Processing and Interpretation of Total Field Magnetic Data

1. Processing and interpreting total field magnetic data, Kevin Rim, Montana • Collected three adjacent grids • Grid #1 used for organization- fill in details later • Removing a least squares best-fit regional plane isolates local anomalies of interest and allows merging of our datasets. Raw data (left) contains Earth’s ambient magnetic field represented by the plane in the center image. Thus, we subtract that plane. The image on the right is the local field which includes corrugation correlated with acquisition as well as signals of interest.

2. Decorrugating removes noise associated with acquisition Residual magnetic field Acquisition noise Ready for interpretation • Decorrugation uses a two step application of orthogonal frequency filters. • It is subjective • the interpreter decides when too much signal is being affected by the filtering.

3. Grid #1 • Decorrugated Total Field Intensity in nanotesla • Color contouring limited to +/- 9 nT otherwise hacksaw blade (NE corner) dominates the contour scheme.

4. Some things to think about – I hadn’t noticed the subtle circular character where the dashed circle is before. The arrows point to anomalies possibly associated with that circle. The interior one looks induced (could be metal) the exterior one might be a reasonable hearth target if there’s no surface sources.

5. There are many processing, interpretation, and visualization steps available to enhance total field magnetic data. • The approach is to highlight anomalies with the character of archaeological targets but which do not have obvious surface sources. • Common interpretational techniques include: • Upward continuation: Recalculate the magnetic results as if they were collected over equivalent but more deeply buried sources. • Analytic signal: calculate the square root of the sum of squares of the horizontal and vertical derivatives of the signal. This is a classic edge detection technique. • Vertical gradient: European practitioners commonly measure the vertical gradient of the field rather than the total field. The vertical gradient is easily calculated from the total field.

6. Upward Continuation – smoothing the results and discriminating against surface sources (historic metal debris, horseshoes, etc.). Both images share same color scale. Original data Upward continued 0.25 metersnote lower amplitudes and less high spatial frequency signal

7. Upward Continuation – smoothing the results and discriminating against surface sources (historic metal debris, horseshoes, etc.). Both images share same color scale. Original data Upward continued 0.5 metersnote lower amplitudes and less high spatial frequency signal

8. Upward Continuation – smoothing the results and discriminating against surface sources (historic metal debris, horseshoes, etc.). Both images share same color scale. Original data Upward continued 1.0 metersnote lower amplitudes and less high spatial frequency signal

9. We use these upward continued results for three main reasons: A slight upward continuation of ½ the line spacing during acquisition further reduces noise from acquisition and surface debris Differencing successive upward continuations allows qualitative depth estimations for anomalies without surface sources. Compare various continuations with the signal from surface sources to assess the likelihood and reliability of targeting deeper similar sources.

10. Equivalent Sources: Differencing upward continuations yields qualitative depth estimates Magnetization from upper 0.25 meters Magnetization from 0.25 to 0.5 meters

11. Equivalent Sources: Differencing upward continuations yields qualitative depth estimates Magnetization from 0.5 to 1 meters Magnetization from 1 meter The sources at 1 meter and deeper appear more geological than archaeological

12. Calculating the analytic signal eases target identification and edge detection - it is the square root of the sum of squared horizontal and vertical first derivatives Total field anomaly analytic signal

13. Example - stone ring observed at surface; a mix of variously magnetized glacial erratics Total field anomaly analytic signal

14. How deep could the observed stone ring be buried and still detectable?Easily detectable from beneath 1 meter of flood deposits As if 1 meter deep Surface feature

15. Grid #1 – Analysis (from inspection of various results) Hacksaw blade at surface Observed stone ring Buried stone ring? Similar to a 3,100±40 B.P. hearth 0.8 meters deep in Yellowstone Curious arcuate anomalies Surface or subsurface debris? 1 5 3 2 4 Total field anomaly

16. Grid #2, Total Magnetic Intensity: regional removed and decorrugated On analysis and inspection during filtering some features appear: Circular features Hearth? Can’t tell these from some boulders without excavation Longer wavelength radial array similar to a Yellowstone feature with a ‘furniture rock’ in the center; could be geologic Each should be assessed relative to field observations 1 3 1 2

17. Grid #3, Total Magnetic Intensity: regional removed and decorrugated On analysis and inspection during filtering some features appear: Circular features A piece of metal at/near surface? Hearth? Only excavation would tell but it is the most likely spot on this grid to investigate Each should be assessed relative to field observations. 1 2 3

18. Guidance for Test Units From the Magnetometer’s Perspective Each anomaly has a source whether historic, geological, or archaeological