Seismic Waveform Tomography for Imaging Complex Fault Zones in the Benue Trough

This study applies seismic waveform tomography to image complex fault zone structure in the central Benue Trough, North Central Nigeria, with the aim of improving the understanding of fault geometry relevant to mineral exploration and structural geology. Fault zones in the Benue Trough control the distribution of barite, lead-zinc, and fluorite mineralisation, but their three-dimensional geometry in the subsurface is poorly constrained by surface mapping or conventional seismic reflection methods due to the steep dip and complex geometry of these structures. This study conducts a seismic tomographic experiment using a grid of 15 shot points and 180 receiver locations covering a 3 by 4 kilometre area centred on a known fault-controlled barite mineralisation zone in Benue State. A 24-kilogram explosive source generates energy recorded by 3-component seismometers at 25-metre spacing. Travel time picks from refracted and reflected arrivals are inverted using FAST first-arrival seismic tomography software. Waveform tomography refinement uses PRONTO acoustic waveform inversion code. Results are compared with surface geological mapping and interpreted in terms of fault zone architecture. Findings reveal two intersecting fault zones characterised by P-wave velocity minima of 1,500 to 2,200 metres per second within their damage zones, contrasting with surrounding rock at 3,000 to 4,500 metres per second. The fault intersection zone at 30 to 60 metres depth corresponds to the highest density of barite vein outcrops at surface. A new north-trending fault splay not visible in surface mapping is imaged at depth, extending to at least 150 metres. The study demonstrates the value of seismic tomography for three-dimensional fault zone characterisation in mineral exploration.

Keywords: seismic waveform tomography, fault zones, Benue Trough, mineral exploration, P-wave velocity.