SEAC: Work in Geophysics
The collection of remote sensing data and its application to archeology through interpretation of survey results is an invaluable tool for providing archeological site evaluations. The Southeast Archeological Center has conducted geophysical surveys at a number of parks in the Southeast including, but not limited to, Castillo de San Marcos, Horseshoe Bend National Military Park, Moores Creek National Battlefield, Dry Tortugas National Park, Natchez Trace Parkway, Blue Ridge Parkway, Vicksburg National Military Park, and Fort Pulaski National Monument (Figure 1.). Each park provides its own unique challenges regarding terrain, soil types, and the type of features trying to be located.
These challenges are met with the array of instruments at SEAC. The instruments used by SEAC include a GSSI 400mhz ground penetrating radar, a GSSI 270mhz ground penetrating radar, a FM256 fluxgate gradiometer, EM-38 ground conductivity meter, and a RM 15-D resistivity meter. Each instrument uses a different method for subsurface detection and is capable of detecting subsurface variations that the others are not. The best possible geophysical survey is one that includes all instruments, so that a comparison of the data yields the greatest possible results.
SEAC archeologist using GPR at Castillo De San Marcos National Monument, St. Augustine, Florida
Figure 2 . GPR time-slice generated by SEAC archeologist from data collected at Fort Jefferson National Monument, Dry Tortugas, Florida.
Ground Penetrating Radar
GPR units are one of several geophysical instruments that archeologists have adopted for non-invasive mapping of archeological site deposits (Bevan 1998, Conyers and Goodman 1997). GPR units depend upon sensing subtle variations in the physical properties of the soil and buried archeological features. In order for a feature to be detected it must have “contrast.” That is, the archeological feature’s effect on or response to the instrument must differ from that of the adjacent soil. The ideal situation is one in which the archeological feature differs sharply from the surrounding soil and that soil is highly uniform. However, the soil or material surrounding archeological features is often varied enough to produce signal variation perceptible to the radar or other instrument. If the naturally occurring variation is as strong as, and has the same characteristics as, that produced by the archeological feature, then the archeological feature’s signal will be lost, indistinguishable from the variation stemming from the surrounding matrix (Nickel 2001).
It is important to emphasize that instruments employed in archeological geophysical surveys do not respond only to the desired targets and, consequently, feature detection depends greatly on the recognition of patterns that match the anticipated form of the archeological target. The challenge in archeological geophysics is to recognize the anomalies produced by the archeological features and distinguish them from the noise produced by responses from the surrounding matrix (Nickel 2001), as well as from anomalies produced by trees and topography.
GPR units operate by transmitting distinct pulses of radio energy from a surface antenna which are reflected off of buried objects, features, or soil structures and then detected back at the surface by a second receiving antenna. GPR systems are capable of producing reliable three dimensional images of the subsurface because feature depth can be determined by measuring the round-trip travel time (in nanoseconds) of the radar pulse before it is recorded at the surface (Conyers 2006). GPR antennas operate on a variety of frequencies between 10 and 1600 MHz, though frequencies in the range of 250 to 400 are the most commonly utilized in archeological prospecting.
Generally, the lower the antenna frequency, the greater the depth into the soil that features can be resolved. However, lower frequency antennas can only resolve very large objects and there is therefore a trade-off between depth of penetration and detail of anomaly resolution.
GPR surveys are conducted by moving the radar antenna along the ground surface in a series of linear transects comprising a larger grid (Figure 4). Two dimensional vertical profiles that display radar reflections from the ground surface to the lowest level of radar penetration are recorded for each of the linear transects. After all of the adjacent transect profiles within a grid are collected, computerized software can be used to combine the profiles and correlate the features, allowing for the production of a three dimensional cube displaying images of buried features and soil stratigraphy under the grid (Conyers 2006). That block can then be horizontally sliced at different depths (or times in nanoseconds) to produce “time-slice” maps displaying subsurface anomalies present at any depth below the ground surface (Figure 4).
FM 256 Fluxgate Gradiometer
A magnetometer is an instrument that generates a strong electromagnetic field by passing a current through an enriched hydrocarbon compound. The FM 256 Fluxgate Gradiometer is a type of magnetometer that uses an enhanced process called the “Overhauser effect”. This refers to a slight change that is made to the proton rich liquid contained in the bottles in the sensor heads that detect magnetic fields. The Gradiometer produces a strong magnetic field in a bubble-shaped area surrounding both instrument and operator, then uses two sensor heads located one-half meter apart to measure any disruption that may be caused by intrusion into that magnetic field. Much like a high-powered metal detector, the magnetometer is highly sensitive to disruptions caused by metallic objects passing through the generated field. However, it has two distinct advantages over standard metal detectors. First, a magnetic gradiometer can accentuate objects that are at shallow depths while discriminating features that are deeper. In addition, the unit has the on-board capability to filter readings based upon the localized magnetic field of the earth in the survey area (Scollar 1990:466-469). While surveys conducted with magnetometers can be problematic, they are useful instruments to the modern archeologist. It must also be considered that small metal artifacts buried near the surface of the ground can register as significant anomalies with the magnetometer.
SEAC archeologist Michael Seibert using the FM 256 Fluxgate
Results of a FM256 gradiometer survey at Horseshoe Bend National Military Park showing the extant postholes of the Red Stick Creeks barricade wall.
EM 38 Ground Conductivity Instrument
The EM-38 is an instrument that allows the operator to measure variability in soil moisture levels, and hence differences in soils and buried features of various materials. It produces a low electrical current that is introduced below the ground surface. Measurement of the relative strength of the generated electrical current as it passes through the subsoil, applied to a function identifying field strength, produces a value which can then be compared to the standard value for the area. When properly tuned for a specific area, the EM-38 allows the user to identify regions within soil profiles that contain differential moisture levels. Readings that deviate from a standard value for the region stand out as anomalies, which may correspond to any number of subsurface features. For example, a series of fired bricks located in a bed of sand will produce a lower conductivity reading due to its reduced moisture content relative to the surrounding soils. The EM-38 has proven useful in identifying burials, foundations, and many other cultural features.
SEAC Archeological Technician Rusty Simmons setting up the EM-38 conductivity meter.
EM-38 data collected at a Chickasaw village site along the Natchez Trace Parkway. The unit is capable of recording data at the 1m (left) and 0.5m (right) depths.
RM15-D Advanced Resistivity
Resistivity was first used in conjunction with archeological investigations in the mid-1940’s, making it the oldest of the geophysical techniques currently in use. Electrical resistivity is predicated on the notion that an electrical current will travel through different objects at different rates of speed, as expressed in Ohms Law (Scollar 1990). The ease with which the current travels through the soil is dependent upon several factors including moisture content, pore spacing, density, and material type. In a resistivity survey an electrical current of known frequency is introduced into the ground; the ease with which the current travels through the ground (apparent resistance) is then recorded at specific locations throughout the survey area (Heimmer 1992; Kvamme 2003; Scollar 1990). The survey will utilize a GeoScan RM15-D Advanced Resistivity Survey System.
After the data is collected the proprietary software package GeoScan will be used to subject the grid to a series of post processing steps intended to improve feature resolution, remove background noise, and accurately identify feature depth and size. The data will then be exported to the contour mapping program Surfer, in which additional manipulations could be made to further resolve the Resistivity anomalies. After processing, the data will be imported into a geographic information system in order to provide real-world coordinates for the new Resistivity data.
SEAC archeological technicians using the RM15-D Resistivity.
RM-15 D resititivy data from HOBE survey, illustrating the path of the Red Stick Creek barricade wall.