Abstract:
Geophysical methods and remote sensing methods have been used in order to delineate the
limestone deposit at Okeluse town in the Eastern part of Ondo state, southwestern Nigeria.
Aeromagnetic data and Landsat 8 OLI image were utilized to explore for any magnetic
signatures and lineaments that are indicative of structures and features that can affect or
displace the limestone formation and ground truthing was carried out using electrical
resistivity method (dipole-dipole profiling and Vertical Electrical Sounding technique) to
delineate the deposit. The magnetic anomaly pattern reflects the geomorphology of the
area as depicted by terrain map of the area, which is characterised by hills and valleys. The
aeromagnetic results indicates that the limestone was deposited in low magnetic intensity
region in form of a graben with residual magnetic intensity values of -0.11 to – 8.89 nT
surrounded by regions in form of hills with relatively high magnetic intensity values of +
0.08 to + 7.35 nT. Euler deconvolution technique using the structural index of 0 and 0.5
revealed the depth of the delineated geologic contact to range between 50 m and 300 m.
Landsat 8 OLI False Colour Composite (FFC) image of bands 257 in RGB was used to
emphasize the limestone region, shape and trend. The observed lineaments occur majorly
in the NE to SW direction indicative of sedimentary trough of down warped and/or faulted
block. Four traverses were established at the two locations where the limestone outcrops
and a total of sixteen (16) VES stations were occupied. The VES results delineated three
geologic layers comprising the topsoil, limestone and shale units. The thickness of the
limestone layer at the two locations varies from 3 to 30 m with an average thickness of
19.5 m. The resistivity of the limestone ranges from 6 to 48 Ohm-m. From the observed
dimensions of the limestone deposit, the estimated probable reserve of the limestone is
843,112,481 metric tonnes. The limestone deposits in Okeluse are not majorly affected by
structures therefore cannot pose any risk during mining as they occur majorly as
overburden materials and can yield an economic profit, except at areas close to an artesian
well and surface water flows.
