Application of Complementary Geophysical Survey Techniques in the Search for Fort Louis at Old Mobile: A Comparative Case Study

Application of five geophysical survey methods - earth conductivity, magnetometry, thermal imaging, electrical resistivity and ground penetrating radar - in the search for archaeological remains of Fort Louis, original capitol of the French colony of Louisiane (1702-1711), has yielded divergent yet complementary results. This project included test excavations to ground truth the geophysical results and to evaluate the relative effectiveness of these five geophysical survey technologies in conditions common to the Gulf Coast plain. Silty soils, a water table within a few meters of the ground surface, and shallow, ephemeral ground disturbances during prehistoric and historic occupations characterize many archaeological sites in the Gulf Coastal plain. Geophysical survey techniques have so far been applied to only a handful of sites in this region, and their application in a systematic, comparative fashion, with follow-up ground truthing, is an even rarer phenomenon. At Old Mobile, none of the geophysical survey technologies discovered French colonial features (although one was found in the course of ground-truth test excavations.) However, three technologies - conductivity, magnetometry, and resistance - did reveal modern, cultural, subsurface features. The results of this case study suggest several general lessons.