UC San Diego

This report documents a project to apply geophysical prospecting techniques at known locations of prehistoric canals in the Gila River Reservation. The project goal was to test the ability of several geophysical prospecting techniques to image buried canals in the Gila River floodplain. Four geophysical prospecting approaches were tested: ground penetrating radar, seismic reflection imaging, magnet gradiometry, and electrical conductivity. These approaches have complimentary abilities to reveal buried objects and structures. Ground penetrating radar transmits high frequency electromagnetic energy into the ground and measures energy reflected from buried interfaces, such as between sand and other soil layers. Seismic reflection imaging similarly looks at reflected energy, but uses seismic waves as an energy source. Magnetometry is sensitive to the magnetic field that is emitted by buried objects and structures; it is particularly sensitive to the presence of burned objects, as well as to the sedimentary fill of a canal. Electrical conductivity uses low frequency electromagnetic energy to detect changes in electrical properties; it is an effective means for detecting soil moisture content changes, such as may be present in a buried canal. Three sites with known prehistoric canals were investigated: GR-441 (Santan Site), GR-1086 (Crown Site), and GR-415. At each site, a prehistoric canal had previously been discovered by excavation. Geophysical prospecting data were collected adjacent to known canal locations so that the ability of each geophysical technique could be tested. Ground penetrating radar had little or no penetration of site soils. It is likely that the high salt content of the Gila River floodplain yielded high attenuation of the 400 Mhz radar signals. Using high-frequency seismic sources produced seismic reflection images with indications of the presence of a prehistoric canal. Seismic images revealed coherent reflections from the sloping sides and bottom of the canal features. These data are interpreted as owing to the contrast between sandy fill and clay layers forming the sides and bottom of the canal. Electrical Conductivity data reveal broad regions of conductivity that follow the general trends of the canals. At each of these sites, regions of high and low conductivity are delimited by the location of the canal. Magnetometry data collected at these sites was largely contaminated by the presence of modern metal fragments. The signal from these strongly magnetic objects located at or near the site surface made it difficult to discern if a low level signal was present owing to the buried prehistoric canal.