Elizabeth L'Heureux

3D Seismic imaging of Massive Sulfides: seismic modeling, data acquisition and processing issues

We present results showing recent trends in the value of modeling studies, acquisition geometry, and processing techniques in seismic imaging of massive sulfides. Forward modeling is valuable in assessing the impact on seismic wave scattering by host rock heterogeneity in delineating exploration targets. Favourable settings for seismic imaging and resolving power for mineral exploration targets are identified. With borehole access, we can take advantage of an acquisition geometry that combines surface and downhole recording for quality control on the velocity field and statics. Improved processing of seismic data demonstrated the usefulness of finding optimal azimuths to characterize the strike, dip and amplitude variations of seismic anomalies. The data used in this paper includes borehole logs and 3D seismic data that were acquired in the Trill area of the Sudbury basin in Ontarion, Canada (Milkereit et al., 2000).

Measuring gravity on ice : An example from Wanapitei Lake, Ontario, Canada

Large lakes have always represented a problem for regional gravity databases; the difficulty of access means gaps or coarse spacing in the sampling. Satellite, airborne, and shipborne gravity techniques are options, but the resolution and/or cost of these systems make them impractical or inaccurate for exploration or environmental studies, where the required resolution is 0.1 mGal/km. In this study, the feasibility of a ground gravity survey over a frozen lake where ice moves because of windy conditions is assessed. Lake Wanapitei, widely accepted as resulting from the impact of a meteorite 37 million years ago, is one of these cases in which the necessity of expanding coverage over poorly sampled regions arose from a significant gap between surface and airborne geophysical maps. Two gravity surveys were completed on the ice of Lake Wanapitei in the winters of 2003 and 2004. To study the structure, longtime series of gravity field measurements were recorded for 98 stations, allowing for improved control over the noise sources in the data. Final processing and integration with an existing regional data set in the area and the application of terrain corrections reduced the amplitude of the circular anomaly from 15 to 9 mGal and its diameter from 11 to 6 km. The feasibility of gravity surveys on ice was assessed, and we determined that for large-scale studies such as this one, the quality of the data, even under noisy conditions, was acceptable. However, for more detailed mapping, calm wind conditions and long time series are required.

Scattering regimes and the influence of heterogeneity on the seismic detection of mineral exploration targets

Summary Frankel and Clayton (1986) describe the effect of Gaussian, exponential and self-similar heterogeneous media on 2D elastic and acoustic wave propagation in crustal scale models. Their results indicate that travel-time anomalies for teleseismic waves are directly correlated with heterogeneity scales, and that these variations become important at scale lengths of tens of kilometers. To investigate these effects in an exploration setting, we used 2D and 3D viscoelastic finite difference models of wave propagation with higher source frequencies and smaller ranges of heterogeneity scales (Bohlen, 2002). The amount of scatter that affects seismic waves as they propagate through the upper crust can be directly related to the scale of heterogeneities within the wave’s path. Traveltime anomalies become non-negligible in the small-angle scattering regime, whereas the influence of back and forward scattering can be seen at much smaller scale lengths. The combined effects serve to mask responses from possible exploration targets embedded in realistic media; when the size of the target is of the same order as the scale length of the background heterogeneity it’s seismic response may be undetectable at the surface even if it represents a strong impedance contrast. In this paper we present the results of a study to assess how well an ideal sulfide-type target in a heterogeneous medium can be imaged, by simulating the seismic response of the target embedded within backgrounds of various degrees of heterogeneity. Motivation comes from 3D seismic data from the Sudbury structure in Ontario, Canada, where several local bright spots within the lower footwall structure indicate possible massive sulfide mineralization (Figure 1). Estimated vertical correlation scales in Sudbury range from less than 1 m to ~3 km (L’Heureux and Milkereit, 2005). The bulk physical property variations used in this study are based on log data from the Sudbury structure.