Geophysical Survey – Additional Information

geophys01Introduction

Since 1987, CRA has routinely incorporated near-surface geophysical techniques in its own cultural resource management projects and has offered these services to clients. CRA owns state-of-the art equipment including duplexed Geoscan FM256 gradiometers; a Geonics, Ltd. EM38 electromagnetic (conductivity) meter; a TR Systems, Ltd. TR/CIA twin-probe resistivity meter; and a Geophysical Survey Systems Incorporated SIR-3000 Ground Penetrating Radar (GPR) equipped with a 400 MHz center frequency antenna and survey cart. Results are processed using a variety of software, importantly Geoplot, RADAN 6, Snuffler, and Surfer, and results are prepared for our clients using Didger, ArcGIS, and AutoCad.

At CRA we stress the use of multiple survey techniques to gain maximum information in a cost efficient manner. 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.

Applicability of Geophysical Surveys

The survey techniques used at CRA are applicable to a wide variety of sites and variable site conditions. We are dedicated to providing service to the fast-paced world of CRM. 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 pastures (mowed and standing), crop stubble, and tilled fields. The rule of thumb is that 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. With proper planning, even sites with obstacles can be surveyed; however, obstacles that impede movement, changing the walking pace, or cause the operator to vary the orientation of the carried instrument can introduce noise into the data and making it difficult to interpret.

3D image showing graves that CRA identified with Ground Penetrating Radar (GPR)

3D image showing graves that CRA identified with Ground Penetrating Radar (GPR)

Fresh agricultural tilling (plowing and disking) does modify the distribution of resistive, magnetic and conductive 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. Geophysical survey does tend to record the effects of agriculture long after a field has been fallowed, though. We regard these crop striations not simply as noise, but as an important record of modern impacts to a site. Nevertheless, in designing field strategies that 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. Strip plowing should be avoided.

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. 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 that 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 produces a major disturbance of the magnetic field surrounding it, at least as it is recorded by a magnetometer or a earth conductivity meter. This obscures the weaker signals that reflect the archaeological targets that we seek (like burned posts, hearths, house floors, etc.). Soil resistivity and GPR can be still be used in these areas.

After considerable field experience with the site type, we have concluded that near surface geophysical surveying techniques of all types can be useful for defining historic cemeteries. There are exceptions: pioneer era cemeteries that have become overgrown with trees and children’s burials can be difficult to identify. Most geophysical techniques can define a cemetery’s past, enclosed limits, and this knowledge is important in certain CRM situations. In certain situations, geophysical techniques can provide precise locations for individual graves, but this depends a great deal on the age of the cemetery, the soils, and the “standardness” of the graves. Not every technique works well for delimiting cemeteries. This becomes glaringly 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 that is erased with weathering over time. However, the use of multiple, complimentary geophysical techniques (e.g. magnetometry and GPR) often produces a more accurate record of a cemetery than simply probing the soil.

A Note on Research Design

Since 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 that 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 collection, 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 a National Register of Historic Places eligible site, cost effective, informed mitigation plans. Often as not, geophysical techniques are used to inform archaeologists where not to dig to achieve the best results during a survey!

Field Protocols (how we collect data)

CRA uses magnetic gradient, electrical resistivity, GPR, 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 CRA projects in 20-m squares and the average survey time for a square is 20–25 minutes. Within a square, readings are taken at measured intervals along transects typically spaced .5–1 meter apart for a total of 1,600–3,200 readings per 20-m square. With both the magnetometers and the earth conductivity meter, readings are routinely taken at 12.5-cm intervals. The GPR routinely takes readings at 2-cm intervals in the horizontal direction with 512–1,024 samples in the vertical direction, vastly increasing the number of readings without appreciably increasing survey time.

Grids are marked with non-magnetic materials, typically plastic pin flags. CRA 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. It is advantageous to remove “field metal” (e.g. horseshoes, bits of farm equipment, etc…) from the site during the field walking stage for the same reasons.

Techniques

  • Magnetrometry, or magnetic gradient survey, utilizing duplexed Geoscan FM256 gradiometers
  • Conductivity, or Electromagnetic (EM), survey using a Geonics EM38
  • Ground penetrating radar (GPR) by means of a Geophysical Survey Systems Incorporated SIR-3000 radar system equipped with a 400 MHz center frequency antenna and survey cart
  • Electrical resistivity with a TR Systems, Ltd., TR/CIA twin-probe resistivity meter

Applications

  • Archaeological site survey
  • Cemeteries
  • Underground storage tanks
  • Pipes
  • Buried concrete and building foundations
  • Roads (testing for sinkholes)