Method and apparatus to compensate a gradiometer having first and second unwanted terms

• US 4,386,318 A
• Filed: 09/26/1980
• Issued: 05/31/1983
• Est. Priority Date: 09/26/1980
• Status: Expired due to Term

• Current Assignee: Her Majesty The Queen In Right of Canada As Represented By The Minister of National Defence
• Original Assignee: Her Majesty The Queen In Right of Canada As Represented By The Minister of National Defence
• Inventors: Vrba, Jiri, Lomnes, Randy K., Fife, Alistair A., Burbank, Max B.
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): Snow, Walter E.
• Application Number:
  US06/191,819
• Time in Patent Office: 977 Days
• Field of Search: 324/225, 324/244, 324/245, 324/246, 324/247, 324/248, 324/249
• US Class Current: 324/244
• CPC Class Codes: G01R 33/022 Measuring gradient

Abstract

A device and method producing a compensated gradiometer signal from an uncompensated gradiometer signal having at least one unwanted term which varies as a function of magnetic field. The device is comprised of a sensing unit for sensing the magnetic field. A generator is provided for producing a first vector signal which is representative of the magnetic field. A function generator is provided for producing a second vector signal which represents a component of one of the at least one unwanted term. A mixer is connected to the generator and the function generator for producing a vector output signal which is equal to the one of the at least one unwanted term. Finally, a subtractor unit is provided which is connected to the mixing means for subtracting the output vector signal from the uncompensated gradiometer signal to thereby produce the compensated gradiometer signal.

Claims

1. A device for producing a compensated gradiometer signal from an uncompensated gradiometer signal having first and second unwanted terms respectively due to (1) imperfections in the construction of the gradiometer and fixed objects which induce magnetization, and (2) eddy currents due to metallic objects located in relatively close proximity to the gradiometer, said first term having a first component B which is a function of the field magnitude and a second component C, and said second term having a first component B which is a function of the derivative of the field and a second component E, comprising:

means for sensing the magnetic field and producing a magnitude signal indicating the magnitude thereof;

first function generating means for producing a first function signal representing said second component C of said first term;

means for mixing said magnitude signal and first function signal to produce a first term signal indicating said first term;

means for differentiating said magnitude signal to produce a differentiated signal;

second function generating means for producing a second function signal representing said second component E of said second term;

means for mixing said differentiated signal and said second function signal to produce a second term signal indicating said second term;

means for adding said first and second term signals to produce a compensating signal; and

means for subtracting said compensating signal from said uncompensated signal to produce said compensated gradiometer signal.

2. A method of producing a compensated gradiometer signal from an uncompensated gradiometer signal having first and second unwanted terms respectively due to (1) imperfections in the construction of the gradiometer and fixed objects which induce magnetization, and (2) eddy currents due to metallic objects located in relatively close proximity to the gradiometer, said first term having a first component B which is a function of the field magnitude and a second component C, and said second term having a first component B which is a function of the derivative of the field and a second component E, comprising the steps of:

placing a gradiometer device in a uniform magnetic field of a first intensity;

placing said gradiometer device in a uniform magnetic field of a second different intensity;

determining the vector magnitude of said first component of said first term which varies directly as a function of magnetic field intensity;

generating a first vector signal equal to said vector magnitude of said first component of said first term;

varying a uniform magnetic field between two bounds to provide a known rate of change in said uniform magnetic field;

determining the vector magnitude of said first component of said second term which varies directly as a function of the rate of change of the magnitude of the magnetic field intensity with respect to time;

generating a second vector signal equal to said vector magnitude of one of said second term which varies directly as a function of the rate of change of magnetic field intensity;

adding said first and second vector signals to produce a compensating signal; and

subtracting said compensating signal from said uncompensated gradiometer signal to thereby produce said compensated gradiometer signal.