Standard Guide for Use of the Metal Detection Method for Subsurface Site Investigations

Importancia y uso:

5.1 Concepts: 

5.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for using the metal detection method for locating subsurface metallic objects. Personnel requirements are as discussed in Practice D3740.

5.1.2 Method—Metal detectors are electromagnetic instruments that work on the principle of induction, using typically two coils (antennas); a transmitter and a receiver. Both coils are fixed in respect to each other and are used near the surface of the earth. Either an alternating or a pulsed current is applied in the transmitter coil creating an alternating or impulsive magnetic field, which induces electrical eddy currents in the earth and in any conductive object (that is, metal) which may be present. The electrical currents flowing in the earth are proportional to electrical conductivity of the medium. These eddy currents, both in the earth and in any metal object, in turn create a magnetic field which induces a voltage in the receiver coil. That voltage is the signal which is amplified and monitored, sampled and/or recorded (Fig. 1). Eddy currents in metallic objects tend to be out of phase with the currents in the earth (for frequency domain) and decay more slowly (for time-domain or pulsed systems).

Note 1: Data are shown for two metal detector coil sizes.

5.4.3 Interferences Caused by Natural and Cultural Conditions: 

5.4.3.1 Sources of noise referred here do not include those of a physical nature such as difficult terrain or man-made obstructions but rather those of a geologic, ambient, or cultural nature that can adversely affect the measurements and hence the interpretation.

5.4.3.2 Natural Sources of Noise—Some kinds of soil/rock, particularly those containing high iron content (often known as mineralized soil) affect receiver coil output strongly enough to indicate the presence of a metal target with certain kinds of metal detectors. Some types of metal detectors provide a means for compensating the output for the ground effect, albeit that these means affect sensitivity for small targets. They usually require the operator to position the detector near the ground (but not near a metal target) and adjust a control until the target signal disappears. Small variations in the soil characteristics and stones (particularly those containing metallic compounds) can cause small changes in the detector output. Often these changes cause small target-like signals, known as “ground noise.” These can confuse the operator because they sound like small targets.

5.4.3.3 Cultural Sources of Noise—Cultural sources of noise can include interference from electrical power lines, communications lines and equipment, underground utilities, nearby buildings, and metal fences.

5.4.3.4 Surveys should not be made in close proximity to buildings, metal fences or buried metal pipe lines that can be detected by the metal detection method, unless detection of the buried pipe line, for example, is the object of the survey. It is sometimes difficult to predict the appropriate distance from the potential sources of noise. Measurements made on-site can quickly yield the magnitude of the problem, and adjustments can then be made.

5.4.3.5 Precaution must also be taken to remove metal from the operator, or to reduce potential effects. Steel-toe boots, respirators, and air bottles can all cause considerable problems with noise.

5.5 Summary—During the course of designing and carrying out a metal detection survey, the sources of ambient, geologic and cultural noise must be considered and the time of occurrence and location noted. The exact form of the interference is not always predictable, as it not only depends upon the type of noise and the magnitude of the noise but also upon the distance from the source of noise and possibly the time of day.

5.6 Alternate Methods—In some cases, the factors discussed above may prevent the effective use of the metal detection method, and other surface geophysical methods (see Guide D6429) such as electromagnetics (Guides D6639 and D6820) or ground penetrating radar (Guide D6432) or non-geophysical methods may be advised to investigate subsurface conditions.

Subcomité:

D18.01

Referida por:

D0420-18, D0420-18, D6429-23

Volúmen:

04.09

Palabras clave:

frequency domain; geophysics; metal detection; metal detector; subsurface investigation; surface geophysics; time domain;