TMI Archaeology

Published on June 2017 | Categories: Documents | Downloads: 29 | Comments: 0 | Views: 356
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Anatomy of Anomalies Total Field Magnetics at a Potential Archaeological Site

Raw and Decorrugated Total Magnetic Field Intensity

The upper image shows the raw total field intensity; dots indicate acquired observations. The lower image shows diurnally corrected and decorrugated results. Horizontal dimensions are meters; the contour interval is 2 nanoteslas (nt).

Analytic Signal of the Total Magnetic Field Intensity

The analytic signal (total gradient) often serves as a convenient method to help locate potential archaeological targets. Here, warm to hot colors mark larger and more likely interesting magnetic sources in the subsurface. The analytic signal, dependent on all three second order directional derivatives, is subject to noise. Distributed sources on the surface could cause any of the spots on the lower image.

Separating Deep and Shallow Equivalent Source Layers

Matched bandpass filtering is an effective way to separate magnetic anomalies arising from different depths. It entails fitting the radially averaged power spectrum of the total field magnetic data with a series of power spectra corresponding to simple equivalent layers at the archaeological site. Often it is the best way to remove the effects of randomly scattered surface debris, particularly if that debris is ferromagnetic. Here, the upper image shows the field from a deep equivalent layer, the lower image is that of the shallow equivalent source; contour interval = 1 nt. Note the effective isolation of the short wavelength signal.

Recommendations and Results – Total Field Magnetics and GPR

The exploration area, in open timber with deadfall, bunchgrass, and some sagebrush contains highly magnetic obsidian boulders distributed on the surface and in the shallow subsurface. The upper set of solid-dashed magenta lines shows the position of a historic road with minor excavation still visible off the map to the east (right). The lower magenta lines outline a near-rectilinear anomaly with high gradient edges. The numbered anomalies indicate recommended positions for test units based on the combined magnetic and GPR results. The results of test units: •Position 1 yielded cultural artifacts •Positions 2, 3, and 4 yielded only boulders. Although each individual anomaly has the character of a boulder with remanent magnetization their concentration and alignment was promising •Position 5 has a fire hearth at about 0.8 meters •Position 6 has a rock concentration dated at 3,090 years.

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