**2. Gravity data and regional/residual separation**

The complete Bouguer airborne gravity data of part of lower Benue trough in Nigeria (**Figure 1**) as acquired and corrected by Fugro Airborne Surveys (FAS) in 2010. The data is issued by the Nigerian Geological Survey Agency (NGSA), Abuja office, Nigeria. The survey was at a terrain clearance of 80 m, along NE–SW oriented flight lines with 4000 m flight line spacing and gridded at 500 m grid spacing. The complete Bouguer anomaly map (**Figure 1**) represents geological

#### **Figure 1.**

*Complete Bouguer gravity anomaly map. Gboko as well as the basement outcrops east of Ogoja are shown. Intrusions and locations of the anticline and syncline are superimposed.*

## **Figure 2.**

*Regional component of the complete Bouguer gravity anomaly map. Gboko as well as the basement outcrops east of Ogoja are shown. Intrusions and locations of the anticline and syncline are superimposed.*

### **Figure 3.**

*Residual component of the complete Bouguer gravity anomaly map. Gboko as well as the basement outcrops east of Ogoja are shown. Intrusions and locations of the anticline and syncline are superimposed.*

information from both deeper crust and shallower sedimentary thicknesses. In order to outline between the two, regional/residual separation is paramount. The regional anomaly (**Figure 2**) with gravity values between less than 31.8 mGal and 37.8 mGal could be interpreted as gravity information due geological formations in the lower crust possibly to the mantle or even lithosphere. The anomaly map (**Figure 2**) is the result of the Gaussian regional filter (low pass) at 50 km cut-off wavelength in MAGMAP tool of Oasis montaj. The other component of the anomaly (residual anomaly) map (**Figure 3**) which is of prime importance to applied geophysicists is obtained by subtracting the regional gravity anomaly (**Figure 3**) from the complete Bouguer anomaly (**Figure 1**) using Isostatic tool menu. This anomaly map (**Figure 3**) shows shorter wavelengths anomalies that can account for varying depth sedimentary basins (lower gravity values) and probably volcanic structures within the sediments (higher gravity values).
