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

Brij Singh,Sanna Juurela,Michał Malinowski,Felix Hloušek,Stefan Buske,Michał Chamarczuk,Emilia Koivisto,Olaf Hellwig,Suvi Heinonen

doi:10.3390/min9050305

Abstract

A 10.5 km2 3D seismic survey was acquired over the Kylylahti mine area (Outokumpu mineral district, eastern Finland) as a part of the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project, which aimed at the development of cost-effective geophysical imaging methods for mineral exploration. The cost-effectiveness in our case was related to the fact that an active-source 3D seismic survey was accomplished by using the receiver spread originally designed for a 3D passive survey. The 3D array recorded Vibroseis and dynamite shots from an active-source 2D seismic survey, from a vertical seismic profiling experiment survey, as well as some additional “random” Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey was characterized by irregular shooting geometry and relatively large receiver intervals (50 m). Using this dataset, we evaluate the effectiveness of the standard time-imaging approach (post-stack and pre-stack time migration) compared to depth imaging (standard and specialized Kirchhoff pre-stack depth migration, KPreSDM). Standard time-domain processing and imaging failed to convincingly portray the first ~1500 m of the subsurface, which was the primary interest of the survey. With a standard KPreSDM, we managed to obtain a good image of the base of the Kylylahti formation bordering the extent of the mineralization-hosting Outokumpu assemblage rocks, but otherwise the image was very noisy in the shallower section. The specialized KPreSDM approach (i.e., coherency-based Fresnel volume migration) resulted in a much cleaner image of the shallow, steeply dipping events, as well as some additional deeper reflectors, possibly representing repetition of the contact between the Outokumpu assemblage and the surrounding Kalevian metasediments at depth.

Highlights

Mineral exploration using 2D seismic surveys is more than three decades old [1].The utilization of 3D seismics for deep mineral targeting started close to the beginning of this century [2]and since several successful case studies have demonstrated the potential of 3D seismics to image geologically complex environments (e.g., [3,4,5,6,7,8,9] and the references therein)
We evaluate the effectiveness of the standard time-imaging approach (DMO-post-stack time migration (PoSTM)/PoSTM and Pre-stack time migration (PreSTM)) in comparison to depth imaging using data from a 10.5-km2 3D seismic survey acquired over the Kylylahti mine area, located within the Outokumpu mineral district in eastern Finland
KPreSDMas as compared imaging, Encouraged by compared to to thethe timetime imaging, we we investigated if the specialized form of Kirchhoff Pre-Stack Depth Migration (KPreSDM), i.e., coherency-based FVM (CBFVM)

Summary

Introduction

Mineral exploration using 2D seismic surveys is more than three decades old [1].The utilization of 3D seismics for deep mineral targeting started close to the beginning of this century [2]and since several successful case studies have demonstrated the potential of 3D seismics to image geologically complex environments (e.g., [3,4,5,6,7,8,9] and the references therein). Depth imaging in a hardrock environment is often hampered by the fact that conventional velocity model-building tools like reflection tomography or migration-velocity analysis [11] cannot be adopted due to the missing coherent reflections. They cannot produce accurate velocity models required for migration. There are some recent successful case studies of PreSDM application to image crystalline rocks in 3D (e.g., [12,13]) Those examples employed a specialized variants of Kirchhoff PreSDM called Fresnel volume migration (FVM) [14,15]

Methods

Results

Discussion

Conclusion