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Preservation Matters: Remote Sensing - Metal Detecting

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Metal detecting is a widely used survey technique in archeology since the late-1950s on sites where metals are present. Metal detectors are relatively inexpensive and effective remote sensing devices that should be part of the tool kit of archeologists working at sites where metal artifacts are likely to be a part of the site assemblage. Metal detectors find metal objects just as shovel tests or test units might be used to discover a site’s content, depth, or boundary. Archeological applications of the metal detector, coupled with precise and accurate recording techniques, are very similar to the well-accepted routine shovel test field survey technique.

There are two types of Metal Detectors. The most used is the very low frequency (VLF) detector which can read buried metals to a depth of 30 cm with the proper coil and soil conditions. Most VLF machines have the capability to identify the type of buried metal. The second is the Pulse Induction (PI) or Zero Voltage Technology (ZVT). This type cannot discriminate metal types nor determine a possible depth, but they can penetrate the ground to a depth of just over one meter and can find objects as small as a straight pin at that depth. Both types can be used for archeological investigation effectively in most soils and field contexts.

Control housing connected to the search coil by a stem wrapped in antenna cable
Diagram depicting the components that comprise a typical metal detection unit

COMPONENTS

Knowing your tools is critical for ensuring a successful survey.

  • All metal detectors work on a basic configurationthat includes a handle, a search coil, an electricalcable, and a control housing for the battery and tuner.
  • The handle allows the operator to swing the coilalong the ground.
  • The search coil contains an antenna that generatesan electromagnetic field. When metallic objectsare near the coil, an electrical current is created which is detected and converted to visual and audible signals.
  • Smaller coils penetrate less deeply compared tolarger coils. The 20 cm (8 in) and 25 cm (10 in)coils are a compromise between the desire for depth and practicality. These coils detect to a depth of 30 to 36 cm (12 to 14 in).
  • An electrical cable connects the antenna in the coilto the control housing and is wound around thehandle stem. It is important to keep the cable wound tightly and securely so that the tuning of the detector does not change during use.
  • Most control housings attach to the handle of thedetector.
Patrick Severts (left) instructing Kirk Cordell (right) on metal detecting imaging displays at Pecos National Historical Park.
Patrick Severts (left) instructing Kirk Cordell (right) on metal detecting imaging displays at Pecos National Historical Park.

PREPARATION

Preparation for metal detecting surveys involves an understanding of detector components, their functions, and practical applications.

  • When a detector is first used in an area, itneeds to be tuned (ground balanced), tothe background level of moisture and metal in the soil. Even with proper tuning, false signals may occasionally be triggered by mineralized rocks, pockets of metallic or other mineralized soils, or even areas of high moisture.
  • The electromagnetic field that detectorscreate penetrates the soil in a cone shape formono coil machines. Mono coils are found on less expensive machines. The more com-mon coil is the Double D. The signal from the Double D goes straight down and does not cone allowing for better coverage at depth. The larger the coil, the greater the electro-magnetic field and the deeper subsurface materials can be detected.

DATA COLLECTIONS

A good method of survey consists of three operations: Metal Detecting, Artifact Recovery, and Provenience Recording.

  • During metal detecting, targets are locatedand marked. A recovery crew follows andcarefully uncovers the objects, leaving them in place. The recording team then plots individual artifact locations, assigns field specimen numbers, and collects the specimens.
  • Visual inspection of the surface can be carriedout simultaneously with the metal detectorsurvey. A metal detector crew may consist of a crew chief, metal-detector operators, and visual inspectors who also flag the targets found by the detectors. Detector operators should walk abreast, following transects across the area to be inspected. While walking, the operators use a sweeping motion over the ground making sure their sweeps overlap the preceding one. Coils should be held as close to and as level with the ground as possible to provide maximum vertical and horizontal coverage.
  • Once an operator locates a target it should bemarked for further investigation or mapping.The flaggers can also visually examine the ground for surface artifacts, allowing the detector operators to concentrate on their machines. Pin flags with plastic shafts can be used to eliminate the problems found with metal shaft pin flags.
Metal detecting instructor, Charles Haecker (front), instructing students at the Metal Detecting for Archeologists Workshop
Metal detecting instructor, Charles Haecker
(front), instructing students at the Metal Detecting
for Archeologists Workshop. The Workshop
was held at Pecos National Historical Park
with field exercises conducted at a Union campsite
related to the Battle of Glorieta Pass.

PINPOINTING

Pinpointing precisely locates objects

  • Recovery crews pinpoint and excavate the artifactlocations marked by the detector team. The usualprocedure is to trowel earth away to expose the artifact using traditional hand tools. Traditional excavation techniques can be employed when site stratigraphy, time, and project design require it.
  • The recovery team should also include a metaldetector to pinpoint the buried target. A metaldetector using a small, 3 to 4 in. (7.5 to 10 cm) diameter coil that allows precise location of the object while it is still in the ground works best for this task. Handheld pinpointers are also available and can be used to minimize the area to be checked and time for artifact recovery.

VISUALIZING

Visualizing field data during a metal detector survey involves recordation of proveniences, GPS locations, and field cataloging data as well as excavation to ground-truth metal detection.

As with any archeological investigation, it is essential to record provenience data to allow for later interpretation of artifact patterns. The recording crew may include a transit operator, a rod holder, or a GPS and data collector, and personnel to assign field-specimen numbers and bag the finds. The recording crew also backfills the excavated holes.

REFERENCES

  • Conner, Melissa and Douglas D. Scott, 1998,Metal Detector Use in Archaeology: An Introduction. Historical Archaeology 32(4): 76-85.
  • Reeves, Matthew and Scott Clark, 2013, OpenMinds, Clearer Signals: Metal Detectorist andArchaeologist Cooperation Takes Another Step. Electronic document, https://sha.org/blog/2013/04/open-minds-clearer-signals-metal-detectorist-and-archaeologist-cooperation-takes-another-step/
  • Register of Professional Archaeologists, 2020,Advanced Metal Detecting for the Archaeologist. Electronic document, http://amda.modernheritage.net/
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Part of a series of articles titled Preservation Matters.

Last updated: February 14, 2022