An example of a 10 x 10 m grid (100 m2) showing 1 m traverse spacing (10 traverses) with 1 m separation between samples (10 samples/traverse) for a 1 x 1 m sampling density.

The area survey is designed to sample a region by acquiring measurements at a uniform spacing along X- and Y-axes in a regular cartesian grid. The line traveled with an instrument is commonly referred to as a traverse (or line), which is walked either in a parallel or zig-zag pattern. Traverses are separated by a uniform distance varying between 0.25 m to several meters, depending on the desired sampling density. Along each traverse geophysical measurements, or samples, are obtained at regular intervals dictated by the nature of the instrument and the desired sampling density. Depending on survey needs and instrumentation, data might be sampled along a traverse every 0.5 to 1 m (resistivity, EM), 0.1 to 0.5 m (magnetometry), or every few centimeters (.01-.25 m, with GPR). High precision surveys employ traverse separations of 0.5 m or even 0.25 m.

Note that measurements are taken in the center of each sampling unit (e.g., a 1 x 1 m square), which facilitates handling of the data arrays when multiple grids are tiled together. With each measurement representing an area each can be viewed as an individual pixel when the data are imaged as a computer graphic.

The choice of a sampling density is a compromise between resolution and the speed of the survey. If one decides a resolution of 0.5 m is necessary as opposed to 1.0 m, then four times as many measurements are required, for example. Although more measurements per square meter improves resolution and the ability to "see" small anomalies it also takes more time. Some instruments (e.g., GPR, many magnetometers) can be moved along a traverse at the same speed independent of the number of samples taken per meter. Laying out more traverses, however, requires more to be walked and therefore more time with any instrument. A partial solution to the speed-of-survey issue is to undertake unequal sampling on X- and Y-axes, for example, separating traverses (X-axis) by 1.0 m and samples (Y-axis) by 0.5 m.

It is emphasized that GPR area surveys always sample measurements at unequal densities along X- and Y-axes. Scan rates are so high that samples (Y-axis) are obtained every few cm while it is not possible to separate traverses (X-axis) at a similar spacing.

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