Geophysical Surveys: Definition & Methods

Instructor: Jennifer Perone

Ms. Perone has taught College Engineering, Ethics, Psychology, Perception, Statistics, Experimental Design & Analysis, Physics and secondary STEM topics for more than 15 years!

Geophysical surveys are the methods used in archaeology to produce a detail image or map of an area. There are many different methodologies, but the most successful surveys involve a solid research design that is well planned.

Geophysical Surveys

As a geologist or archeologist, how can you identify resources without sampling or disturbing surfaces? How can you detect buried features in difficult-to-reach areas, or without excavation?

Geophysical surveys are archaeological methods that use ground-based physical sensing techniques to produce a detail image or map of an area. These methods are neither invasive nor destructive, an important goal when surveying culturally sensitive sites such as cemeteries.

GPR visual representation of a cemetery. Yellow arrows show discrete objects, red arrows show smaller disruptions, blue lines show horizontal reflectors like bedrock

Data Collection and Processing Practices

Surveying of any site, regardless of instrumentation, begins by marking off the site into survey grids. The surveyor will walk the instrument along, taking readings at regular intervals. Reference points placed along the corners of each grid mark the collected data, minimizing positioning error.

Data processing and imaging allow the conversion of raw numeric data into interpretable maps. Geophysical data may be rendered as graphics, helping to intuitively recognize cultural and natural patterns and to visualize physical features that are causing detected anomalies.

Types of Geophysical Surveys

There are several types of geophysical surveys and each has different applications and equipment. They may be conducted from the air, on the ground or down drill holes.

Airborne geophysical surveys may involve magnetic, radiometric, gravity and/or electromagnetic surveys and are conducted from helicopters or light aircraft in a grid pattern.

Ground-based surveys may take one of many forms:

Seismic Surveys

Seismic surveys involve placing geophones (sensors connected to wires) in strategic patterns to provide information about the properties of rocks several kilometers below the earth. The survey team will induce vibrations using truck-mounted vibrating weights or small explosives. The vibration is measured as it is passed through layers of the earth.

Ground Penetrating Radar

In Ground Penetrating Radar (GPR), the radar signal is an electromagnetic pulse that is directed into the ground. Subsurface objects cause reflections in the data and the travel time indicates the depth of the object.

Ground penetrating radar equipment
GPR

GPR is best utilized in sandy soils without a lot of clay, silts or material that will cause metallic interference. Rocky terrain or heterogeneous sediments can further cause problems.

Magnetic Surveys

Magnetic surveys specify variations of the earth's magnetic field due to the presence of magnetic minerals. Small variations in the magnetic content of specific metals allow technicians to interpret rock types and assist in identifying resources.

Magnetometers provide better resolution of small, near-surface features. They respond strongly to iron and steel, brick, burned soil and many types of rock. It's even possible to detect very subtle anomalies caused by disturbed soils or decayed organic materials, like from a long ago campfire.

A magnetic gradiometer map of a hearths in a prehistoric campsite in Wyoming, USA, later verified by excavation.

Radiometric Surveys

Radiometric surveys measure gamma rays that are continuously emitted from the earth via natural decomposition of common radiogenic materials. You can therefore assess gamma radiation from the top 30 centimeters of the ground. This may be performed from the air or directly on top of the ground to identify metallic and industrial minerals.

Gravity Surveys

Gravity surveys use a gravimeter to measure the gravity field to determine variations in rock density. A technician must take gravity measurements during set intervals of distance and record the precise height at each location. These surveys are used to identify areas that have mineral or energy resource deposits.

Induced Polarity (IP) Surveys

Induced Polarity (IP) surveys induce an electrical field in the ground and quantify the conductivity and resistivity of the subsurface. Identifying changes in the electrical currents allows technicians to identify changes caused by different rocks and minerals. These surveys are used to find metallic minerals.

Electromagnetic (EM) Surveys

Electromagnetic (EM) surveys induce an electrical magnetic field to measure the 3-D variations in conductivity within the near-surface rock and soil. These measurements may be used to locate metallic minerals and to explore the groundwater and salinity patterns of a certain region.

In archaeology, changes in underground conductivity may indicate buried features. Conductivity meters respond strongly to metal. This can be a disadvantage when the metal is extraneous to the archaeological record, but can be useful when the metal is of archaeological interest.

Electromagnetic conductivity survey

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