Gilles Bellefleur

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.

Application of self-correcting tomographic inversion to a borehole radar test survey

Variations of transmitter power and instrumental time drift often observed during borehole radar surveys are not usually monitored by the commercially-available acquisition systems. These variations may create artifacts in tomograms and lead to erroneous interpretation if not taken into account. The Self- Correcting Tomographic Inversion (SCTI) is a technique that jointly recovers these source variations together with the velocity or attenuation distribution. It assumes that the transmitting time To and the source strength Ao may be considered constant only at each transmitter position. The problem results in a linear system of equations where the usual Jacobian matrix should be augmented by sparse columns with non-null elements corresponding to the respective transmitter positions only; thus the parameter vector (slowness or attenuation coefficient distribution) can be appended with the to (or log Ao) values for these transmitter positions. Synthetic and survey data examples demonstrate that the conventional inversion algorithm produces artifacts mainly located along the transmitter and the receiver boreholes and towards the corners of the tomogram. The magnitude of the artifacts depends on the distance between transmitter and receiver boreholes. The SCTI technique reduces the amplitude of these artifacts while recovering the transmitter drift. Two crosshole surveys with inter-changed transmitter-receiver positions were also performed to evaluate reciprocity. The resulted tomograms are slightly different, but the overall images seem to be improved. However, the SCTI seem to diminish the discrepancy between the reciprocal values. Meanwhile, the SCTI also recovers a very suitable variation for the transmitter parameters. We have also attempted to monitor the drift of these transmitter parameters by control measurements at the ground surface at different times during the survey with different antenna separations. It shows that the variation of to and Ao is of the same order as resulted from the SCTI method.

Corrected GPR velocity and attenuation tomography of artifacts due to media anisotropy, borehole trajectory error, and instrumental drifts

Using standard inversion algorithm, velocity and attenuation tomograms can show artifacts which compromise interpretation. These artifacts can be due to errors in borehole trajectory measurements, medium anisotropy, T0 (initial time) or A0 (initial amplitude) drifts. In order to cancel these artifacts, the error sources can be introduced as unknown parameters in inversion algorithms (Hollender, 1999). In this paper, we present results obtained with crosshole radar data, recorded in a limestone quarry. Using the appropriate algorithms, all the artifacts have been cancelled and tomograms show clearly subhorizontal structures in agreement with the quarry stratification. In our data set, results do not reveal significant trajectory error, and T0 and A0 drifts are low. However, the presence of a velocity and attenuation anisotropy appears clearly on the tomograms. In the case of attenuation tomograms, the high anisotropy rates could be explained by the cumulative effect of the partitioning of energy due to reflection and transmission mechanisms at interfaces, and medium anisotropy.

Potential pitfalls in amplitude inversion revealed by overlapping crosshole radar surveys

Crosshole radar measurements were taken in 4 vertical boreholes aligned on a N-S profile. During the survey, the receiver was kept in one borehole at the extremity of the profile while the transmitter was moved along the 3 other boreholes. Using this overlapping survey geometry, several first arrival amplitudes at different transmission distances were measured for specific raypath angles. These amplitudes depend on the traveled distances and raypath angles. Several factors, such as the antenna radiation pattern, partitioning of energy at interfaces, and anisotropy of the media, explain the variations of the first arrival amplitudes with raypath angles. The antenna radiation pattern and anisotropy can be taken into account prior to, or during tomographic procedures. However, effects from partitioning at interfaces and residual amplitudes from the theoretical antenna radiation function used in data reduction, may be misleadingly introduced into a tomographic algorithm. Owing to the overlapping geometry, a correction combining these 2 factors is statistically estimated and applied to the data. The direct arrival attenuation tomograms obtained from the corrected data are more representative of the local geology.

SEISMIC MODELING OF HETEROGENEITY SCALES OF GAS HYDRATE RESERVOIRS

The presence of gas hydrates in permafrost regions has been confirmed by core samples recovered from the Mallik gas hydrate research wells located within Mackenzie Delta in the Northwest Territories of Canada. Strong vertical variations of compressional and shear velocities and weak surface seismic expressions of gas hydrates indicate that lithological heterogeneities control the lateral distribution of gas hydrates. Seismic scattering studies predict that typical horizontal scales and strong velocity contrasts due to gas hydrate concentration will generate strong forward scattering, leaving only weak energy to be captured by surface receivers. In order to understand the distribution of gas hydrates and the scattering effects on seismic waves, heterogeneous petrophysical reservoir models were constructed based on the P-wave and S-wave velocity logs. Random models with pre-determined heterogeneity scales can also be used to simulate permafrost interval as well as sediments without hydrates. Using the established relationship between hydrate concentration and P-wave velocity, we found that gas hydrate volume content can be determined by correlation length and Hurst number. Using the Hurst number obtained from Mallik 2L-38, and the correlation length estimated from acoustic impedance inversion, gas hydrate volume fraction in Mallik area was estimated to be 17%, approximately 7x10 m free gas stored in a hydrate bearing interval with 250,000 m lateral extension and 100 m depth. Simulations of seismic wave propagation in randomly heterogeneous models demonstrate energy loss due to scattering. With the available modeling algorithm, the impact of heterogeneity scales on seismic scattering and optimum acquisition geometries will be investigated in future studies.