UNIVERSITY OF TORONTO

Faculty of Arts and Science

DECEMBER EXAMINATIONS 2000

JGP438H1 F

Duration – 2 hours

Non-programmable calculator allowed. Do 5 out of 8 questions. All questions are of equal value.
  1. Explain the Bouger correction for gravity data (purpose and method). Why is gravity data useless without additional geological information? Estimate the peak anomaly and approximate anomaly width over a localized body at depth 1000 m, of volume 106 m3, and density 200 kg/m3 less than the host rock (G = 6.67. 1011 in SI units).
  2. Why are four electrode systems used for resistivity surveys rather than two electrodes? What types of electrodes are desirable and why? Define apparent resistivity (what does it mean?). Sketch and label the Wenner and Sclumberger configurations, showing and labeling electrodes, wires, transmitter and receiver, and relevant distances.

  3. Using a sketch of the transmitted current waveform and of the received voltage waveform, show how the same field configuration can be used to measure both resistivity, IP and SP by indicating which properties of the received waveform are measured. Why do massive conductors have a larger effect on resistivity measurements than disseminated ones, and vice versa for IP (se diagrams if helpful)?
  4. Aeromagnetic surveys produce raw data which, like gravity data, requires corrections and processing before it is really useful. Explain the benefits of and the physical meaning of a) reduction to the pole and b) downward continuation. Why would data from a vertical component flux-gate magnetometer not be as good for locating magnetized sub-surface bodies at the equator as data from a proton magnetometer (use a labeled diagram in your explanation).
  5. Describe the sources available for seismic surveying, and indicate the area of application each would be best suited for (and why, if you know).
  6. How would sedimentary rocks compare with crystalline rocks (in general) with respect to a) seismic velocity b) resistivity c) susceptibility d) IP response e) density. Define terms a) through e). Borehole logs of each of these can be used to aid the interpretation of above-ground geophysical surveys. Explain (in as specific a way as possible) how borehole seismic logs assist seismic reflection interpretation.
  7. Electromagnetic surveys can be done in the time domain or the frequency domain. What does this distinction mean? Which domain is the VLF method? Define in-phase and quadrature EM response, and indicate which of these is best suited for sensing a poor conductor. How would you quantify "poor"?
  8. What is a gamma-ray spectrum, and what information can one get about rock composition by measuring a gamma-ray spectrum? What depth of rock does a gamma-ray scintillometer typically respond to?