Nonlinear Seismic Tomography for Characterising Basement Complexity in the Nigerian Shield

This study develops and applies a nonlinear seismic tomography methodology for characterising the velocity structure and complexity of the Nigerian Shield basement, addressing a critical gap in the knowledge of Precambrian crustal architecture in West Africa. The Nigerian Shield encompasses diverse Archean and Proterozoic terranes assembled during the Pan-African orogeny, creating complex basement velocity structure that affects the performance of seismic surveys in basement complex terrain, groundwater exploration accuracy, and mineral exploration targeting. Existing linearised tomography methods inadequately resolve the strong velocity contrasts and small-scale heterogeneity of Nigerian basement conditions. This study develops a nonlinear iterative approach combining ray-based first-arrival tomography with waveform-based refinement, using an adaptive mesh parameterisation that concentrates model parameters in high-gradient zones identified by the ray-based solution. The method is validated on a synthetic Nigerian-terrain-representative model. Field data are acquired using a dense regional seismic refraction network comprising 12 shots recorded by 86 broadband seismometers deployed along a 120-kilometre line traversing migmatite-gneiss, schist, and granite units in Ogun and Kwara States. Resolution analysis uses a checkerboard test and posterior uncertainty quantification. Findings reveal Vp velocity contrasts of 0.8 to 1.4 kilometres per second over lateral distances of 500 to 2,000 metres at basement level, significantly larger than previously estimated from first-arrival modelling alone. Mylonite shear zones show velocities 15 to 20 percent lower than surrounding rocks, creating characteristic tomographic signatures that can be used as exploration guides. The study contributes an original nonlinear tomographic methodology with demonstrated performance advantages for basement complex imaging.

Keywords: nonlinear seismic tomography, Nigerian Shield, basement complexity, velocity heterogeneity, Pan-African orogeny.