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Archaeological Case History with The EM38B

Introduction

The recently introduced EM38B simultaneously measures the electrical conductivity of the terrain (in mS/m), to a depth of about one and a half meters, and the magnetic susceptibility (in ppt) to a depth of one-half meter, essentially yielding two surveys for the price of one. This note describes data taken with this instrument at Pecos National Historic Park, New Mexico, U.S.A.

Site NM11

At this site a pit-dwelling (probably ca 800AD), discovered accidentally, had been confirmed with a 1m x 1m test excavation. The objective of the EM38B survey was to determine whether the dwelling, typically of the order of a few meters in diameter, would be detected as an anomaly in either terrain conductivity or susceptibility. A further objective was to locate the dwelling with respect to the test excavation. Thus, the survey grid consisted of four lines of length 30 m, each centered on the test excavation, and each rotated 45 degrees clockwise from the previous line.

While an anomaly is present in the susceptibility data, in this case the conductivity data clearly shows an essentially circular feature centered at the survey grid origin. There are also indications of a small extension of the anomaly to the east, and possibly of the test excavation just below and to the left of the grid center. Diameter of the circular