The figure illustrates an apparatus for inducing an electric current in an electrically conductive object or layer of material and measuring the magnetic field generated by the current (that is, the induced magnetic field) to obtain information about the object or layer. The apparatus can be used, for example, to detect metal objects buried underground.
Like older apparatuses developed for the same purpose, this apparatus includes (1) a primary electromagnet coil excited by alternating current to generate an excitatory alternating magnetic field in the object or layer of interest and (2) a sensing electromagnet coil to measure the induced magnetic field. Heretofore, it has been common practice to mount the primary coil as far a possible from the sensing coil to prevent the excitatory magnetic field from overwhelming the relatively weak induced magnetic field at the sensing coil; unavoidably, this results in a large apparatus. The present apparatus is more compact because the primary and sensing coils are mounted close to each other; indeed, they are mounted concentrically. Despite the proximity of the coils to each other, the excitatory magnetic field does not overwhelm the weak induced magnetic field at the sensing coil; on the contrary, as explained below, the coils are designed so that the net excitatory magnetic flux through the sensing coil is zero.
More precisely, there are two primary coils: a larger (outer) and a smaller (inner) one. The sensing coil is the smallest coil and lies within the inner primary coil. All three coils are mounted in a disk of polycarbonate or some other suitable electrically nonconductive material. The outer and inner primary coils are connected electrically in series, at opposite polarity, to a low-impedance ac power source. Thus, when power is applied, the two primary coils generate alternating magnetic fields of opposite polarity.
The magnetic fields generated by both primary coils at typical measurement distances (much greater than the diameter of the outer primary coil) are given by the well-known dipole approximation. In a typical design, the magnetic dipole moment of the outer primary coil is much greater than that of the inner primary coil, so that at typical measurement distances, the net excitatory magnetic field is dominated by the portion generated by the outer primary coil. However, near the center, where the sensing coil lies, the opposing magnetic fields from the two primary coils can be made approximately equal in magnitude, so that the net excitatory magnetic field can be made zero or nearly so within the sensing coil - in effect, placing the sensing coil in a magnetic cavity. By suitable choice of the numbers of turns and the radii of the inner and outer primary coils and the radius of the sensing coil according to established equations of electromagnetism, one can achieve a balance between the opposing excitatory fluxes intercepted by the sensing coil, so that the net excitatory flux through the sensing coil is zero.
In operation, a replica of the ac excitation is digitized and sent to a microcomputer. The electromagnetic force generated in the sensing coil by the induced magnetic field is amplified, then also digitized and sent to the microcomputer. In the microcomputer, the amplitude and phase of the signal from the sensing coil are compared with those of the ac excitation. The relative-amplitude and relative-phase information is displayed on an output interface. This information can be interpreted in terms of properties of the object or material to which the excitatory magnetic field has been applied.
This work was done by I. J. Won of Geophex Ltd. for Stennis Space Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
I. J. Won, PhD
605 Mercury Street
Raleigh, NC 27603-2343
Refer to SSC-00074