Magnetic Methods and Results

1. Magnetic Methods and Results Daniel Boesner Wendy Key Geol 692 May 2009

2. Main Points • Introduction • Method Background and Theory • Field Area • Field Methods • Equipment • Data Collection • Processing • Results • Conclusions

3. Method Background and Theory • Magnetometer utilizes the precession of spinning protons to measure the total magnetic intensity of the Earth’s magnetic field. • Values vary geographically due to the dipole nature of the Earth’s magnetic field. • Iron-rich minerals and other metals produce localized perturbances in magnetic field values which are detected by the magnetometer. Breiner, S., 1973 Simple model of the Earth’s magnetic field Proton procession magnetometer

4. Magnetic Contour Map Breiner, S., 1973

5. Iron-rich materials create perturbances in the magnetic field Breiner, S., 1973 Effects of depth on anomaly Breiner, S., 1973

6. Field Area Construction of the Truckee and V-Line Canals began in 1903 and 1904 respectively as part of The Newlands Project which used the combined waters of the Truckee and Carson Rivers to irrigate over 400,000 acres of western Nevada desert (Simond, 1996). http://www.waterhistory.org/histories/reclamation/newland/

7. Field Methods-Equipment • Scintrex Envi-Mag proton procession magnetometer • Roving and base station magneto-meters employed

8. Field Methods- Data Collection • Total field data collected every 0.5 s by the rover and every 2 s by the base station. • Stations were established at specified locations along 5 lines as follows: • Lines 1 and 3 (Length of the V-line Canal and Truckee Canal): 100 m station intervals • Lines 2 & 5 (Re-Evaluations): 25 m intervals • Line 3 (Abandoned Levee): 50 m intervals • Stations were established to facilitate the collection of GPS locations and to allow for preliminary analysis of total field and noise anomalies in the field.

9. Field Methods- Data Collection • To better differentiate anomalies from observable cultural “contaminants” detailed maps were made locating cultural features. • Anomalous areas were identified in the field by plotting total field measurements collected at each station.

10. Processing • Oasis Montaje by Geosoft was used to evaluate the data. • Position data between stations were calculated under the assumption that stations are the end points of a straight line. • Noise values greater than 1 nT were removed in order to eliminate anomalies generated by noise sources. • Data was reduced-to-pole (RTP) using an inclination of 63.5° and a declination of 14.21° (IGRF, NOAA).

11. Noise >1 nT Removed Raw Data RTP Effects of Processing

12. Line 2 Line 5 Lines 1 and 3 Results- V-Line Canal

13. Results- Truckee Canal Line 4

14. Limitations • A variety of surface and subsurface interpretations are possible for each recorded anomaly. • Burger et al. (1992) suggest a distance of 20+ m should be maintained to reduce the effects of cultural “contaminants”.

15. Conclusions • Data anomalies may be unreliable due to the volume of cultural “contaminants” particularly along the Truckee Canal. • Total field measurements along the Truckee Canal are more extreme than those along the V-Line Canal. • Anomalies observed along the V-Line Canal could not be readily attributed to apparent cultural or geologic sources.

16. Questions?

17. References • Burger, R.H., Sheehan, A.F., and Jones, C.H., 1992, Introduction to applied geophysics: Exploring the shallow subsurface: New York, W.W. Norton & Company, 554 p. • Simonds, Wm., J., 1996, The Newlands Project: Third Draft: Bureau of Reclamation History Program <http://www.cityoffernley.org/index.asp?NID=339> (Accessed: 26 April, 2009). • Breiner, S., 1973, Applications manual for portable magnetometers: Sunnyvale, CA, Geometrics, 58.