The use of magnetotellurics for mineral exploration: an experiment in the Chibougamau region of Quebec

1992 ◽  
Vol 29 (4) ◽  
pp. 621-635 ◽  
Author(s):  
Ping Zhang ◽  
Michel Chouteau
Keyword(s):  
Mineral Exploration ◽  
Three Dimensional ◽  
Central Section ◽  
Southern Boundary ◽  
Near Surface ◽  
Metasedimentary Rocks ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Abitibi Subprovince ◽  
Geophysical Information

Ten magnetotelluric (MT) soundings were recorded in the range 300 Hz to 200 s along an 11 km profile across a long, well-defined, east–west sub vertical conductor in the vicinity of Chibougamau. This conductor had been recognized by the airborne transient electromagnetic technique and confirmed by other ground electromagnetic methods. The primary purpose of the MT survey was to further understand and constrain the structure of the conductor, especially its extent at depth. This study is the first to demonstrate the utility of the MT method for mineral exploration in Abitibi subprovince.The subsurface of the survey area can be roughly divided into a resistive northern section, a conductive central section (containing the conductive sheet), and a resistive southern boundary. Although some data are distorted by static shifts or three-dimensional effects, there is evidence that the data from the central conductive section are relatively free of static distortions. Therefore, taking into account existing geological and geophysical information and with extensive two-dimensional modelling, it is concluded that a conductive sheet with conductivity of 2 S∙m−1 and a width of 25 m should extend to a depth of no more than 750 m; an additional conductive block is required below this sheet. Cores from boreholes have shown that the conductivity of the near-surface sheet is mainly caused by sulfide minerals and graphite. It is also believed that graphite in the metasedimentary rocks under the central section may be responsible for the conductivity at depth.There is a clear boundary in terms of geoelectric characteristics between the northern part of the MT profile which is located in the Optica subprovince and the southern MT profile located in the Abitibi subprovince. Under the Optica subprovince, there are no structures other than a very resistive block with a thickness of at least 20 km. In contrast, conductive layers are found in the upper crust in the western and central parts of the Abitibi subprovince.

Interpretation of 3-D effects in long‐offset transient electromagnetic (LOTEM) soundings in the Münsterland area/Germany

Geophysics ◽  
1992 ◽  
Vol 57 (9) ◽  
pp. 1127-1137 ◽  
Author(s):  
Andreas Hördt ◽  
Vladimir L. Druskin ◽  
Leonid A. Knizhnerman ◽  
Kurt‐Martin Strack
Keyword(s):  
Integral Equation ◽  
Thin Sheet ◽  
Three Dimensional ◽  
Equation Modeling ◽  
Data Sets ◽  
Sign Reversal ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Long Offset ◽  
Geological Situation

The interpretation of long‐offset transient electromagnetic (LOTEM) data is usually based on layered earth models. Effects of lateral conductivity variations are commonly explained qualitatively, because three‐dimensional (3-D) numerical modeling is not readily available for complex geology. One of the first quantitative 3-D interpretations of LOTEM data is carried out using measurements from the Münsterland basin in northern Germany. In this survey area, four data sets show effects of lateral variations including a sign reversal in the measured voltage curve at one site. This sign reversal is a clear indicator of two‐dimensional (2-D) or 3-D conductivity structure, and can be caused by current channeling in a near‐surface conductive body. Our interpretation strategy involves three different 3-D forward modeling programs. A thin‐sheet integral equation modeling routine used with inversion gives a first guess about the location and strike of the anomaly. A volume integral equation program allows models that may be considered possible geological explanations for the conductivity anomaly. A new finite‐difference algorithm permits modeling of much more complex conductivity structures for simulating a realistic geological situation. The final model has the zone of anomalous conductivity aligned below a creek system at the surface. Since the creeks flow along weak zones in this area, the interpretation seems geologically reasonable. The interpreted model also yields a good fit to the data.

scholarly journals Three-Dimensional Stefan Equation for Thermokarst Lake and Talik Geometry Characterization

2021 ◽  
Author(s):  
Noriaki Ohara ◽  
Benjamin M. Jones ◽  
Andrew D. Parsekian ◽  
Kenneth M. Hinkel ◽  
Katsu Yamatani ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Ground Subsidence ◽  
The Arctic ◽  
Permafrost Region ◽  
Boundary Area ◽  
Near Surface ◽  
Thermokarst Lakes ◽  
Thermokarst Lake ◽  
Permafrost Thaw ◽  
Transient Electromagnetic

Abstract. Thermokarst lake dynamics, which plays an essential role in carbon release due to permafrost thaw, is affected by various geomorphological processes. In this study, we derive a three-dimensional (3D) Stefan equation to characterize talik geometry under a hypothetical thermokarst lake in the continuous permafrost region. Using the Euler equation in the calculus of variations, the lower bounds of the talik were determined as an extremum of the functional describing the phase boundary area with a fixed total talik volume. We demonstrate that the semi-ellipsoid geometry of the talik is optimal for minimizing the total permafrost thaw under the lake for a given annual heat supply. The model predicting ellipsoidal talik geometry was verified by talik thickness observations using transient electromagnetic (TEM) soundings in Peatball Lake on the Arctic Coastal Plain (ACP) of Alaska. The lake width-depth ratio of the elliptic talik can characterize the energy flux anisotropy in the permafrost although the lake bathymetry cross section may not be elliptic due to the presence of near-surface ice-rich permafrost. This theory suggests that talik development stabilizes thermokarst lakes by ground subsidence due to permafrost thaw while wind-induced waves and currents are likely responsible for the elongation and orientation of thermokarst lakes in certain regions such as the ACP of northern Alaska.

3D inversion for multipulse airborne transient electromagnetic data

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. E401-E408 ◽  
Author(s):  
Yunhe Liu ◽  
Changchun Yin
Keyword(s):  
Mineral Exploration ◽  
Synthetic Data ◽  
Underground Structures ◽  
Sensitivity Matrix ◽  
Near Surface ◽  
Power Pulse ◽  
Data Inversion ◽  
Transient Electromagnetic ◽  
1D Modeling ◽  
The Time Domain

Multipulse airborne transient electromagnetic (ATEM) systems transmit one high-power pulse and one low-power pulse containing more high-frequency EM signals. Such systems have better near-surface resolutions while maintaining the depth of exploration of other conventional systems. ATEM systems are especially suitable for geologic mapping and mineral exploration. The inversion of multipulse ATEM data has been mainly limited to 1D modeling, which is not suitable for complex underground structures. We have investigated an algorithm for 3D multipulse ATEM data inversion based on direct Gauss-Newton optimization with quite-fast convergence. The forward problems were solved in the frequency-domain based on the secondary scattered electrical field equation, and then the inverse Fourier transform and the convolution with transmitting waveform were applied to calculate the arbitrary waveform response and sensitivity matrix in the time domain. To optimize the number of computations and memory, we further used an EM “footprint” concept in our inversions to reduce the forward model size and sparse the sensitivity matrix. The inversion results of synthetic data showed that our 3D algorithm is very effective for inverting the multipulse data with results combining advantageous resolutions of different transmitting pulses. Finally, we applied our algorithm to invert real survey data obtained at McMurray, Alberta, Canada, to further test its effectiveness.

The effect of near‐surface superparamagnetic material on electromagnetic measurements

Geophysics ◽  
1982 ◽  
Vol 47 (9) ◽  
pp. 1315-1324 ◽  
Author(s):  
G. Buselli
Keyword(s):  
Transient Response ◽  
Soil Cover ◽  
Lateritic Soil ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Electromagnetic Measurements ◽  
Delay Times ◽  
Residual Response ◽  
Made In

The development of an instrument that enables transient electromagnetic (TEM) measurements to be made to voltage levels of 1 μV/A and less has enabled the detection of an anomalous transient response in some areas with lateritic soil cover. This anomalous transient causes apparent resistivity values derived from the measured transient decay to decrease at late delay times in areas where the known geology indicates the values should increase with delay time toward the resistivity value of the basement. The main cause of the anomalous transient has been identified as the response of superparamagnetic material in the lateritic soil cover. Both field and laboratory measurements of the voltage M induced by this transient, show a [Formula: see text] time dependence. This is the same behavior reported previously for magnetic viscosity over a longer time scale. Measurements of magnetic susceptibility of material separated magnetically from soil samples taken at areas where a residual response is measured, show that over a wide temperature range (from −196°C to 590°C) the susceptibility increases with temperature, confirming the presence of superparamagnetic particles. The anomalous transient response is localized to within 3 m of the transmitter loop; it is consequently detected only by loop configurations where the receiver loop is in proximity to the transmitter loop. The effects caused by the presence of a superparamagnetic response within 3 m of the transmitter loop apply to all electromagnetic methods, whether the measurements are made in the time or frequency domain.

Transient electromagnetic response of a three‐dimensional body in a layered earth

Geophysics ◽  
1986 ◽  
Vol 51 (8) ◽  
pp. 1608-1627 ◽  
Author(s):  
Gregory A. Newman ◽  
Gerald W. Hohmann ◽  
Walter L. Anderson
Keyword(s):  
Integral Equation ◽  
Frequency Domain ◽  
Digital Filters ◽  
Three Dimensional ◽  
The Body ◽  
Late Time ◽  
Near Surface ◽  
Decay Spectrum ◽  
Transient Electromagnetic ◽  
Central Loop

The three‐dimensional (3-D) electromagnetic scattering problem is first formulated in the frequency domain in terms of an electric field volume integral equation. Three‐dimensional responses are then Fourier transformed with sine and cosine digital filters or with the decay spectrum. The digital filter technique is applied to a sparsely sampled frequency sounding, which is replaced by a cubic spline interpolating function prior to convolution with the digital filters. Typically, 20 to 40 frequencies at five to eight points per decade are required for an accurate solution. A calculated transient is usually in error after it has decayed more than six orders in magnitude from early to late time. The decay spectrum usually requires ten frequencies for a satisfactory solution. However, the solution using the decay spectrum appears to be less accurate than the solution using the digital filters, particularly after early times. Checks on the 3-D solution include reciprocity and convergence checks in the frequency domain, and a comparison of Fourier‐transformed responses with results from a direct time‐domain integral equation solution. The galvanic response of a 3-D conductor energized by a large rectangular loop is substantial when host currents are strong near the conductor. The more conductive the host, the longer the galvanic responses will persist. Large galvanic responses occur if a 3-D conductor is in contact with a conductive overburden. For a thin vertical dike embedded within a conductive host, the 3-D response is similar in form but differs in magnitude and duration from the 2-D response generated by two infinite line sources positioned parallel to the strike direction of the 2-D structure. We have used the 3-D solution to study the application of the central‐loop method to structural interpretation. The results suggest variations of thickness of conductive overburden and depth to sedimentary structure beneath volcanics can be mapped with one‐dimensional inversion. Successful 1-D inversions of 3-D transient soundings replace a 3-D conductor by a conducting layer at a similar depth. However, other possibilities include reduced thickness and resistivity of the 1-D host containing the body. Many different 1-D models can be fit to a transient sounding over a 3-D structure. Near‐surface, 3-D geologic noise will not permanently contaminate a central‐loop apparent resistivity sounding. The noise is band‐limited in time and eventually vanishes at late times.

Thermomechanical Coupling of a Hyper-viscoelastic Truck Tire and a Pavement Layer and its Impact on Three-dimensional Contact Stresses

2021 ◽  
pp. 036119812110171
Author(s):  
Angeli Jayme ◽  
Imad L. Al-Qadi
Keyword(s):  
Contact Stress ◽  
Contact Area ◽  
Three Dimensional ◽  
Interaction Model ◽  
Rolling Resistance ◽  
Contact Stresses ◽  
Thermomechanical Coupling ◽  
Temperature Profiles ◽  
Near Surface ◽  
Truck Tire

A thermomechanical coupling between a hyper-viscoelastic tire and a representative pavement layer was conducted to assess the effect of various temperature profiles on the mechanical behavior of a rolling truck tire. The two deformable bodies, namely the tire and pavement layer, were subjected to steady-state-uniform and non-uniform temperature profiles to identify the significance of considering temperature as a variable in contact-stress prediction. A myriad of ambient, internal air, and pavement-surface conditions were simulated, along with combinations of applied tire load, tire-inflation pressure, and traveling speed. Analogous to winter, the low temperature profiles induced a smaller tire-pavement contact area that resulted in stress localization. On the other hand, under high temperature conditions during the summer, higher tire deformation resulted in lower contact-stress magnitudes owing to an increase in the tire-pavement contact area. In both conditions, vertical and longitudinal contact stresses are impacted, while transverse contact stresses are relatively less affected. This behavior, however, may change under a non-free-rolling condition, such as braking, accelerating, and cornering. By incorporating temperature into the tire-pavement interaction model, changes in the magnitude and distribution of the three-dimensional contact stresses were manifested. This would have a direct implication on the rolling resistance and near-surface behavior of flexible pavements.

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