MULTI-MODE GPR SURVEYS INFORM AND BENEFIT FROM ARCHAEOLOGICAL EXCAVATIONS: DELAWARE RIVER MARGIN, NEW JERSEY

Geophysical imaging is increasingly being used to execute archaeological investigations at prehistoric Native American sites, however post-excavation surveys are rare. These can be highly informative due to detailed ground-truth offered by the excavation and documentation of these archaeological sites, compared to the point-source nature of many geological investigations. We present a comprehensive pre- and post-survey 800 MHz ground-penetrating radar (GPR) investigation of the Late Pleistocene/Early Holocene-aged Site-28WA550 (Snyder Complex, New Jersey). The original surveys were used to establish site formation context and to assess the viability GPR imaging in a plowed setting with minor lithological contrast and localized buried targets. A topographic high between two post-glacial paleo-channels formed the basis for identifying a landform suitable for human habitation. It contained transient stability surfaces and artifacts associated with the Paleoindian cultural period. Post-excavation surveys revealed an upstream extension of a 2.5-m-deep paleo-depression. Additionally, surveys were conducted for the first time along the perimeter of an active excavation (“wrap-around” scan mode), across one of the excavated levels (extending the depth of penetration), and in a side-looking mode up the trench wall. These collectively allow a quasi-3D visualization that can be compared with the exposed stratigraphy. In addition to (semi)continuous reflection patterns, point-source diffractions allow the targeting of more directed excavations for potential artifacts both beneath the levels and into the wall. Despite the homogeneity of oxidized silty sand horizons with minor pedogenetic overprinting, strong signal return in 2D images (B-scans) was correlated with distinct shifts in sediment properties (e.g., magnetic susceptibility). In addition, a nearby filled 4.5x3.0 m excavation was easily detected through truncation and backfill characteristics, with implications for identifying old test pits and trenches at other sites. Our study demonstrates the value of pre- and post-excavation georadar surveys at archaeological sites lacking extensive ephermal or permanent structures or earthworks, which may otherwise discourage the applications of geophysical imaging.