Modern frequency-domain electromagnetic induction (EMI) sensors for detection of unexploded ordnance (UXO) use wideband transmitter circuitry design. The inductor is driven using a sinusoidal signal from a time-varying voltage source. This driving voltage produces sinusoidal current in the transmitter coil at the desired frequency or frequencies. Producing low frequency sinusoidal transmit waveforms requires that the power amplifiers internally dissipate a large amount of heat energy because of the inefficiency of generating a sinusoidal waveform. This effectively precludes the operation of commercial systems at frequencies below 30 hertz (Hz).
The objective of this SERDP Exploratory Development (SEED) project was to design a frequency-domain EMI UXO sensor using a direct current (DC)-driven transmitter coil. The transmitter coil mechanically rotates (using slip rings) about one of its axes. As the coil rotates, a distant object sees a sinusoidal time-varying magnetic inducing field at the rotation frequency. A pair of identically shaped receiver coils spaced equidistant above and below the transmit coil is used as a gradient detector.
The system performance parameters were modeled using a sophisticated electromagnetic software modeling utility and verified by making measurements with a single receiver. Once all parameters were verified and the second receiver coil was installed, two similar test objects (one hollow and one solid) were used to evaluate the system performance. These secondary magnetic fields induced in the test objects were measured and processed by the digital signal processor (DSP) lock-in amplifier and presented as Inphase and Quadrature results. These Inphase and Quadrature results from the electromagnetic system were recorded as a function of frequency between 0.5 and 12 Hz. A GEM 3 instrument was then used to collect data from 30 Hz up to 10 KHz and the two systems’ data were joined producing one continuous response function from 1 Hz up to 10 KHz.
The benefit of this approach is predicated upon the importance of new information to be acquired in the extremely low frequency (ELF) region pertaining to a metallic object's shape, composition, and wall thickness. This information may be critical in differentiating intact UXO from ordnance and explosives (OE) scrap. Substantially reducing the number of false alarm excavations will provide substantial savings in UXO clearances. (SEED Project Completed - 2007)