A MULTIFREQUENCY ELECTROMAGNETIC INDUCTION SURVEY OF A HISTORIC HOME SITE

An electromagnetic induction survey was conducted at the Strawbery Banke Museum in Portsmouth, New Hampshire. The purpose of the survey was to locate the remains of the Pecunies’ house, and more specifically, the location of a mikveh (Jewish ceremonial bath) reputed to have been located in the basement. The sedimentary overburden consists of nearshore sand overlying marine clays disturbed by over three hundred years of continuous occupation. The site was surveyed using a Geophysical Survey Systems Inc. Profiler EMP-400, in vertical dipole moment, measuring EM frequency response at 13khz, 11khz, and 8khz. Three surveys were conducted over two non-sequential days; the first relying on internal WAAS corrected GPS readings over a defined grid area and with low, inline orientation. The second survey repeated the first, with the EMP-400 held in low, broadside orientation, and the third using internal GPS readings and low, inline orientation over a larger area with no pre-defined grid. The orientation of the EMP-400 had an effect on data acquisition speed, slowing the user down significantly, allowing for greater data point density. Data point density averaged approximately 6 points/m². The inphase, quadrature, and conductivity data were contoured with quadrature data proving less useful. Several anomalies are apparent including the root mass of a recently cut ash tree. Although the foundation of the house could not be determined, a deeper anomaly in the southwest corner of the grid may be the remains of the mikveh. In addition, the location of a pottery kiln of the Marshall Pottery, active on the site during the mid-18^th century, may be present in the southeast corner. The northeastern section of the grid contains several additional anomalies which may represent the remains of late-18^th century to mid-20^th century structures.