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(a) Random station locations (b) Profile lines (c) Grid stations

Profile 1 Profile 2

(a) Random station locations (b) Profile lines (c) Grid stations

Profile 1 Profile 2

(d) Station distribution in grids, profile lines and randomly in an area with mixed topography (image modified from htt://ssed.gsfc.nasa.gov/tharsis/shademap.html)

FIGURE 6.9 Station arrangements: (a) random distribution of stations, (b) stations along profile lines, (c) a grid of stations, and (d) mixed station arrangements in a complex topographic setting.

(d) Station distribution in grids, profile lines and randomly in an area with mixed topography (image modified from htt://ssed.gsfc.nasa.gov/tharsis/shademap.html)

FIGURE 6.9 Station arrangements: (a) random distribution of stations, (b) stations along profile lines, (c) a grid of stations, and (d) mixed station arrangements in a complex topographic setting.

The fundamental questions that must be asked prior to any survey are as follows: (1) What is the maximum depth of penetration?, (2) What are the line and trace spacing (horizontal resolution)?, and (3) What is the resolution needed to achieve the goals of a survey?

6.4.1 Mode of Sensor Transport

EM system sensors and recording equipment may be hand carried, backpacked, pulled along in a handcart, or mounted on a vehicle (aircraft, all-terrain-vehicle, or some other type of motorized vehicle). In any case, registering the measurement locations for future reference is one of the most difficult issues related to any type of field sensor measurements. This may be accomplished by establishing measurement locations (stations) over a grid or along profile lines, as shown in Figure 6.9, or by recording a Global Positioning System (GPS) measurement at each station. If the topography is rugged, or if the landscape is cluttered with trees, roads, fences, or other cultural features, then it may be necessary to locate stations in pattern that appears to be more-or-less randomly dispersed, as shown in Figure 6.9a. GPS measurements are usually employed to record the location of dispersed stations. Figure 6.9d illustrates an environment where grids of stations and profile lines are used in the flat terrain, and randomly dispersed station locations are employed in the rugged terrain. Measurements along profile lines are often utilized for locating linear features, such as buried pipes, in the subsurface.

6.4.2 Station Spacing

A good rule of thumb for establishing an adequate trace measurement spacing is that the spacing should be less than one quarter of the size of the smallest object that is to be detected by the survey.

This value is twice the Nyquist sampling frequency and should be adequate for most situations. Technically, "oversampling" is never a real problem, but measuring more stations than necessary costs extra money and should be avoided for economic reasons. Economics and other practical considerations (e.g., spatial survey accuracy) are the only limitations on using very small distances between stations.

6.4.3 Depth of Penetration

Surveys should be designed so that the instrument depth of penetration is twice that required to see the deepest object anticipated in the survey area. The depth of penetration varies with the separation of transmit and receive coils (Geonics Limited [Date Unknown]—Technical Note TN-31). However, from a practical point of view, the only way to know the depth of penetration for a particular survey site is to make an initial guess based on estimations from the skin depth calculations (Equation (6.2)). The skin depth can be calculated using the following relationship:

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