Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Falling Creek Ironworks

Magnetic Field Gradient Survey

See also: Summary Page - Resistance Survey - GPR Survey

The results of the magnetic field gradient survey are presented graphically as grayscale shaded maps. In the magnetic survey maps, the light shades represent magnetic field gradient (vertical component of the vector field) lows. The dark shades represent magnetic field gradient highs.

The magnetic survey results are presented using two images, one clipped at +- 0.5 standard deviations (Figure 1), the other clipped at +- 3 standard deviations (Figure 2). Figure 1 displays more subtle magnetic anomalies, while Figure 2 is intended to display and emphasize extremely high amplitude large-scale anomalies. Anomalies of interest are outlined in Figure 1.

Magnetic survey map of Colonial Ironworks
Figure 1

 

High-amplitude magnetic anomalies at Colonial Ironworks
Figure 2

1. A high-amplitude, large wavelength bipolar anomaly centered at 18N, 27E is outlined in red. This anomaly has strong positive and negative components (+280, -220 nT) aligned with magnetic north. The anomaly has a relatively large wavelength in profile, indicative of a greater depth below surface for the anomaly source. The characteristics of this anomaly make it a strong candidate for a large, iron rich, in-situ buried thermo-remanent archaeological feature such as a blast furnace. Note: this anomaly is more readily visible in Figure 2.

2. A large high amplitude anomaly, centered at 25N, 35E is outlined in yellow. This anomalous region is fan shaped, approximately 30 meters wide in the east-west direction and 15 meters in the north-south direction. Magnetic field gradient values range from approximately +225 to -100 nT. The source of this anomaly is likely the slag and charcoal deposit identified during previous investigations at 44CF7 (MacCord, 1964; Higgins, 1995). Note: The high amplitude anomaly centered at 13N, 43E is a modern drainage culvert that runs underneath the access road.

3. Two low-amplitude, large wavelength bipolar anomalies (centered at 34N, 54E and 46N, 84E) are outlined in light blue. They have positive and negative components that are aligned with magnetic north, a relatively strong positive component (+ 30 to 50 nT) to the south and a relatively weak negative component to the north (- 8 to 10 nT). Both anomalies have a large wavelength in profile, making them appear "diffuse". Larger wavelengths are often indicative of a greater depth below surface for the anomaly source. The characteristics of these anomalies make them strong candidates for smaller buried in-situ thermo-remanent archaeological features (bloomeries, chaferies, kilns etc).

4. Two linear magnetic field gradient highs centered at 29N, 72E and 45N, 72E are outlined in dark blue. The southern anomaly is a set of parallel linear features separated by 1-2 meters. These anomalies have maximum amplitudes of approximately 15-20 nT. The source of these anomalous regions may be linear archaeological features (walls, trenches, etc).

5. Numerous high-amplitude, short wavelength bipolar anomalies are outlined in green. These anomalies are aligned to magnetic north and have strong positive and negative components (+-50 to 100 nT). The high amplitude and short wavelength of these anomalies indicate they are probably caused by ferrous iron objects at or close to the ground surface, however their north-south orientation suggests that they might be caused by in-situ thermo-remanent archaeological features.

6. Many other high amplitude bipolar anomalies are present in the magnetic data which appear to be randomly oriented, and high amplitude anomalies which seem to lack bipolarity. Many of these are due to modern iron objects, many of which were observed on the surface. It is possible, however, that some of these are due to archaeological features which have been disturbed, or iron objects associated with the 1619-1622 iron works.

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