Cost-Effective Seismic Exploration: 2D Reflection Imaging at the Kylylahti Massive Sulfide Deposit, Finland

Abstract

We show that by using an advanced pre-stack depth imaging algorithm it is possible to retrieve meaningful and robust seismic images with sparse shot points, using only 3–4 source points per kilometer along a seismic profile. Our results encourage the use of 2D seismic reflection profiling as a reconnaissance tool for mineral exploration in areas with limited access for active seismic surveys. We used the seismic data acquired within the COGITO-MIN project comprising two approximately 6 km long seismic reflection profiles at the polymetallic Kylylahti massive sulfide mine site in eastern Finland. The 2D seismic data acquisition utilized both Vibroseis and dynamite sources with 20 m spacing and wireless receivers spaced every 10 m. For both source types, the recorded data show clear first breaks over all offsets and reflectors in the raw shot gathers. The Kylylahti area is characterized by folded and faulted, steeply dipping geological contacts and structures. We discuss post-stack and pre-stack data processing and compare time and depth imaging techniques in this geologically complex Precambrian hardrock area. The seismic reflection profiles show prominent reflectors at 4.5–8 km depth utilizing different migration routines. In the shallow subsurface, steep reflectors are imaged, and within and underneath the known Kylylahti ultramafic body reflectivity is prominent but discontinuous.

Sparse 3D Seismic Imaging in the Kylylahti Mine Area, Eastern Finland: Comparison of Time Versus Depth Approach

“…Data were recorded quasi-continuously (20 h per day) with a 2 ms sampling interval for 30 days. During this period, other components of the COGITO-MIN survey took place, i.e., 2D seismic acquisition along two profiles (Line A and Line B) using both Vibroseis and dynamite sources [28] (Figure 1), and vertical seismic profiling (VSP) in underground boreholes, with dedicated Vibroseis and dynamite sources also on the surface [20]. In order to improve source density and create a sparse 3D active-source survey to benchmark results of seismic interferometry, additional Vibroseis and explosive sources were placed wherever permitted ( Figure 1).…”

“…In the first approach, a final NMO stack was migrated using 3D Stolt migration (PoSTM) with a constant velocity of 5400 m/s (combination of steps 2a and 3 in Table 2). We chose the velocity of 5400 m/s based on processing of COGITO-MIN 2D seismic data [28]. In case of 2D data, this velocity provided better fit between mapped geological contacts at the surface and the migrated reflections.…”

Seismic imaging across fault systems in the Abitibi greenstone belt – an analysis of pre- and post-stack migration approaches in the Chibougamau area, Quebec, Canada

“…The Kirchhoff prestack time or depth migration (PSTM or PSDM) method has also been utilized in crystalline rock environments (e.g., Malehmir et al, 2011;Singh et al, 2019), but computational complexity and the requirement of a detailed velocity model limited the wide application of a PSTM algorithm (Fowler, 1997). In addition, strong scattering of seismic waves, low S/N ratios, and small-scale changes in acoustic impedance within crystalline rock environments rendered both PSTM and PSDM algorithms less popular in a crystalline rock environment (Salisbury et al, 2003;Heinonen et al, 2019;Singh et al, 2019;Braunig et al, 2020). An important, somewhat neglected issue is the effect of survey geometry on processing results and whether it is possible to adjust the processing flow to compensate for underperformance caused by the survey geometry, for example the effect of crooked survey.…”

Editorial for Special Issue “Seismic Methods in Mineral Exploration”

“…In comparison, standard time-domain processing and imaging failed to produce convincing image of the top~1500 m of the subsurface, which was the primary interest of the survey. Heinonen et al [6] show that an advanced pre-stack depth-imaging algorithm based on Fresnel volume migration (FVM) recovers meaningful and robust seismic images from 2D seismic data acquired with sparse shot points. This suggests that 2D seismic reflection profiling with sparse geometry can be used as a reconnaissance tool for mineral exploration in areas with limited access for active seismic surveys.…”

“…This suggests that 2D seismic reflection profiling with sparse geometry can be used as a reconnaissance tool for mineral exploration in areas with limited access for active seismic surveys. Heinonen et al [6] obtained these results by varying the number of shot points used in the depth migration algorithm on two approximately 6 km long seismic reflection profiles, acquired with 20 m source spacing at the polymetallic Kylylahti massive sulfide mine site, Finland. Steep reflectors in the shallow (<500 m) part of the seismic sections are imaged especially well in the FVM migration result and correlate with black schist interlayers with mica schist observed in the geological map.…”