Archaeology: Geophysical Investigations
Geophysical Investigations: Additional InformationSince 1987, CRAI has routinely incorporated near-surface geophysical techniques in its own cultural resource management projects and has offered these services to clients and other firms throughout the mid-south. CRAI owns state-of-the art equipment, Geoscan Research FM36 and FM256 fluxgate gradiometers and a Geonics, Ltd. EM38 electromagnetic meter. Results are routinely processed using a variety of software, importantly Geoplot and Surfer, and results are prepared for our clients using Didger, ArcView, and AutoCad. |
||||
 |
Survey specialist and Senior Archaeologist, R. Berle Clay Ph.D., RPA, has a long history of involvement in geophysical survey beginning in 1984 and has published on strategies of using survey techniques in field archaeology (Clay 2000). He is a continuing contributor to the National Park Service yearly workshops on geophysical techniques. At CRAI we stress the use of multiple survey techniques in a cost efficient manner to gain maximum information on sub-surface archaeological structures. Using high speed, high-density data collecting field equipment and up to date processing software, we have a continuing commitment to increasing the utility and accuracy of geophysical survey data in both cultural resource management (CRM) projects and non-CRM archaeological research. Our clients include universities in three states, Federal and State agencies, and private applicants both in the United States and Europe. |
|||
Applicability of Geophysical SurveysThe survey techniques used at CRAI are applicable to a wide variety of sites and variable site conditions. This is due to choice on our part because of our dedication to providing service to the fast-paced world of cultural resource management. The "ideal" survey site would be a manicured lawn with no ornamental plantings, buildings, or any other above ground obstructions. We have yet to survey such a site. The techniques we use are applicable to open woodlands, provided there is no undergrowth or the undergrowth can be removed, various types of pasture (mowed and standing), crop stubble, and tilled fields. The rule of thumb is if the site can be walked at a speed of approximately one meter per second without major obstacles (like downed trees and low hanging tree limbs), it can be surveyed. Obstacles that impede movement, changing the walking pace, or cause the operator to vary the orientation of the carried instrument, introduce noise into the data making it difficult to interpret. Fresh agricultural tilling (plowing and disking) does modify the distribution of magnetic and conductivity constituents in the soil and this disturbance is reflected as cultivation or crop "marks" when such fields are surveyed. These tend to become less prominent over the months as the topsoil "stabilizes and homogenizes" due to weathering. However, geophysical survey does tend to record the effects of agriculture long after a field has been fallowed. We regard these crop striations as an important record of site impacts and not simply noise. Nevertheless in designing field strategies which will involve geophysical survey, we recommend that surface scarification (for example to facilitate controlled surface collecting) not be used, at least until the geophysical surveys have been completed. Speaking from our mid-America experience, geophysical surveys may be conducted at all times of the year. We have encountered most problems with extreme heat but we use specific techniques to control the effects of heat that can cause sensitive electronic equipment to drift affecting the quality of the data. For best results the ground should be dry underfoot so that the operator does not collect mud while walking which can distort magnetic readings in certain types of soils. Frankly we have never tried survey with a snow cover! Electromagnetic surveys, while they are affected by total soil moisture (importantly, a heavy rain during an EM survey can shift the measurement of earth conductivity significantly), appear to be far less susceptible to total soil moisture than soil resistivity surveys that tend to be unproductive when the soil is either saturated or extremely dry. The techniques we use, because they both involve magnetics, have circumscribed utility in urban contexts with a record of building, demolition, and rebuilding. The problem is metal targets of all sorts. The random metal target (like the wire pin flag beloved by archaeologists!) produces a major disturbance of the magnetic field surround it, at least as it is recorded by a magnetometer or a earth conductivity meter. This obscures the much less strong signals that reflect the archaeological targets that we seek (like burned posts, hearths, house floors, etc.). Finally, after considerable field experience with the site type, we have concluded that near surface geophysical survey techniques of all types are of marginal utility in defining historic cemeteries. There are exceptions and any archaeological geophysicist can point to them with pride. Furthermore, most techniques can define aspects of the cemetery as a whole, for example it's past, enclosed limits, and this knowledge is important, perhaps sufficient in certain CRM situations. However, as far as providing precise location on individual graves consistently, site after site in different soil conditions, they tend to do a less than adequate job which becomes glaring apparent when a historic cemetery is excavated as part of its relocation (a practice which we find is becoming increasingly important in CRM archaeology). The reasons for this seem to be that the act of historical burial is a very "brief" geophysical episode and, depending on soil type and grave depth, one that generally causes minimal geophysical modification of the soil or, as in the case of agricultural tillage, modification which is erased with weathering through time. We would be far more successful in locating a grave if our ancestors put on a clambake in the open grave as part of ritual feasting before they buried their revered ancestor and many of our First Nations forbearers did just that! A Note on Research DesignBecause they involve intensive ground coverage, the near surface geophysical survey techniques we use are generally not cost effective in phase I, site discovery surveys. They have an obvious place in phase II, site evaluation. Integrated as the "leading edge" of a phase II evaluation which may involve intensive shovel testing, excavated units, and surface scrapes, they provide a wealth of data which can be used to inform the traditional field techniques which follow in the evaluation. Geophysical surveys never replace conventional data collecting techniques. They can significantly reduce the cost of phase II evaluation by, for example, eliminating area disking and controlled surface collecting which is labor and time intensive and, to the extent that they aid in a better definition of site parameters, lead to both better evaluations of significance and, in the case of National Register Eligible site, cost effective, informed mitigation plans. Field Protocols (how we collect data)CRAI uses magnetic gradient and earth conductivity survey techniques that have proven applicability to a wide range of survey conditions and all seasons. Normally, geophysical survey data are collected on CRAI projects in 20m squares and the average survey time for a square is 15-20 minutes. Within a square, readings are taken at measured intervals along transects typically 1m apart for a total of 1600 readings per-20m square. With both the magnetometers and the earth conductivity meter, readings are routinely taken at 25cm intervals. Using new technology we hope to take readings routinely at 12.5cm intervals doubling the number of readings without increasing survey time. Grids are marked with non-magnetic materials, typically plastic pin flags. CRAI has found that a major source of "noise" in magnetic and electromagnetic surveys is wire pin flags used by archaeologists in early stages of their field research. We recommend that plastic flags or wooden stakes replace wire pin flags if geophysical survey is contemplated at any future point in the research program at a given site. |
||||
Examples of applying geophysical capabilities to archaeological surveysCahokia Mounds, near St. Louis, Missouri - use of both Geonics EM38 earth conductivity meter and Geoscan Research FM 256 fluxgate gradiometer duplexed with a FM 36 gradiometer to enhance interpretation Carty Site, near Columbus, Ohio - use of both gradiometer and earth conductivity meter to enhance interpretation Daniel Boone's Station, Athens, Kentucky - geophysical survey locates sub-surface structure features prior to excavation The Hollywood Site, Tunica County, Mississippi - use of geophysical survey in location and interpretation of house sites at a Mississippian town McConnell's Station, Bourbon County, Kentucky - geophysical survey locates sub-surface structure features prior to excavation The Widow Gibson Cabin, near Perryville, Kentucky - geophysical survey pinpoints the location of the cabin prior to archaeological assesment of the structure More examples of EM38 Results. Click on the thumbnails to see a larger image. |
 |
 |
 |
Learn more about geophysical surveying...* PDF format. Requires Adobe Acrobat Reader. Download for free at the Adobe webpage. Conductivity (EM) Survey: A Survival Manual by Dr. R. Berle Clay Working with the EM 38 Earth Conductvity Meter by R. Berle Clay The Place of Geophysical Survey in Contemporary Fieldwork by R. Berle Clay Recent Geophysical Surveys of Archaeological Sites
|