metal detectors etc

7/29/2019 Metal Detectors etc

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METAL DETECTORS

A wide variety of "metal detectors" are commercially available today; they have theadvantage of being easy to use and most cost only a few hundred dollars. The larger thesearch coil, the deeper the penetration; however coins and small metal objects can be

detected only a few inches deep and very large metal objects only to depths of a few feet.Non-metal objects are not detected. Some areas are too "noisy" for metal detectors."Noise" can originate from power lines, or from obscuring signals caused by nearbyparked cars, scattered nails, re-bar or metallic litter at the site. Highly mineralized areasare difficult to work in, and certain rocks such as iron-rich basalt can be troublesome formetal detector work.

Metal detectors are "active" instruments. A battery-powered transmitter in the unitradiates a relatively low-frequency alternating current signal into the ground by means ofa transmitting coil. If the signal from the transmitter encounters any type of conductingmetal or mineral in the ground an induced current flows in the subsurface target. This

induced current then re-radiates a weak signal back to the surface. The latter signal is out-of-phase with the transmitted signal and thus is easily detected by a receiving coil.Modern metal detectors have circuitry for carefully balancing out any direct signalleakage between transmitter and receiver coils and for discriminating between large andsmall, shallow or deep, and ferrous or non-ferrous metals.

The simpler instruments of this type are useful for "coin shooting" at old ghost town sites,or archaeological sites (on land or under the sea), and for locating gold or silver depositswithin a quartz vein in a lode mine. Small objects such as coins usually must lie within afew inches to a foot of the surface to be detected by metal detectors.

The sensitivity of metal detectors is a steep function of the coil diameter, however withlarge coils and ample transmitter power larger metal objects can be located to depths of10 or 15 feet using metal detectors. Claims for detection at greater depths as well asidentification of metals by type are suspect.

RESISTIVITY METHOD

The resistivity method of subsurface exploration is powerful but often tedious to employunless an automated instrument is available. The method is simple: Current is introducedinto the ground through one pair of electrodes. Current flow between these electrodesfans out through the ground in a pattern and intensity that depends on the conductivity ofthe ground and any stratification or obstacles that lie in the vicinity of the electrodes. Asecond pair of electrodes is then used to quantitatively measure the voltage pattern on thesurface resulting from the current flow pattern of the first set of electrodes. A number ofdifferent electrode configurations are used in practice, but in simplest form the operatortakes measurements along a straight line ("traverse"), moving his electrodes in pairs. Hethen repeats the measurements along a parallel line until the area of interest has beencovered with a rectangular grid of electrode positions. If multiple electrodes are used and


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the results recorded automatically at the push of a button, the area to be examined can besearched more efficiently, and also probed at various depths at the same time. (As a ruleof thumb, the depth of maximum sensitivity for resistivity sounding is about 1.5 times theelectrode spacing in typical arrays). A crew of two can easily study an area of perhaps1000 square meters in a day. Typical electrode spacings might be 0. 3 to 1.0 meters for

shallow targets.

Once the resistivity data has been collected, a simple computer program quicklygenerates a three-dimensional map of ground electrical resistivity or conductivity.Targets most easily seen on resistivity surveys are cavities or voids, but buried walls andfilled trenches can often be mapped. The target depth divided by the diameter of thetarget should be less than 3 or 4 for best sensitivity, though some experts claim to be ableto detect targets with a depth to diameter ratio of 9 or more. Boulders, geologicalstratifications and water-table depth can also be successfully located by the use ofresistivity by selecting appropriate electrode spacing to allow the probing current to enterthe ground to the appropriate depth. Resistivity meters employed in oil prospecting are

often powered by large generators using very high voltages and electrodes spacedperhaps hundreds of meters or kilometers apart, but instruments suitable forarchaeological use are battery powered, easy to use, and usually priced under $1500.Resistivity instruments no different than those used by professional geophysicists, butwith fancy labels attached, are often found advertised for five times the price of standardinstruments. Let the buyer beware!

GROUND-PENETRATING RADAR (GPR)

Radars designed for probing into the earth typically operate from 30 to 300 MHz-thefrequency being determined by the length of the dipole antennas used. It is necessary touse relatively low frequencies because the earth almost always is a good absorber of radarwaves. Unfortunately, low frequencies imply long probing wavelengths and longwavelengths imply low resolution. A very short pulse is used allowing accuratemeasurement of depth to the target, however the antenna beam is very broad (90-120degrees usually) and can not easily be narrowed because the antennas become too big andbulky.

Very often GPRs are mounted on a small wheeled cart which is hand towed across thearea of interest, that is, if the search area is reasonably flat and relatively free of brushand boulders. The echoes are displayed in a continuous strip oscilloscope false colorrecord for ease of interpreting results. In recent years the state of the art in GPRtechnology has been greatly improved by computer signal processing methods, since theperformance of these radars is almost always "clutter limited." Clutter signals areunwanted reflections, off-axis echoes, and multiple scattering echoes. These signalsobscure the target of interest under bands of signals but in many cases digital processingimproves radar performance by many orders of magnitude.


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When cart-mounted radar can be used, an experienced operator can often traversing largeareas of surface at a site in a single day. The radar output can be recorded on a standardhome video tape for archiving and detailed study, and also printed out on strip-chartpaper for immediate on site analysis. GPRs are usually limited not only by clutter but alsoby attenuation of the radar signal in the soil. This is most severe in clay soils and damp

soils where the salt content is high. The depth of penetration at some sites may be lessthan 1 foot, or under favorable conditions, many tens of feet or even hundreds of feet.Commercial cart GPRs are priced from about $18,000 to $40,000 and operator trainingand experience is necessary to interpret the records.

Very often cart radars can not be used because of rugged surface terrain. Or perhaps thearea to be explored is underground---inside a tunnel or cistern or along a confined areasuch as a hillside. Portable individual transmitting and receiving dipoles are useful insuch cases. But the data must now be recorded point by point, usually by taking Polaroidphotos. Targets of interest can be triangulated and mapped if these targets can be viewedfrom various aspect angles. Portable GPRs are well suited for discovering cavities and

voids, and when soil attenuation values are low they can detect caves, tombs, orchambers one hundred feet or more in depth. Interpretation of GPR records of all types isunusually difficult requiring operator skill and experience for satisfactory results.

HIGH-FREQUENCY SEISMIC SOUNDING

Sound waves are not easily coupled into soils, except at very low frequencies (a fewHertz, or cycles per second) but at higher frequencies sound waves can be used in rock orsolid walls as a helpful diagnostic tool. Frequencies used for probing in bedrock or stoneare generally 1000 to 30,000 Hertz (cycles/second). A coupling gel, or mud layer, isnecessary to couple the seismic signal into and out of the transmitting and receivingtransducers and this makes field measurements somewhat time consuming unless only afew locations are to be surveyed. High-frequency sounding is especially useful forfinding tombs and voids in areas of high radar signal absorption. For example, the Valleyof the Kings in Egypt has very high radar attenuation, but the same limestone can beprobed with high-frequency sound waves for distances well beyond 100 feet. Measuringthe thickness of a wall or pillar is readily done with this method. High-frequency seismicsounding instruments are not presently commercially available, but can be custom builtfor about $10,000.

MAGNETOMETRY

The earth's magnetic field is slightly disturbed by some kinds of archaeologicalanomalies such as fired clay pottery. The magnetic signals associated with archaeologicalfeatures are very small and easily obscured by trash metals, power lines, nearbyautomobiles, and the like. Magnetometers are most suited for remote, isolated sites awayfrom modern buildings and debris. Magnetometers cost from about $1500 to $10,000 andcan be used in pairs (a "differential magnetometer" to subtract out all but the wanted


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signals. Modern magnetometers are sensitive to field changes of about 1 gamma-theearth's weak magnetic field intensity is of the order of 50,000 gammas. Magnetometryhas been successfully used to located imported stone at some well-known archaeologicalsites. Fired mud brick has a reasonably high magnetic anomaly and of course ferrousmaterials such as one might expect at an Iron-Age or later site, give rise to very large

magnetic anomalies.

MICROGRAVITY

Gravity is one of the weakest of all forces found in nature. Yet, the earth's gravity field isvery slightly altered by such features as subsurface voids or caves. Suitable gravitymeters, known as "microgravimeters" cost of the order of $50,000 and require a veryexperienced trained operator. Point by point measurements must be made, which may betime consuming. The data must be carefully corrected for such things as surfacetopography and diurnally varying "earth tides." For these reasons gravity surveys have

been little used in archaeology to date.

AERIAL PHOTOGRAPHY AND IMAGERY

Conventional aerial (stereo-pair) photos of a site are very useful, as has been suggested,since outlines and features not visible from the ground frequently show up in aerialphotos. Thermal infra-red (IR) imagery requires a scanner, usually cooled by liquidnitrogen, (instrument cost $15,000 to 50,000), but surface temperature differences of asmall fraction of one degree can be measured. At night radiation cooling of the ground isnot uniform if there are subsurface features that impede or enhance heat flow. Inadditional to diurnal heating and cooling, seasonal heat flow temperature changes canoften be detected providing information on deeper archaeological anomalies. Heat flowthrough rock and soil is very slow---rock is an excellent heat insulator---so infra- redmeasurements give information about temperatures near the surface, not abouttemperatures deep within the earth. In spite of the limitations, false-color images showingtemperature contours can thus provide interesting clues for the archaeologist at somesites, especially if such measurements can be made carefully at periodic intervals throughan entire year. The Temple Mount in Jerusalem is an ideal site for on-going thermal infra-red imaging studies and Tuvia Sagiv, an architect from Tel Aviv, has already obtainedsome fascinating thermal IR images of the Temple Mount area. These can all be donefrom a distance or from the air.

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