Survey Design

Spatial Sampling Density

Sampling density refers to the line and station spacing used for a particular survey. The optimum line and station spacing depends significantly on the dimensions of the target, its depth and the system being used to perform the survey. As expected, smaller ordnance items generate more compact TEM anomalies; especially if they are buried at shallow depths. It is important to use a sampling density which accurately characterizes TEM anomalies without adding unnecessary time and expense.

UXO surveys for target picking are generally performed along a set of parallel lines. For EM61 instruments, the optimum line and station spacings are bounded by the width of the transmitter coil. Thus if the transmitter coil has a width of 1 m, the optimum line and station spacings should both be less than 1 m. For common EM61 surveys, a line spacing of 0.5-1 m is used. And depending on walking speed, station spacings range from 10-20 cm. Similar line and station spacings may be applied to other TEM instruments.

After the initial survey has been performed, cued interrogation data are frequently collected over a set of picked targets. In this case, the sampling density may be increased in order to more accurately characterize the anomaly and improve classification.

Excitation Orientation of the Target

The excitation of a buried target occurs parallel to the inducing field. Thus in order to accurately recover polarizations \(L_{x'},L_{y'}\) and \(L_{z'}\) for the target, we must excite the target significantly from multiple angles. Ideally, the target would be excited from 3 orthogonal directions; thus assuring the data contains significant contributions from each polarization.

For systems with multiple orthogonal transmitter loops (Metal Mapper), this aspect of survey design is not an issue. For single loop transmitters however (EM61), perpendicular survey lines have historically been added to excite targets from multiple sides. This is done especially when collecting cued interrogation data. With the advances of modern systems with multiple orthogonal transmitter loops, cued interrogation data can be collected from a stationary position.