A seismic survey in a permafrost environment: challenges in imaging sediment hosted massive sulphide Zn-Pb deposits

Abstract

Summary In this study, seismic data collected in 1998 in the Red Dog mine (Alaska, USA), illustrates the challenges that can occur in acquisition and processing when targeting sulphide ore bodies in a permafrost environment. The region is also unique due to the large quantity of barite. In a permafrost environment, strong surface waves are the main challenge faced when processing. In this paper an fk filter was used to attenuate as much of the surface wave as possible. Strong surface waves, weak high velocity direct waves, and low signal to noise ratios characterize seismic data targeting shallow (200-400m) reflectors in permafrost environments. After reprocessing of the 1998 seismic data an image was obtained for the barite/sulphides that correlated well with the top and bottom of mineralization given from borehole data. Optimal seismic survey design and processing for a 2D/3D survey has been investigated through a 2D seismic modeling study. The modeling study investigated the feasibility of using high resolution seismic techniques to image massive sulphides that are overlain by thick barite in a sediment host rock. Petrophysical parameters for our geologic model, including p-wave velocity and density, were derived from logged borehole data in the Red Dog area. Using a 2D elastic finite difference wave modeling code we were able to generate synthetic high resolution seismic data. The main challenge presented in this type of geologic setting, as mentioned above, is the presence of a strong surface wave that interferes with shallow target reflections. An fk filter can be used to remove surface wave energy provided trace spacing is small enough (4-5m). Additionally, making use of an optimum offset time window to avoid surface and direct waves when processing, has implications for 3D seismic survey design and is an alternative to fk filtering.