ADVANCES IN SHALLOW GEOPHYSICAL PROSPECTION ON A FLOODPLAIN SITE IN SONORA, MEXICO: IDENTIFYING IRRIGATION CANALS USING MAGNETIC GRADIOMETRY

La Playa (SON F:10:3), in northern Sonora, Mexico, is one of the largest identified Early Agricultural period (2100 B.C.-A.D. 50) archaeological sites. During this period in the southwestern United States and northwestern Mexico, people began to modify their environments for farming. La Playa has remains of an extensive Early Agricultural Period irrigation canal system. Erosion at La Playa is severe, and only eroded remains of irrigation canals have been identified. Intact canals, water retention features, and cultivated soils had not been located for study. Various methods of shallow geophysical prospection were used to detect intact agricultural features buried by alluvium.

At similar sites, Ground Penetrating Radar (GPR) surveys have been successful in locating large canals. Attempts at La Playa have been unsuccessful due to soil moisture and a high percentage of silt and clay. Magnetometer surveys have also been unsuccessful due to the inability to detect low magnitude magnetic anomalies. Recently, magnetic gradiometry was chosen as a method of subsurface detection due to its ability to detect sensitive subsurface magnetic anomalies without effect from soil moisture or sediment size. A gradiometer records the difference between two magnetic sensors at a fixed distance from one another, thus removing unwanted signals from the Earth’s magnetic field, local geology, and diurnal magnetic changes.

Although previous prospection methods failed, they aided in informing the appropriate geophysical technique for this particular site. This paper presents recent magnetic gradiometry data from La Playa. Intact irrigation canals, water retention features, reservoirs, and possible cultivation plots were successfully located beneath the alluvium. This data set has significantly contributed to the understanding of land management strategies by imaging the extent and morphology of the canal network and relationship to other agricultural features. This technique has changed future excavation strategies from those which focus primarily on salvage archaeology, to strategies which can incorporate the testing of specific archaeological features before they are destroyed by erosion.