BCGS Technical Talk – February 20, 2019
This month we have two exciting speakers.
- Sarah Devriese, Condor North Consulting
- Shawn Letts, Anglo American plc
Date/Time: Wednesday, February 20, 2019 @ 4:30pm PST
Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver
Title & Abstracts:
Sensitivity-based data reduction of large 3D DC/IP Surveys
Sarah G. R. Devriese*, Robert Ellis, Ken E. Witherly, Condor North Consulting ULC, Geosoft
Survey companies are increasingly favouring large 3D DC resistivity (DC) and induced polarization (IP) surveys to aid in exploration programs. We present a sensitivity-based data winnowing algorithm that can reduce the amount of data from 3D DC/IP surveys to decrease the inversion computation time but retain similar levels of resolution in the final model. The method is based on the sensitivity of the data to the model space and designed to retain equal sensitivity coverage throughout the model space. The algorithm is tested on both synthetic and field examples.
Interpretation and Benefits of Full-Tensor Squid-Magnetometer Data
Shawn Letts1* Joel Jansen1, Louis Polomé3, Anre Vorster2
1Anglo American plc
2De Beers Group of Companies
3Spectrem Air Pty Ltd
Anglo American and De Beers have been supporting the development of low-temperature Super-conducting Quantum Interference Device (SQUID) magnetometers for over 15 years, with most of the research undertaken by the Leibniz Institute of Photonic Technology (IPHT) in Jena, Germany. This partnership lead to the development of a TEM sensor able to measure precisely the late-time step-response of the earth. The principal advantages of SQUID sensors for TEM surveys are: direct measurement of the magnetic field (hence the step-response of a transmitted square-wave), high magnetic field resolution at low frequencies, and a flat frequency response from DC to 10kHz.
Early research also focussed on passive magnetic-field measurements, including the development of a Full-Tensor Magnetic Gradiometer (FTMG). Advantages of measuring the full tensor over the total-field include: higher spatial resolution; a five-fold increase in information about the earth’s magnetic field, including information about magnetic remanence; and insensitivity to diurnal effects.
Standard cryogenic Dewars (cryostats) are vertical cylinders for ease of use, but this shape is not optimal for airborne surveys owing to the high-drag of the vessel. To overcome this drawback, De Beers and Anglo American funded IPHT to develop and construct a horizontal-cylinder cryostat that easily and aerodynamically fits into a fixed-wing pod or a conventional rotary-wing towed bird. The new system was trialed by De Beers over four projects during 2018, totaling ~ 17,000 line-km.
The results of these surveys corroborate the advantages predicted by forward modelling, being increased spatial resolution and a much-improved interpretational capability of the causative geology. The accompanying presents survey results from a highly complex terrain in southern Africa to illustrate the aforementioned advantages of FTMG.