Geophysical Survey and Subsurface Site Characterisation at a Pre-colonial Settlement in Borgu, Kwara State

Geophysical survey methods offer non-destructive tools for characterising subsurface archaeological features at densely settled or ecologically sensitive sites where conventional excavation would be logistically or ethically constrained. This study presents an integrated geophysical survey of a pre-colonial settlement mound in Borgu, Kwara State, using earth resistance survey, magnetometry, and ground-penetrating radar (GPR) to map subsurface architectural and depositional features before a limited programme of targeted test excavation. The surveyed mound covers approximately two and a half hectares and rises to a maximum height of four and a half metres, with surface ceramics spanning a range consistent with occupation from the early second millennium CE to the early colonial period. Magnetometry survey across the full mound surface identified over thirty dipolar anomalies consistent with fired features including hearths, kilns, and possible iron smelting installations. Earth resistance mapping of the upper mound revealed a rectilinear pattern of high and low resistance anomalies interpretable as a compound enclosure plan with associated internal structures. GPR survey along four transects identified high-amplitude reflectors at approximately one and a half metres depth consistent with mud brick or packed clay architectural surfaces. Targeted test excavation at four anomaly locations confirmed fired clay hearth features, a collapsed adobe wall sequence, and iron slag accumulation consistent with geophysical predictions in three of four cases. The study demonstrates the high information yield of pre-excavation geophysical characterisation and recommends its adoption as standard practice in Nigerian settlement archaeology. Keywords: geophysical survey, magnetometry, GPR, Borgu, pre-colonial settlement.