April 2017 Technical Talk

BCGS Technical Talk – April 20, 2017

Speaker : Kit Campbell, Campbell & Walker Geophysics Ltd.

Christopher (Kit) Campell’s professional life encompasses more than forty years of diverse experience in all facets of airborne and ground geophysics. Campbell & Walker Geophysics Ltd. emphasizes airborne geophysics program management and QA/QC, mapping and interpretations, forward and inversion modeling and applied geophysical research.

Title: Recent applications of airborne gravity gradiometry in mineral and petroleum applications; examples and lessons learned

Date/Time: Thursday April 20, 2017 @ 4:30pm

Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver

Abstract:

Three examples from two recent helicopter-borne gravity gradiometry surveys are presented: the first for kimberlites in the Slave Province, the second two from a survey flown for Sedex Pb-Zn-Ag mineralization in the Kechika Trough, BC.

An innovative approach to establishing bathymetry control in a portion of the Canadian Shield is discussed.

We then move on to discuss the critical application of terrain corrections as applied in a relatively rugged survey area. Finally, again in a region of rugged terrain, we address the derivation of the tensor components using Fourier and Equivalent Source methods, and discrepancy in results.

March 2017 Technical Talk

BCGS Technical Talk – March 16, 2017

Speaker : Joel Jansen, Anglo American

Title: The HVSR passive seismic method in mineral exploration geophysics and operational geosciences

Date/Time: Thursday March 16, 2017 @ 4:30pm

Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver

Abstract:
Near-surface passive seismic methods can be used in mineral exploration and geotechnical site-investigations to map unconsolidated regolith and thus bedrock topography to depths in excess of 100m. Passive seismic surveys measure ground motions created by ‘passive sources’ (vehicle movements, mill vibrations, etc.) rather than from ‘active sources’ (explosives or mechanical ground strikes) commonly used in conventional reflection-seismic surveying. Because the energy source is weaker, passive seismic measurements are made over tens of minutes rather than tens of seconds in the latter case. Vertical resolution, especially of thin beds, is not as good as for active-source seismic-reflection or ‑refraction surveys, but logistically, passive seismic surveys are much less costly and challenging to undertake and with almost no environmental impact.

Specifically, the Horizontal to Vertical Spectral Ratio (HVSR) method utilises the fact that the unconsolidated regolith between the free-air surface and bedrock typically acts as a waveguide for surface waves. It has been shown that vertically-polarised surface waves (i.e. Rayleigh waves) resonate in the waveguide at specific frequencies while horizontally-polarised surface waves (i.e. Love waves) do not. By taking the ratio of the vertical to horizontal power spectrums and isolating the peak frequency, the thickness (t) of the unconsolidated regolith can be determined using the simple equation:

formula_1    (1)

where Vs is the shear-wave velocity of the regolith and fo is its fundamental resonant-frequency.

Knowing fo in building and bridge construction is of great importance, because if the resonant frequency of the structure matches that of the soil then there is a greater risk of collapse during an earthquake, as seen in Mexico City in 1985.

From a mineral-exploration perspective, the near-surface passive method has an obvious role to play in gravity surveying by providing a thickness-of-cover estimate beneath each station that can then be stripped from the gravity response to give a clearer picture of the true bedrock anomaly. It can also be used to corroborate depth-to-bedrock estimates from other methods, such as EM or resistivity. Where Vs is unknown, it can be estimated by making measurements in areas where the thickness of cover is otherwise known (e.g. next to boreholes).

Alternatively, in areas where the depth to the hard layer is or should be known, the method could be used to detect regions with varying Vs, as might be expected in a water-saturated zone behind a tailings dam.

This presentation expands on the background to the method and discusses several examples.

 

February 2017 Technical Talk

BCGS Technical Talk – February 14, 2017

Speaker : Jan Franke

Dr. Jan Francke has spent his 27-year career working solely with deep GPR systems and applications, mainly in mineral exploration.  His experience spans 86 countries in every environment, ranging from Siberia to Libya.  He has authored dozens of technical papers and book chapters on GPR and conducts workshops on long-range GPR worldwide.

Title: A review of ground penetrating radar applications to mineral exploration

Date/Time: Tuesday February 14, 2017 @ 4:30pm

Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver

Abstract:
The interest in mineral exploration applications for GPR has risen significantly in recent years, due partly to extraordinary claims of performance made by some promoters.  Although all radar systems are governed by the same physics, advances in electronics over the last decade have enabled much deeper penetration than was previously possible, from pocket-sized wireless systems operated by iPhones.

The talk will provide an overview of GPR’s history and principles, as well as its use in mineral exploration.  A review of applications of the technology will highlight successes, with penetration to hundreds of metres in non-polar environments, as well as unexpected failures in seemingly suitable settings.

Also addressed will be recent claims of extreme penetration through any environment using “lased” EM energy or “megawatt” transmitters.  These instruments and their data will be analysed within the context of accepted physics.  The talk will conclude with an overview of the future of deep GPR, including advances such as real-time sampling, novel transmission modulations, instrument miniaturisation, multi-frequency systems, hybrid EM-GPR systems as well as new deployment platforms such as UAVs.

 

AMEBC / BCGS Roundup 2017 Short Course: Geophysics for Geologists

The BCGS Executive are pleased to provide details about the upcoming two-day Geophysics for Geologists short course at Roundup 2017 to be held on January 21 and 22, 2017.

Course Outline:

Day 1 (AM): Induced Polarization and Resistivity

Day 1 (PM): Gravity, Magnetics, and Radiometrics

Day 2 (AM): Electromagnetics and Magnetotellurics

Day 2 (PM): Inversion, Modelling, and Interpretation

The full course outline, list of speakers, individual coarse module summaries, and registration details can be found on the AMEBC Roundup 2017 website.

Geophysics for Geologists

 

KEGS/BCGS Roundup Breakfast 2017

KEGS/BCGS Roundup Breakfast – Tuesday, January 24, 2017

SPEAKER: Dr. Martyn Unsworth, Professor of Geophysics, University of Alberta

TITLE: A geophysical view of mountain building: perspectives from the Andes, Himalaya and Antarctica

DATE/TIME: Tuesday, January 24, 2017

LOCATION: Fairmont Waterfront Hotel, Princess Louisa Room, Vancouver

REGISTRATION: Online at www.kegsonline.org (Deadline Jan 20, 2017)

ABSTRACT:

Geophysical imaging has proven to be a valuable tool in understanding the geological processes that occur within plate boundaries. Magnetotelluric (MT) exploration is especially useful in these studies since it can measure electrical resistivity, a rock parameter which is sensitive to the presence of fluids such as water or partial melt. In this presentation I will describe how MT has been used to study mountain building. In subduction zones, MT has defined the pathways taken by molten rock from the mantle to the surface where it is erupted by volcanoes. It has also detected some of the largest magma bodies on Earth and shown that the melt can have a high water content. The Himalaya and Tibetan Plateau were formed by the collision of the Indian and Asian plates and MT studies have imaged regions of molten rock that are sufficiently weak to flow. A final example will focus on Mount Erebus in Antarctica, where MT data are being used to image the magma system of a volcano which is not located on a plate boundary.

Erebus-084-IMG_5036-ATLAS