Apparatus for measuring magnetic flux density resulting from galvanic current flow in subsurface casing using a plurality of flux gates

Claim 1

Apparatus for locating corrosion on ferromagnetic casing vertically traversing subsurface earth formations by measuring the magnetic flux density resulting from the longitudinal flow of d.c. galvanic current within said casing; comprising: an elongated body member adapted to traverse said casing; a plurality of flux gates positioned within said body member and aligned with the axis of sensitivity of each of said flux gates at a right angle to said longitudinal galvanic current flow for detecting the magnetic flux density resulting from said galvanic current flow, each of said flux gates comprising a drive coil and a sense coil; a plurality of sample and hold circuits, each of said sample and hold circuits connected to a sense coil for providing a plurality of d.c. signals functionally related to the detected magnetic flux density; an oscillator circuit switchably connected to said drive coils and connected to said sample and hold circuits for providing a drive signal to said drive coils and a synchronization signal to said sample and hold circuits; a plurality of filter circuits, each filter circuit connected to a sample and hold circuit; and a summing amplifier connected to said plurality of filter circuits providing an output voltage signal functionally related to the composite of the d.c. signals, said composite voltage signal representing the magnetic flux density resulting from said galvanic current flow; and means for transmitting said composite voltage signal to a surface location.