Ground Metal Detection Reconnaissance
by Rich Green
From 1993 to present, Historic Archaeological Research contains directed and or supervised several archaeological investigations designed around using metal detector science. The objective of each project has been to systematically recover metal artifacts in conjunction with typical archaeological reconnaissance methods. The author has nearly thirty-five years experience with metal detecting devices; the past seventeen have been dedicated to this specific application and purpose. This is a 2009 revision of the original writing and is intended to provide a broad outline for archaeologists considering this approach to survey of historic period sites.
Metal Detector Classification
The following is a brief discussion and description of the metal detectors used by HAR; although, there is no intention to go into great detail on this topic. There are perhaps hundreds of makes and types of metal detectors manufactured by several dozen domestic and foreign manufacturers, using a like number of features offered on this large variety of instruments. So that you can reduce variables and control survey parameters, metal detection instruments are selected for complimentary performance characteristics which are fundamental to archaeological reconnaissance. Those types of detector have been classified as Type I, II and III. Special purpose instruments may also be useful under certain circumstances and are discussed in further detail below.
Both the Type I and II metal detectors are VLF motion, full range discriminating detectors. Since we're not usually worried with the instruments ability to discriminate (identify and reject) most metal items, discrimination control really isn't a crucial issue. In fact, detectors created for use by hobbyists to locate coins etc are typically made from features that make them lower than desirable for archaeological survey work. Instruments with visual or audio target identification and or additional discrimination enhancements have been observed with that author to skew sampling uniformity. These discrimination features should either be manually defeated by adjustment, or this kind of metal detector should be avoided altogether.
Metal detectors for use in archaeological survey are selected, not for that facility to determine the probable type of material, but rather for inherent sensitivity and a critically quick response to all metal objects, particularly those found near one another. The kinds of detector we have found most useful are considered more basic in design and function yet, when working in tandem, possess the ideal characteristics for thorough artifact recovery.
Type I VLF: Type I instruments operate near the 5-6 KHz range. These detectors have an affinity for iron artifacts and exhibit exceptional sensitivity to metal items rather expensive from the conductivity spectrum i.e. brass, copper and silver.
Type II VLF: The Type II detectors generally offer an operating frequency near 10-15 kHz. These instruments are intrinsically sensitive to metal targets of mid range conductivity for instance aluminum, lead, nickel; gold and small irregularly shaped artifacts. The Type II is typically less sensitive to small iron artifacts making this a better choice for working in and around concentrations of ferrous debris.
Type III Multi-frequency: Recent innovations in metal detector design exploit the response characteristics of both the Type I and Type II instruments by simultaneously utilizing multiple frequencies. This may be accomplished differently by various manufacturers; however, for the purpose of this discussion, the Type III circuit design is considered a combination of the Type I and II detectors.
Hand-held Magnetometer: This device is made to locate ferrous metals only and is used to identify the presence of iron and other ferromagnetic materials. The hand-held magnetometer is an extremely sensitive detector capable of identifying artifacts such as small as nails near the top, as well as larger objects at much greater depths.
Two-box type Metal Detector: It is a Transmit-Receive (TR) type metal detector efficient at locating metal objects that have relatively large surface areas up to extreme depths. The two-box will detect both ferrous and non-ferrous metals; however, it won't indicate on small metal artifacts of any kind, and is typically used in conjunction with or instead of the magnetometer to identify larger deeply buried items. This instrument might also be used to locate voids, and is useful if it is necessary to inductively trace linear features like utility lines.
Survey Methods
Metal detection technicians may work either alone or using the assistance of an excavator. In either case, permitting the technician to indulge in the recovery of detected artifacts has been confirmed to be both expeditious and, perhaps more importantly, provides immediate operator feedback critical to the metal detection process.
Each metal detector, no matter type classification, is adjusted for response to all metal artifacts unless otherwise instructed by a supervisor. Under some circumstances, it may become essential to purposely reject concentrations of debris or items too numerous to collect, plus a representative sample could be deemed appropriate. Types of this include modern metal surface waste, disintegrated wire fence material or concentrations of near-surface building debris. Otherwise, all detected metal items are recovered and collected by the operator and or the excavation personnel that have been assigned to assist the technician.
All artifacts of interest are assigned a field specimen number and the location of recovery is marked by a non-conductive pin flag for subsequent mapping. Any unidentified items are accepted as artifacts and are collected for laboratory analysis. The artifact collection bag includes all pertinent information including the date, operator's name, detector type, axis of sweep and the depth of the recovered item. Field specimen bags are subsequently collected and recorded in an FS log. Items recovered but not assigned field specimen identification are collected and counted permitting statistical breakdown of metal artifact density within the survey area.
Wide Area Survey: Large parcels of land can be sampled by deploying metal detector operators to survey multiple parallel transects. This Type I and II instruments, by virtue of different operating frequencies, can be positioned relatively close together without cross-talk and spurious interference. As a rule, Type I and II detector technicians are positioned alternately at five meter intervals. Technicians using Type III instruments may have to be placed in line where minimum interference is obtained. This is a over adequate technique for quickly sampling larger areas that are not immediately slated for intensive metal artifact recovery. Pin flags placed at recovered artifact locations often indicate distribution patterns and areas of concentration which will merit further more intensive investigation methods.
2-2-90 Method of Intensive Metal Detection: HAR has refined an intensive metal detection survey procedure that we have now dubbed the 2-2-90 Method. A grid is established of discretionary size, usually 10-15 meter units using the former being greater desirable. Units larger than 15-meters are difficult for operators to cover completely and needs to be avoided. Technicians are assigned to grid units and also the survey axis is determined by instrument type. Typically, the Type I operators survey units along the N-S axis and technicians using Type II instruments investigate along the E-W axis. The Type III detector might be substituted for either from the other two types.
Each grid square is examined in overlapping sweeps resulting in 100% initial coverage. Following completion of a unit, a 2nd technician with an opposite type detector is assigned to survey machine perpendicular for the first operator's sweep. Operator skill and detector performance are integrated in this fashion. This technique results itcoverage by 2 operators using 2 complimentary detectors, at a 90 degree angle of approach. Coverage is improved exponentially and either leads to a rise of collected artifacts or more thoroughly substantiates no metal remains within a given survey unit.
Metal detection reconnaissance, in conjunction with classical archaeological survey, is a pragmatic method that clearly enhances artifact sampling and archaeological research of historic period sites. A version of a these techniques ought to be considered in every cases where metal artifact distributions are anticipated. For more detailed information, please do contact HAR.