Browsing by Author "Kolaj, M."

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Inductive electromagnetic data interpretation using a 3-D distribution of 3-D magnetic or electric dipoles
(Society of Exploration Geophysicists, 2017-05-25) Kolaj, M.; Smith, Richard S.
In inductive electromagnetics, the magnetic field measured in the air at any instant can be considered to be a potential field. As such, we can invert measured magnetic fields (at a fixed time or frequency) for the causative subsurface current system. These currents can be approximated with a 3D subsurface grid of 3D magnetic (closed-loop current) or electric (line current) dipoles whose location and orientation can be solved for using a potential-field-style smooth-model inversion. Because the problem is linear, both inversions can be solved quickly even for large subsurface volumes; and both can be run on a single data set for complementary information. Synthetic studies suggest that for discrete induction dominated targets, the magnetic and electric dipole inversions can be used to determine the center and top edge of the target, respectively. Furthermore, the orientation of plate targets can be estimated from visual examination of the orientations of the 3D vector dipoles and/or using the interpreted location of the center and top edge of the target. In the first field example, ground data from a deep massive sulfide body (mineral exploration target) was inverted and the results were consistent with the conclusions drawn from the synthetic examples and with the existing interpretation of the body (shallow dipping conductor at a depth of approximately 400 m). A second example over a near-surface mine tailing (a near-surface environmental/engineering study) highlighted the strength of being able to invert data using either magnetic or electric dipoles. Although both models were able to fit the data, the electric dipole model was considerably simpler and revealed a southwest−northeast-trending conductive zone. This fast approximate 3D inversion can be used as a starting point for more rigorous interpretation and/or, in some cases, as a stand-alone interpretation tool.
Mapping lateral changes in conductance of a thin sheet using time-domain inductive electromagnetic data
(Society of Exploration Geophysicists, 2013-11-05) Kolaj, M.; Smith, Richard S.
With the inductive electromagnetic geophysical method, the laterally varying conductance of thin sheet models can be estimated either through a direct transform of the measured data or through inversion. The direct transform (called the simplified solution) does not require grid or line data and is simple enough to be performed in the field because the conductance at a location is calculated directly from the ratio of two measured magnetic fields (the vertical spatial and temporal derivative of the vertical magnetic field) at that location. However, the simplified solution assumes that the secondary horizontal magnetic fields are zero and/or that the sheet has a uniform conductance. Our nonapproximate solution (called the full inversion) does not make these assumptions, but requires gridded data, measurements of the secondary horizontal magnetic fields, and more complicated inversion algorithms. Through forward modeling, we found that the full inversion provides better results than the simplified solution when the spatial gradient of the resistance is strong and/or when the horizontal magnetic fields are large. Because the simplified solution may be preferable due to its simplicity, we introduce two unreliability parameters, which assess the unreliability of the conductance calculated using the simplified solution. A comparison of the simplified solution and full inversion in a fixed in-loop survey collected overtop a dry tailings pond in Sudbury, Ontario, Canada, revealed that there were small differences around large conductance contrasts, which coincided with elevated unreliability parameters. The simplified solution is recommended if fast in-field interpretations are required, or additionally, as a first-pass survey that can be performed with sparse station spacing to identify areas of interest. Denser grid data can then be collected, for the more reliable full inversion, over areas of interest and/or zones where the simplified solution is expected to be unreliable as predicted by the unreliability parameters.
A multiple transmitter and receiver electromagnetic system for improved target detection
(Society of Exploration Geophysicists, 2015-06-22) Kolaj, M.; Smith, Richard S.
We have developed an alternative strategy for the inevitable deeper inductive electromagnetic (EM) exploration, which will be required as shallow deposits are exhausted. Rather than using very large magnetic moment ground loops, measurement stations are repeated using many smaller sized loops with smaller moments. The multiple transmitter data are then weighted and summed into a single high signal-to-noise ratio (S/N) composite transmitter. The composite transmitter can be thought of as a postprocessing method that uses the collected multitransmitter data to construct/simulate a transmitter, which maximizes the coupling to a particular target. The appropriate transmitter weights to use will depend on the target location and geometry, and, as such, different weighting schemes allow for the construction of different composite transmitters, each of which will maximally highlight different targets. We have assumed no prior knowledge of the location and orientation of the exploration targets, and we constructed composite transmitters for each possible location of a discretized subsurface and 324 possible target orientations (dipole embedded within a fully resistive medium). A modified difference of squares and a dipole look-up table was used to assess the fit between each composite transmitter and the suggested target location and orientation. Synthetic studies using conductive plate target(s) embedded within a fully resistive medium found that the target locations and orientations could be accurately determined and that the S/N of the composite transmitter was significantly higher than that of standard fixed-loop ground and airborne surveys. In a ground time-domain EM field test, 23 transmitter positions were used, and a shallow target (conductive dike) could be identified using the developed methodology. The composite transmitter data we produced was considerably easier to interpret and had a larger amplitude than that of any one single transmitter.
Robust conductance estimates from spatial and temporal derivatives of borehole electromagnetic data
(Society of Exploration Geophysicists, 2014-05-01) Kolaj, M.; Smith, Richard S.
The conductance of an infinite uniformly conductive thin sheet can be calculated using the ratio of the temporal gradient and the spatial gradient in the normal direction of any component (or combination of components) of the secondary magnetic field. With standard borehole electromagnetic (BHEM) systems, the temporal gradient can either be measured or readily calculated from transient-magnetic-field data, and the spatial gradient in the normal direction can be estimated using adjacent stations. Synthetic modeling demonstrates that, for a finite thin sheet, the magnitude of the field provides a robust and reliable apparent conductance in typical three-component BHEM survey configurations. The accuracy in which the apparent conductance can be calculated is hindered by low spatial gradient signal values and can only be reliably estimated where the fields are large (i.e., in close proximity to the target). In a field example of BHEM data collected over a massive sulfide deposit in Sudbury, Ontario, Canada, the spatial gradient could be calculated over a roughly 100-m-wide zone, and a consistent apparent conductance could be calculated at each delay time using the magnitude of the field. Increases in the apparent conductance with increasing delay time are likely due to currents migrating into more conductive parts of the body. The apparent conductance values were also consistent with Maxwell models and time constant derived conductance estimates. This simple and robust apparent conductance is ideal as a first-pass estimate for target discrimination, grade estimation, and starting values for forward and/or inversion modeling.