We compared three shallow-penetrating EM systems by making surveys along identical<br>grids over a 1940s-era, abandoned, buried bunker (?) at the Denver Federal Center. These<br>systems were the Geophex GEM-2, GSSI GEM-300, and Geonics EM3 1. All three systems<br>detected certain objects, including likely foundations for large tanks, possible underground<br>openings, and present-day electric light poles. A feature of particular interest was a long, narrow<br>anomaly that we think may reflect a buried cement wall. This wall shows up on the quadrature<br>(or quadrature-phase, q-p) component as a relative high at low frequencies and as a relative low<br>at high frequencies. At intermediate frequencies it was invisible on the q-p component. In<br>particular, this includes 9.8 kHz, the operating frequency of the EM3 1. Although the EM3 1 did<br>pick up indications of the wall on its in-phase (i-p) component, the strong signatures of the many<br>bunker features drove its i-p values off scale over much of the survey area, so that they were<br>relatively less useful than the i-p maps from the other two systems. An advantage of the GEM-2<br>and GEM-300 instruments, therefore, is that they can be used to sweep a range of frequencies<br>and more clearly detect features such as the (cement block without rebar?) bunker wall. A<br>disadvantage, however, is that neither GEM-2 nor GEM-300 appear to be well calibrated.<br>Conductivity values from interpretations of their multifrequency data sets, made using a standard<br>EM modeling program, seemed unreasonable. We conclude that the programmable<br>multifrequency systems, such as GEM-2 and GEM-300, have an advantage for reconnaissance of<br>certain archaeological sites, but that better-calibrated systems like the EM3 1 should be used<br>where it is important to know true conductivity values, as in studies of geology, water resources,<br>or ore deposits.


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