Michel Chouteau

3D gravity inversion using a model of parameter covariance

In a three-dimensional (3D) gravity inversion, there is a need to restrain the number of solution by adding constraints. We present an inversion method based on the stochastic approach. It attempts to minimize an objective function consisting of the sum of the data misfit squared norm weighted by the data error covariances and the model squared norm weighted by the parameter covariances. The minimization of the latter allows reconstruction of structures of different shapes. The algorithm also includes depth weighting and may use borehole data. The effect of the selected parameter covariance matrix is illustrated in a comparison of two inversions for a dipping short dyke. These covariances can be determined either a priori using available geological data or experimentally by fitting density variograms. Depth weighting prevents the inversion from yielding undesirable shallow density structures when using ground survey data only; however, it cannot provide a good delineation of the bodies. Using borehole data in addition to surface data improves the determination of the geometry and position of the bodies. For the latter, the depth weighting is replaced by weights that are dependent on the norm of the kernel terms. Density estimates, from outcrops samples or well logging data, can be set fixed where known within the model. Density positivity constraint can also be included when needed. Gravity data from the Blake River Group in the Rouyn-Noranda mining camp (Quebec, Canada) is used to show the performance of the method, with the inclusion of borehole and surface density data.

Estimation of hydraulic conductivity of an unconfined aquifer using cokriging of GPR and hydrostratigraphic data

Abstract Densely sampled geophysical data can supplement hydrogeological data for estimating the spatial distribution of porosity and hydraulic conductivity over an aquifer. A 3D Ground Penetrating Radar (GPR) survey was performed over a shallow unconfined aquifer consisting of a coarse to medium sand sequence overlying an impermeable clay layer. The site is instrumented with piezometers and water levels are frequently monitored. Vertical determination of moisture and granulometry at a resolution of 10 cm were made at a few locations. The GPR reflection times were correlated with piezometric and stratigraphic information; cokriging of both data yields the spatial distribution of the radar velocities within the layers. Porosity and hydraulic conductivities are estimated using the Complex Refractive Index Method (CRIM) and Kozeny–Carman formulations, respectively. A pumping test and a tracer test, both done using a well in the center of the survey zone, provide a measure of the average hydraulic conductivity and its anisotropy. The results from cokriging in the saturated zone show that the estimated parameters agree very well with the measured hydrogeological data. The geometric mean of the porosity is close to the laboratory measurements. The geometric mean of the GPR-derived hydraulic conductivities fits the values obtained from the pumping and tracer tests. The range of estimated hydraulic conductivities is quite large and indicates that flow could be faster or slower than the one predicted from the pumping test in some places. Radar attenuation is also found to be a good indicator of porosity distribution. From the observed (high) GPR attenuations and electrical conductivities of water sampled in the piezometers, porosity is determined using Archies formula. In the vadose zone, moisture content estimated from the GPR velocities using either CRIM or Topp formulations agree well with the ones from the laboratory measurements. Cokriging of the radar reflection times and of the hydrogeological/stratigraphic data leads to an accurate estimate of the radar velocities with a precision and a spatial resolution much higher than the CDP technique. Within the limits of the interpretative models, porosity, saturation and hydraulic conductivities can accurately be estimated with a high spatial resolution over the survey zone.

bh_tomo-a Matlab borehole georadar 2D tomography package

bh_tomo is an open source borehole georadar data processing and ray-based 2D tomography software package developed at the Ecole Polytechnique of Montreal. bh_tomo runs under Matlab version 7.0 and above, and is therefore portable between computer operating systems supported by Matlab. To perform the tomographic inversions, bh_tomo includes an implementation of the classical LSQR algorithm, as well as an implementation of the recent geostatistical inversion scheme developed at the Ecole Polytechnique of Montreal. One important motivation behind the development of bh_tomo was to ease the data processing sequence necessary to perform tomographic inversion of georadar amplitude data, especially when measured between many adjacent boreholes. The software package relies on a mini database and comprises interactive modules to manage, process and interpret the data.

3D stochastic inversion of gravity data using cokriging and cosimulation

A new application has been developed, based on geostatistical techniques of cokriging and conditional simulation, for the 3D inversion of gravity data including geologic constraints. The necessary gravity, density, and gravity-density covariance matrices are estimated using the observed gravity data. Then the densities are cokriged or simulated using the gravity data as the secondary variable. The model allows noise to be included in the observations. The method is applied to two synthetic models: a short dipping dike and a stochastic distribution of densities. Then some geologic information is added as constraints to the cokriging system. The results show the ability of the method to integrate complex a priori information. The survey data of the Matagami mining camp are considered as a case study. The inversion method based on cokriging is applied to the residual anomaly to map the geology through the estimation of the density distribution in this region. The results of the inversion and simulation methods are in good agreement with the surface geology of the survey region.

Integrating TDEM and MT methods for characterization and delineation of the Santa Catarina aquifer (Chalco Sub-Basin, Mexico)

Abstract Magnetotelluric (MT) and time domain electromagnetic (TDEM) surveys were undertaken in the region of Santa Catarina, located in the Chalco Sub-Basin of the Mexico Basin. The objective was to constrain the geometry of the fresh water aquifer and confirm the continuity of the basaltic flows between the volcano and the sedimentary basin. In order to define the stratification at depth with an emphasis on the geometry of the main aquifer, 11 MT and 5 TDEM soundings were recorded along a north–south profile. Interpretation of MT soundings show that the bedrock is located at a depth of at least 800–1000 m. Using TDEM apparent resistivity curves to constrain the high frequency MT data, three main layers were defined overlying the bedrock. These layers are, from the surface to bottom, a 20- to 40-m-thick layer of sands, ash and clay, followed by a very conductive 200-m-thick layer of sand and ash, saturated with highly mineralized water and, finally, a zone with gradually increasing resistivities, corresponding to the main aquifer. The TDEM data, the magnetic transfer functions and the 2D MT model also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This feature is likely to be highly permeable fractured basaltic flows, evidenced also in one of the water wells. To verify the presence of fractured basalts below the volcano ranges, 38 TDEM soundings were collected on the flanks of the Santa Catarina range. Layered models obtained from the TDEM soundings enabled an assessment of a major conductive zone (1–10 Ω m) at depth. Two hypothesis are envisaged and the nature of this zone is attributed either to a clayey layer or to fractured basaltic flows. If the latter possibility is confirmed, this continuous zone could provide a channel by which the water contaminated by the Santa Catarina landfill may leak into the basin.

A dc resistivity and IP borehole survey at the Casa Berardi gold mine in northwestern Quebec

In the spring of 1996, a direct current (dc) resistivity and induced polarization (IP) borehole survey was carried out at the Casa Berardi gold mine in northwestern Quebec to study the spatial extent of the economic disseminated zone of an auriferous quartz vein type orebody. Crosshole pole-pole and single-hole pole-dipole configurations were used to delineate the geometry of the body associated with the Casa Berardi fault system. Since the spatial data sampling was insufficient for 3D inversion, the interpretation has been done using 3D dc and IP forward modeling. Model changes were applied iteratively to match synthetic with field data. Sensitivities provide information on how to alter the models efficiently. Furthermore, they indicate the significant regions of the model, giving evidence on where the model is meaningful. A model study using a simple prismatic block structure is shown to enhance understanding of the physical response associated with the two types of borehole survey. The algorithms used for the interpretation offer a so-called grid-independent electrode positioning technique, which is a helpful modus operandi to significantly facilitate the simulation process. The result is a resistivity and chargeability model that produces the observed physical response and incorporates all known geological a priori information. Particularly, the IP response carries detailed information on the appearance of the orebody, whereas the potential response reflects the resistivity contrast between metavolcanic and metasedimentary rocks at the Casa Berardi fault.

Deep conductivity structure in Abitibi, Canada, using long dipole magnetotelluric measurements

Magnetotelluric data recorded in the period range 10²–2.104s using 100 km telluric dipoles in western Quebec, Canada, yield impedance estimates free from galvanic distortion and static shift. Analysis of the apparent resistivity, phase and magnetic transfer function data favors a regional 1D anisotropic model for the subsurface resistivity structure in the Abitibi Subprovince; lower resistivity direction of the anisotropy is EW, parallel to the main fault system. Joint 1D inversion of both principal impedances indicates a conductivity anisotropy in the upper and middle crust and in the upper mantle. It might originate for all depths from the same tectonic mechanism, mainly a NS shortening followed by an EW dextral shearing in the Archean.

Massive sulphide delineation using borehole radar: tests at the McConnell nickel deposit, Sudbury, Ontario

Abstract Ground-penetrating radar (GPR) surveys were undertaken in three boreholes intersecting the McConnell massive Ni–Cu sulphide deposit near Sudbury, Ontario, Canada. Radar data were acquired using single-hole and crosshole configurations to assess the potential in delineating massive sulphide deposits. Single-hole radargrams acquired with 22 and 60 MHz antennae display reflections from the upper and lower boundaries of the deposit in areas where the contact between the host rock and ore body is sharp. Direct arrival traveltimes selected from crosshole measurements between two shallow boreholes were used for tomographic reconstruction of inter-hole velocities. The crosshole data set also contains late arrival reflections from the ore deposit. The reflections were migrated using the velocity distribution obtained from tomography, to estimate the position of the upper surface of the deposit between the two boreholes. The deposit is imaged over a small area near its intersection with a shallow borehole. The survey geometry could not provide information on the position of the ore body at mid-distance between the boreholes, i.e. in areas of most interest to geologists.

A link between deformation history and the orientation of reflective structures in the 2.68–2.83 Ga Opatica belt of the Canadian Superior Province

True three-dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked. This is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth, dip, and depth of a reflector and the medium velocity. Results from Lithoprobe line 48, located in the Opatica belt of the Archean Superior Province, differentiate reflectors with two distinct orientations, which coincide with the attitudes of two deformational fabrics mapped at surface. Assuming a connection between reflectivity and strain induced by tectonic processes, the reflectors with NNE strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the Opatica belt. Other reflectors, which strike ENE-WSW and dip shallowly to the north and to the south, indicate that most of the reflectors in the southern Opatica and beneath the Abitibi greenstone belt at middle and lower crustal levels formed during a later, approximately N-S shortening event. Mantle reflections previously interpreted as a relict suture of an Archean subduction zone dip to the north at around 30°–45° and are also associated with this N-S event. The distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the Opatica crust was reworked during the N-S shortening event, although a region of the middle and lower crust, characterized by the earlier D1 reflectors, is preserved in the central part of the belt.

Delineating mineralization and imaging the regional structure with magnetotellurics in the region of Chibougamau (Canada)

Tensor magnetotelluric (MT) soundings were carried out to delineate the geometry of an extended shallow conductor detected by airborne electromagnetic method in the mining camp of Chibougamau (Canada). Careful distortion analysis of the impedance tensors from eleven sites indicate that the subsurface is reasonably 2-D within the 0.0025–10s range. The recovered regional impedances are corrected for static shifts employing 2-D inversion for minimum structure. The modeled resistivity cross‐sections show a far more complex conductivity structure than anticipated. The sheet‐like shallow conductor lies near the boundary between the conductive southern section located in the Abitibi subprovince and the resistive northern section located in the Opatica subprovince. It extends to a depth of 250 m. Two other limited conductive zones are imaged respectively between 0.4 km and 3 km and between 1.5 km and 4 km. We propose that the conductors are associated with sulfide mineralization and graphite in shear zones. They ...