Archaeological sites are sensitive areas that can be easily disrupted by excavation or inadvertently destroyed by development. Geophysics uses remote sensing technology to measure variability in electromagnetic radiation, which is emitted or reflected off objects buried in the ground. As a result, geophysical techniques can locate and delineate buried archaeological sites before they are inadvertently damaged or lost. Because these techniques investigate the subsurface in detail without the labor, time, and expense of soil removal, using them can also save time and money.
Geophysics can be used in a wide range of settings to study the subsurface, from sandy beaches to impervious parking lots. The success of remote sensing with geophysical instruments depends largely on the instruments used. Factors such as soil type, moisture content, and site disruption, such as highways and electrical lines, affect different instruments in different ways. Thus a project's setting dictates the most appropriate instrument to use.
Cardno JFNew's CRM specialists have expertise in a wide array of geophysical survey techniques:
- Ground-Penetrating Radar (GPR) transmits radar (microwave) energy into the ground and records the reflections that are produced. GPR provides real-time depth information, producing the exact depth below surface where buried deposits are located. It is not affected by modern day activities such as electrical lines or automobile traffic. Best suited for dry soils with little to no clay content, GPR is used to identify all different types of features including modern piping and holding tanks, and archaeological features such as house foundations, hearths, and living floors. It is often used in historic cemeteries to identify grave locations.
- Magnetometry measures the amount of magnetism of the soil and subsurface at any given point in time and space. Because burning increases the amount of magnetism in any given object, magnetometry is often used to identify prehistoric hearths or houses made from baked mud or clay. It also is quite effective in identifying metal objects. Because it is extremely sensitive and proximity to unrelated metal can distort the instruments readings, magnetometry is best suited to areas isolated from automobile traffic or in close proximity to electrical lines.
- Resistivity measures how strongly soil opposes the flow of an electric current. Materials with high water content or those that retain moisture conduct electricity well, so they have low resistivity against a current. Because they usually allow for good drainage or lack moisture, hard, dense archaeological materials like stone walls, mounds, banks, cobble, rubble, paving, rubble-filled pits, and stone coffins and tombs typically have high resistance. Ditches, soil-filled pits, hut circles, foundation trenches, graves, gullies and furrows, and metal pipes all exhibit low resistance. Because water content greatly affects resistivity readings, it is best used in dryer areas and soils.
- Electromagnetic (EM) Conductivity is the opposite of Resistivity; it measures the ability of soil to conduct electricity. Areas that have high conductivity can indicate potential for buried materials. Like magnetometry, conductivity meters respond strongly to metal, which may be disadvantageous when metal is extraneous to the archaeological record. While both conductivity and resistivity detect the same types of archaeological materials, conductivity allows for much faster data acquisition because no physical contact to the soil is necessary, and it can be used in conditions unfavorable to resistance meters, such as hard rocky ground or pavement.
For a list of Frequently Asked Questions on Cultural Resources, visit here.