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:

    (1)

where V_s is the shear-wave velocity of the regolith and f_o is its fundamental resonant-frequency.

Knowing f_o 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 V_s 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 V_s, 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.

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.

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.

http://roundup.amebc.ca/events/geophysics-for-geologists/

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.

BCGS Technical Talk – December 15, 2016

Speaker: Gary Tipper, Skytem

Title: New generation of helicopter time domain electromagnetic systems for mineral exploration in rough terrain

Date/Time: Thursday December 15, 2016 @ 4:30pm

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

Abstract:
Advances in technology for helicopter borne Time Domain Electro-Magnetic survey systems in the last 10 years has allowed the Mining industry the ability to acquire high resolution data in areas historically very difficult to access.

This presentation will focus on some of the past and ongoing technical challenges associated with survey design, drape mode versus contour mode flying, helicopter and pilot performance, technical limitations of past helicopter AEM systems and data quality issues of previous public domain BC AEM surveys.

It will show examples of past system geometry and design, claimed system dipole moments and calculations of Effective Dipole Moments (EDM), based on the wave forms of the different commercial AEM systems, and realistic depth of investigation.

It concludes with details of the new SkyTEM AEM systems for providing high resolution data in these challenging areas and recent case study of a new generation system flying in British Columbia.