Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Prehistoric Houses at CA-ORA-116

Newport Bay, Orange County, California

CA-ORA-116 is an Intermediate period (ca. 300 BC - AD 700) midden and habitation site at Newport Bay, Orange County, California. Lewis Somers of Archaeo-Physics conducted a geophysical survey of the site as part of a multi-disciplinary study of the site by Statistical research, Inc.

A number of archaeological features were identified on the basis of the geophysical data. This allowed archaeologists to pinpoint Features for excavation and to interpret excavation results within the greater context of the site.

Excavation at CA-ORA-116 was guided in part by the results of the resistance survey. The top image shows the Highpass filtered resistance data in color. The bottom image shows excavation units and identified house features overlaid on the resistance data.

In the image below, excavation units are overlaid in blue; the high resistance features circled in yellow were identified as house pits. These results show a clear correlation between resistance anomalies and prehistoric features.

Two interpretations of the magnetic field gradient data emphasize anomalies of different magnitudes. Prehistoric cultural features are likely to produce only very weak magnetic anomalies, typically less than 10 nanoTeslas (positive or negative) from the local mean. Anomalies of this magnitude can be difficult to detect in the presence of ferrous iron. Even small pieces of modern iron or steel debris can create anomalies of several hundred nT, obscuring any weak anomalies in the vicinity.

With appropriate processing it is possible to distinguish weak anomalies even in the presence of strong anomalies caused by ferrous iron. The top image shows the magnetic data within the expected range of prehistoric features. For example, house features identified by excavation (see resistivity map) are associated with weak magnetic highs. Some anomalies within this range are obviously historic in origin, such as the two parallel linear magnetic lows caused by modern vehicle tracks. While anomalies of modern origin may not directly inform us of prehistoric site use, they can reveal modern disturbance which may impact the site. The bottom image shows high amplitude magnetic anomalies which are caused by historic iron debris.

Reference: House Pits and Middens, a Methodological Study of Site Structure and Formation Processes at CA-ORA-116, Newport Bay, Orange County, California. Donn R. Grenda, Christopher J. Doolittle, and Jeffrey H. Altschul, eds. Technical Series 69, Statistical Research, Inc. Tuscon, AZ; Redlands, CA 1998.

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