LSGI then uses up-to-date RES2DINV and RES3DINV inversion software to transform apparent resistivity into geologically meaningful inverted ("true") resistivity interpretations.

 

LSGI also uses advanced techniques to remove edge effects and display voxel images, sections, plans and selected benches of resistivity (or any type of data) in 2D and 3D.

 

 

 

 

 

 

Large Loop Electromagnetic Surveys  The Athabasca Basin in Northern Saskatchewan has been (and still is) the proving ground for methods aiming to detect and resolve deep conductive targets at depths up to (and exceeding?) 1000 meters. Survey designs and survey equipment are still evolving to improve depth of investigation, target resolution and cost reduction.

 

Living Sky Geophysics (LSGI) uses advanced mathmatical modeling and inversion techniques to interpret ground and airborne EM data surveys. Due to the complexity and non-uniqueness of EM models, interpretations are based on using known geological parameters to constrain models and inversions. Interpretations are also based on Occam's Razor ("the simplest solution is probably the correct one.").

 

Maxwell from EMIT (ElectroMagnetic Imaging Technology at www.electromag.com.au ) is used as the base for EM interpetations. Plate models are fitted with a controlled inversion proces to interpret Fixed Loop and Moving Loop EM data. where possible both X and Z components are used to get a "best fit" over a range of EM channels. For Fixed Loop surveys, oposing loops are inverted simultaneously. Layered Earth Inversions are performed on in-loop soundings using the CSIRO BEOWULF Module with Maxwell EM software.

 

 

 

SQUID EM

The SQUID (Superconducting QUantum Interference Device) consists of a small sensor (typically a couple of cm in size) which becomes a super conductor at low temperatures ~ 69 degrees Kelvin for HTS and ~ 4 degrees Kelvin for LTS applications. The sensor is located within a cryostat and is cooled with liquid nitrogen for HTS SQUIDS and liquid helium for LTS SQUIDS.

SQUID EM sensors have been in service for ground electromagnetic surveys for approximately 10 years, including the CSIRO LANDTEM HTS SQUID system and the proprietary JESSY LTS SQUID sensor developed for Anglo American Plc.

Conventional Induction coil recevers measures the time derivative of the magnetic field resulting from electric currents induced in the ground. With a square transmitter waveform, This measurement of dB/dt is an approximation of "impulse response". By measuring B-field TEM responses, one measures the time-integral of impulse response which is called "step response".

 

                   SQUID B-Field advantages

• Unparalelled acuracy - Sources have cited acurracy improvements of up to 10 to 20 times that of conventional Induction coil recevers.
• Ability to measure increasingly later time gates, resulting in better definition of highly conductive targets at increasingly greater depths. For base metal sulphide targets with high conductivities, the later time gates are crucial to defining deeper zones or targets that are undetected below other less conductive shallow bodies or conductive overburden.
• Preferential attenuation of fast decays - it is easier to observe the response of a good conductor in the presence of a weaker conductor such as a host, overburden or less conductive bedrock feature. The response of a good conductor is observed in a B-field TEM survey earlier in time than it is in an equivalent dB/dt survey which means that it is more likely to be above the noise level.
• The increased accuracy of the measurement may aid in the design of a more focussed EM survey array with a smaller transmitter loop to further reduce the background or layered response. Additionally, the higher accuracy of the resulting data collected with the SQUID sensor will certainly result in more accurate models and interpretations of the data for exploration purposes.

    

SQUID EM surveys offer advantages for both deep and shallow applications for resolution and data quality. (Images from Woods et. al 2009).

 

Airborne EM Surveys  consist of fixed wing or helicopter borne surveys. With some exceptions many of these surveys are brute force detection surveys and require ground follow-up for targeting purposes. These surveys are invaluable for determining conductor trends, extent and conductivity bright spots for focussing gound work. In some instances (depending on the actual response) it is possible to develop targets from the arborne dataset. LSGI uses a calculated Tau derived from the decays of the airborne dataset. the calculated Tau's are manually filtered for spike rejection to produce the Tau maps. Some of the products available are: grids and maps of selected representative channels of the data, Conductivity Tau Maps, 1D Inversions (CDI's), and Model Plate Interpretations with Maxwell (where possible). The example below shows a series of 2D plate models combined into a 3D composite model using VTEM data over a nickel-copper sulphide deposit.

 

 

 

Potential Field Surveys  consist of passive surveying of magnetic and gravity fields. This is most often done on a project scale or on a local ground scale to target specific structural features. Magnetic surveys are often carried out in conjuntion with airborne EM surveys. Analysis of these surveys with advanced mathmatical techniques can provide complementary background information (if not direct targets) on many types of exploration projects. Potential Field analsis techniques use directional xyz derivatives for determining solutions for the portential field source. It is important to have good quality data for these analyses. While noise (sytem or geologic or cultural) can be filterd out, there is a loss of information. Current acquisition technology usually acheives success in keeping noise to a minimum.

 

After determining and possibly adjusting for noise, Living Sky Geophysics (LSGI) uses reduction to pole, euler deconvolution and SED techniques to determine geological contacts and to locate and interpret both direct and inferred structure and structural trends.

 

 

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