the number of turns of the coil of each of the first and second sensor means so as to supply a current output from the third sensor means that is twice the current output from each of the first and second sensor means.

5. The apparatus of claim 1, wherein the processing means comprises means for comparing the phases of the first and second resultant signals.

6. The apparatus of claim 5, wherein the processing means supplies to the indicating means a signal corresponding to the phase difference between the first and second resultant signals.

7. The apparatus of claim 6, wherein the processing means includes means for shifting the phase of the first resultant signal with respect to the second resultant signal prior to said comparing.

8. Apparatus for locating an obscured elongated object carrying current excited therein by a source of electromagnetic energy coupled to the elongated object, the current in said object producing a circumferential magnetic field that parasitically induces a current in a nearby object, whereby an interfering magnetic field is produced, said apparatus comprising first, second, and third sensor means arranged to measure at first, second, and third locations, respectively, a component of said magnetic fields and for producing corresponding first, second, and third signals, respectively, said third sensor means being located midway of a line connecting said first and second locations, said sensor means being movable as a group relative to said objects and relative to said source, means for combining the signals from the sensor means so as to produce a first resultant signal representative of subtraction of the magnetic field components measured at the third location from the magnetic field components measured at the first location and so as to produce a second resultant signal representative of subtraction of the magnetic field components measured at the third location from the magnetic field components measured at the second location, and so that twice the magnetic field components measured at the third location is effectively subtracted from the sum of the magnetic field components measured at the first and second locations, means for processing the resultant signals, and means responsive to the processing means for indicating the location of the object.

9. The apparatus of claim 8, wherein the processing means comprises means for detecting amplitude values of the first and second resultant signals and producing the sum of the amplitude values, and wherein the indicating means is responsive to said sum.

10. A method of locating an obscured elongated object, comprising inducing in the object a current from electromagnetic energy propagated from a remote source so as to produce a circumferential magnetic field, providing first, second, and third magnetic field sensors positioned at the vertices of an isosceles triangle with the first and second sensors located at the base of the triangle, moving said sensors as a group relative to the object and relative to the source, employing said first and second sensors to measure a magnetic field component of said circumferential magnetic field and to produce corresponding first and second signals, respectively, portions of said signals being due to measuring by said first and second sensors of a magnetic field component of said propagated electromagnetic energy, employing said third sensor to measure said magnetic field component of said propagated electromagnetic energy and to produce a corresponding third signal, combining said signals so as to produce a first resultant signal representative of subtraction of the magnetic field component measured by the third sensor from the magnetic field components measured by the first sensor and so as to produce a second resultant signal representative of subtraction of the magnetic field component measured by the third sensor from the magnetic field components measured by the second sensor, processing the resultant signals, and indicating in response to said processing the location of the object.

11. The method of claim 10, wherein said processing comprises measuring a phase difference between said first and second resultant signals, and providing an output signal corresponding to said phase difference.

12. A method of locating an obscured elongated object, comprising coupling a source of electromagnetic energy to the object and exciting a current therein which produces a circumferential magnetic field, providing first, second, and third magnetic field sensors at respective locations with the third sensor location midway of a line connecting the first and second sensor locations, moving the sensors as a group relative to said object and relative to said source, employing the sensors to measure magnetic field components at said locations and to produce corresponding signals, combining the signals so as to produce a first resultant signal representative of subtraction of the measured magnetic field components at the third location from the measured magnetic field components at the first location and so as to produce a second resultant signal representative of subtraction of the measured magnetic field components at the third location from the mcasured magnetic field components at the second location, processing the resultant signals, and indicating in response to the processing the location of the object.

13. The method of claim 12, wherein the processing comprises detecting amplitude values of the first and second resultant signals and producing the sum of the amplitude values, and wherein the indicating is responsive to said sum.

14. A method of locating an obscured elongated object, comprising inducing in the object a current from electromagnetic energy propagated from a remote source so as to produce a circumferential magnetic field, providing first, second, and third magnetic field sensors, positioning the first and second sensors so as to measure both a component of said circumferential magnetic field and a component of a magnetic field of said electromagnetic energy at first and second locations, respectively, positioning the third sensor relative to the first and second sensors so as to measure said magnetic field component of said propagated electromagnetic energy at a third location and so that the third sensor is relatively insensitive to the circumferential magnetic field, moving said sensors as a group relative to the object and relative to the source, producing respective first, second, and third signals from said sensors as the result of the measurements at the respective locations, combining said signals so that the signal produced by said third sensor compensates for the portions of the signals produced by said first and second sensors corresponding to the magnetic field component of said propagated electromagnetic energy measured by said first and second sensors, and processing the combined signals so as to indicate the location of the object.

15. A method of locating an obscured elongated object in the presence of a nearby further object, comprising coupling a source of electromagnetic energy to the elongated object and exciting a current therein which produces a circumferential magnetic field that induces a current in the further object parasitically and thereby produces an interfering magnetic field, providing first, second, and third magnetic field sensors at respective locations, moving the sensors as a group in said fields relative to said objects and relative to said source, employing the sensors to measure components of said magnetic fields at said locations and to produce corresponding signals representative of the measured magnetic field components, combining said signals so that the signal from said third sensor compensates for the portions of the signals from said first and second sensors that correspond to the interfering magnetic field measured at said first and second locations, and processing the combined signals so as to indicate the location of the elongated object.

16. A method in accordance with claim 15, wherein said third location is midway of a line connecting said first and second locations, and wherein said signals are combined so as to subtract twice the signal from the third sensor from the sum of the signals from the first and second sensors.