Detecting Archeological Artifacts from the San Salvador Settlement in Colton, California, Using Ground Penetrating Radar and Magnetic Gradiometry

In the mid-1800s, San Salvador was situated alongside the Santa Ana River in what is now Colton, CA. The settlement had grown to be the largest non-native settlement between New Mexico and Los Angeles until the river washed away or buried all adobe structures and settlers' belongings during the Great Flood of 1862. Historical archives reveal that a significant portion of San Salvador is buried approximately 3 meters beneath sandy river deposits of the vacant, 200-acre lot of Pellissier Ranch.

We are conducting Ground Penetrating Radar (GPR) and ground-based magnetic gradiometry surveys in search of buried adobe structural remains, household items composed of wood and iron, and metallic farming equipment. The GPR and gradiometer equipment that we are using for this study are expected to be particularly effective in imaging structures and objects buried at depths down to 5 meters. Contemporary debris from trespassers on site may result in false-positive gradiometer results for anomalies related to San Salvador. We therefore focus more heavily on GPR results in our study. Previous studies by other researchers at southwestern sites have shown GPR to be successful at imaging "adobe melt" off adobe structures, which are otherwise difficult to image due to the highly similar characteristics of soil and adobe.

Using a GSSI SIR 4000 control unit equipped with a 400 MHz antenna, we image an "adobe melt" signature at a depth of ~2.5 meters and a canal signature buried at ~1.5 meters deep. Additional structural GPR anomalies vary in size from 8-35 meters at a depth of 1-3 meters. GPR profiles are continuing to be collected utilizing an additional 350 MHz HyperStacking antenna, along with concentrated gradiometer surveys. The ultimate goal of our archeo-geophysics research is to provide data to the Spanish Town Heritage Foundation, which could potentially lead to the recognition of the cultural importance of this site, before the city of Colton develops Pellissier Ranch.