We have designed, fabricated, and field-tested a new, unique, monostatic, broadband, electromagnetic sensor for subsurface geophysical investigation. The sensing unit consists of a pair of concentric, circular coils that transmit a continuous, broadband, digitally-controlled, electromagnetic waveform. The two transmitter coils, with precisely computed dimensions and placement, create a zone of magnetic cavity (viz., an area with a vanishing primary magnetic flux) at the center of the two coils. A third receiving coil is placed within this magnetic cavity so that it senses only the weak, secondary field returned from the earth and buried targets.This monostatic configuration has many advantages including (1) compact sensor head, (2) a large transmitter moment, (3) high spatial resolution, (4) no spatial distortion of an anomaly common to bistatic sensors, (5) circular symmetry that greatly simplifies mathematical description, and, therefore, (6) simplified forward and inverse modeling processes. Three prototype GEM-3 units have been built and tested at various environmental sites, including those containing unexploded ordnance and land mines.
An estimated 110 million landmines, mostly antipersonnel mines laid in over 60 countries, kill or maim over 26 000 people a year. One of the dilemmas for removing landmines is the amount of false alarms in a typical minefield. Broadband electromagnetic induction spectroscopy (EMIS) is a promising technology that can both detect and identify buried objects as landmines. By reducing the number of false alarms, this approach significantly reduces costs associated with landmine removal. Combining the EMIS technology and a broadband EMI sensor, the scientific phenomenology that has potential applications for identifying landmines, unexploded ordnance, and hidden weapons at security checkpoints can now be explored.
We examined amplitude and frequency changes in shallow seismic‐reflection data associated with simple source‐parameter modifications for the sledgehammer. Seismic data acquired at three sites with different near‐surface geology show the potential effects of varying the hammer mass, the hammer velocity, the plate mass, and the plate area. At these study sites, seismic amplitudes depend on plate‐surface area and on hammer mass but not heavily on hammer velocity or plate mass. Furthermore, although the total bandwidth of the recorded data was independent of source parameter changes, the peak frequency at one site was increased approximately 40 Hz by increasing the area of the plate. The results indicate that the effects of modifying the source parameters for the sledgehammer are site‐dependent. The experiments described are quick, cheap, and simple, and can be duplicated by others at prospective sites to answer site‐specific questions.
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