Gravity Anomaly Modelling for Deep Crustal Structure Investigation Beneath the Awka-Onitsha Axis, Anambra State

Understanding the deep crustal architecture beneath the Awka-Onitsha axis in Anambra State is fundamental to interpreting the tectonic evolution of the Anambra Basin and its potential for deep-seated mineral and energy resources. This study performs forward and inverse modelling of Bouguer gravity anomaly data to investigate deep crustal structure beneath the Awka-Onitsha corridor. Gravity data were extracted from the Bureau Gravimetrique International database and supplemented with ground-truth measurements at forty-five additional stations along a 90 km north-south profile. Data were processed for free-air, Bouguer, and terrain corrections before generating anomaly maps and profiles. Regional and residual anomaly separation was achieved using polynomial trend surface analysis. Two-dimensional forward modelling was performed using GM-SYS software, with initial models constrained by published seismic refraction and deep borehole data for the region. The modelled crustal section reveals a three-layer crustal structure comprising a sedimentary cover (2.3 to 2.5 g/cc), a crystalline upper crust (2.7 to 2.8 g/cc), and a lower crust (2.9 to 3.0 g/cc). A significant gravity low of negative 25 mGal over the Awka area is modelled as a thick sedimentary prism (up to 5 km) overlying a granitic upper crust. A sharp gravity gradient marks the transitional boundary between the Anambra Basin and the Onitsha Basement High. Moho depth beneath the axis is estimated at 32 to 37 km. These models contribute to a refined understanding of crustal architecture with implications for resource assessment.

Keywords: gravity modelling, Bouguer anomaly, crustal structure, Anambra Basin, Moho depth