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Geophysical Methods
The results obtained from geophysical surveys are determined a great deal by how they are measured. Factors such as line (& station) spacing, orientation, arrays, measurement parameters (frequency, sampling, etcetera) and even equipment play a large part in what can be retrieved from data. "The earth is a low pass filter." is often used in geophysics. The literal translation of this is that resolution decreases with depth and larger targets and contrasts are needed for detection at depth. A well designed survey can improve detection and resolution of most any target. Each geophysical method focuses on a particular property.
- EM = conductivity
- RESISTIVITY = resistance
- MAG = susceptibility
- GRAVITY = density
The examples below illustrate some of the types of geophysics and LSGI's work to extract and improve information from the data.
The DC Resistivity method is used to image subsurface resistivity. A curent is applied to the earth and voltages are measured at remote locations. The geometry of the measurement array is usually designed to acheive subsurface imaging based on desired depth of investigation. It is customary to design the array to exceed the desired depth of investigation to ensure proper imaging of the desired target. Many types of arrays can be utilized depending on the desired resulting knowledge from 1D (sounding) to 1D lateral mapping (gradient) to 2D (sections) and 3D (volumes). The measured quantity is an "apparent" resistivity and often contains noise, including current and potential electrode effects as well as array geometrical effects.
Living Sky Geophysics (LSGI) pre-processes resistivity to remove noise, including current and potential electrode effects. The example shows current and potential low level electrode noise removed from a pole-dipole resistivity profile. This eliminates the 'electrode striping' often seen in apparent resistivity data and improves the inverted data fit significantly.
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.
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.
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. Conventional Induction coil recevers measures the time derivative of the magnetic field (dB/dt) or 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 are:
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Accuracy improvements of up to 10 to 20 times that of conventional Induction coil recevers.
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Ability to measure increasingly later time gates, resulting in better definition of highly conductive targets at increasingly greater depths.
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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.
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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.
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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.
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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. (Woods et. al 2009).
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 AutoTau derived from the decays of the airborne dataset. 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 fields consist of magnetic and gravity fields. Surveys are 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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