Archaeological Geophysics

Archaeological geophysics is an investigation into the physical properties of the Earth –and changes in those physical properties– to detect archaeological remains and anthropogenic changes to the landscape.  These techniques are non-invasive (unlike traditional excavation) and, when deployed on an archaeological survey, they make it possible to cover large areas of interest relatively quickly and with minimal cost.  These techniques detect and precisely map subsurface archaeological features, and in some cases can provide data for interpreting the functions of particular archaeological structures.  While geophysics has been applied to archaeological questions for the better half of a century, considerable advancements in battery-life, cost-effectiveness, and portability, and the refinement of the range and detection ability of these instruments makes rapid data collection and on-site (or at least in field) processing with laptop computers possible.  These advancements make large-scale archaeogeophysical surveys not only feasible, but effective and integral tools for modern archaeological projects.

The KAMBE project has employed a range of techniques at both Kalavasos-Ayios Dhimitrios and the Maroni settlement cluster, however, due to the characteristics of the local soils and the architectural building materials, we have found that magnetometry and ground-penetrating radar (GPR) provide the most useful data.  For this reason, these techniques have become the backbone our our geophysical surveys.*

Magnetometry

Magnetometers detect small magnetic field (and changes in these magnetic fields) caused by subsurface archaeological features, often due to the presence of weakly magnetized iron oxides.  When materials are heavily-fired (usually > 600°C although this depends on the material), they acquire a remnant magnetization aligned with the direction of the earths magnetic field at that point in time.  Archaeological features, including hearths, kilns, metallurgical facilities, and even fired-clay bricks, can be detected using a magnetometer.  Human interactions with their landscape also tend effect the magnetic enrichment of surface soils through the deposition of organic waste, fires and burnt materials, and the deposition of fired artifacts.  The KAMBE Project has employed both a Geometrics G-858 MagMapper optically pumped cesium-vapor magnetometer and a Geoscan FM256 fluxgate gradiometer (see below).

G-858 optically pumped cesium vapor magnetometer in foreground, Summer 2008 [GSSI 400 MHz GPR in background].

 These two instruments function slightly differently, with the G-858 records the total magnetic field at a given location, while the FM256 functions in gradient mode, with two magnetic sensors arrayed vertically (in the white square column).  Each of these sensors records the magnetic field, and by subtracting the measurements from each, we are able to essentially remove data “noise” caused by nearby magnetic objects, and focus on signals caused by subsurface objects.

Geoscan FM256 fluxgate gradiometer survey, Summer 2013

Ground-Penetrating Radar (GPR)

Ground-penetrating radar instruments transmit high-frequency electromagnetic radio (radar) pulses into the ground, which bounce of buried architecture, objects or geology and are reflected back to the receiving antenna. Signal reflections are stronger when there is a large difference in the dielectric permittivity of adjacent materials.  The instrument records the time elapsed since the initial radar pulse and the intensity of the reflected radar wave, making it possible to estimate the depth of the reflective materials beneath the surface.  Compiling these reflections makes it possible to produce “profiles” of subterranean features.  By collecting data along closely-spaced transects, it is possible to stitch these profiles together through various interpolations and then to create horizontal “time-slices” where time substitutes for depth, essentially imaging data in an “archaeological plan” view.  These slices can enhance the interpretation of GPR data because archaeological features with regular shapes, such as roads and floors (of interest to our project) are more easily recognized in plan view than in vertical section.  We have employed a GSSI SIR-3000 GPR with both 270 MHz and 400 MHz antennae, a Malå X3M with a 500 MHz antenna and a Sensors and Software Noggin GPR with a 250 MHz antenna.

GSSI SIR-3000 GPR with 400 MHz antenna and survey cart at KAD, March 2010

Time slices from Units 3 and 4 (a 40m x 30 m survey area) at Kalavasos-Ayios Dhimitrios collected during the 2008 field season with a GSSI SIR-3000 and 400 MHz antenna (processed by Jeff Leon)

*It is worth noting that electrical resistivity surveys have been performed with some success during the early spring at KAD by John Hunt (unpublished).  Unfortunately, the lack of moisture in the soils during the mid-summer KAMBE field seasons precludes its use.

 

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