Geophysics and Mapping. presented by: Stephen Brown

Transcription

1 Geophysics and Mapping presented by: Stephen Brown

2 Recommended book for INIGEMM Geophysics for the mineral exploration geoscientist, by Michael Dentith and Stephen Mudge, Cambridge University Press, 2014.

3 Characterization of Earth Geological view Variations of bulk chemistry Geophysical View Variations in physical properties A particular area of geology may appear: homogeneous to a geologist but heterogeneous to a geophysicst or vice versa

4 Geological and Geophysical Observations The two methods are complementary, but may appear to be contradictory Must resolve differences by looking at the important influences over each observation For example, porosity and and pore fluids don't play a large roce in geological descriptions, yet influence physical properties sensed remotely by geophysics greatly Some geophysical techniques measure a physical property directly, but interpretations of all methods are influenced by contrasts and relative spatial changes Fields of rock physics or petrophysics bridge the gap

5 Categories of Geophysical Methods Gravimetric Magnetic Radiometric Seismic Electromagnetic Satellite remote sensing

6 Gravity and magnetic methods: Detect differences in density and magnetism by measuring variations in the Earth's natural fields Radiometric method: detects variations in natural radioactivity allowing estimates radio-element content of rocks

7 Seismic methods: Detect variation in elastic properties manifested in variations in the speed of sound and the attenuation or dampening of vibrational energy Electromagnetic methods: Low frequency (~ DC): flow of electric current through rock pores estimate conductivity or resistivity High frequency (radar): speed and attenuation of radio waves contrasts in dielectric properties

8 Electrical methods continued: Induce electric current, measure voltage Resistivity surveys: either at nearly DC or higher frequency Measure naturally occurring DC or low frequency voltages Self potential Measure naturally occurring time varying voltages or measure time varying magnetic fields induced by electric current flow Magneto-tellurics

9 Satellite remote sensing Passive imaging of optical reflections in different light wavelength or color bands Transmit radio frequency energy and receive reflections (radar) Can get responses sensitive to variations in vegetation, mineralogy, temperature sensing, dielectric contrasts and attenuation

10 Measurement types & responses Airborne, ground, borehole Natural sources Artificial sources

11 What to plot Magnitude vs gradient

12 Measurement influences Signals from Something else Topography Near surface effects

13 Measurement influences Terrain or topography vertical + side effects

14 Responses Anomaly shape and size

15 Responses asymmetry

16 Responses Measurement of vector fields

17 Responses ambiguity

18 Responses ambiguity

19 Responses ambiguity

20 Responses Ambiguity example Add observations From drilling

21 Interpretation Measurement is sum of everything

22 Combining measurements Combine physics to reduce ambiguity

23 Data analysis Sampling and filtering

24 Data analysis Smoothing and filtering

25 Measurements to maps

26 Measurements to maps Looking for recognizable patterns and features

27 Measurements to maps

28 Measurements to maps

29 Measurements to maps

30 Measurements and maps Careful combination of structural geology, aeromagnetic surveys, and GIS

31 Tiny magnetometer Magnetic anomalies and mineralization

32 Inexpensive Sensor resolution = 1/10 1/250 of anomaly size

33 Design components

34 Breadboard

35 Prototype Ready for testing

36 First test Conclusion: calibration needed!

37 Testing and troubleshooting Developed calibration procedure Found nasty wiring problem and fixed it Tested battery life > 7 hours per charge Tested and adjusted data averaging parameters Wrote software for calibration and data I/O ready for GIS plotting Worked on interpolation and map making

38 It's working now! After much testing with the magnetometer group - Steve, Vicente, Diego, Andrés, Santiago - Finally a survey of old Quito Airport

39 - with some limitations (1) resolution is minimal anomalies must be large (2) magnetometer senses motion best to stand still (3) GPS has location uncertainties, so making maps is tricky