Background: Aeromagnetic data of the Ikogosi warm spring region was used to calculate the basal depth of the magnetic layer (Curie point depth) in the region. The warm spring issues from a crossing of fractures from a metasedimentary suite of Effon Psammite formation which form part of the Precambrian basement complex in Nigeria. Method: The adopted computational method transforms the spatial data into the frequency domain and provides a relationship between radially average power spectrum of the magnetic anomalies and the depths to the respective sources. Heat flow density and equivalent depth extent of heat production from radioactive isotopes in the area were also evaluated. Results: The average Curie point depth for the Ikogosi warm spring area is 15.1 ± 0.6 km and centres on the host quartzite rock unit. The computed equivalent depth extent of heat production provides a depth value (14.5 km) which falls within the Curie point depth margin and could indicate change in mineralogy. The low Curie point depth observed at the warm spring source is attributed to magmatic intrusions at depth. This is also evident from the visible older granite intrusion at Ikere -Ado-Ekiti area, with shallow Curie depths (12.37 ± 0.73 km). Conclusions: Results indicate that the area is promising for further geothermal explorations.

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In reply to Geothermal Energy 2:11 comments about possible errors in our recent paper Geothermal Energy 2:6, 1-21 with title 'Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring -Ekiti State, southwestern Nigeria', we show that there are no errors in the published paper. Our choice of 55 × 55 km block dimension slide across the magnetic anomaly map is consistent with the derived Curie point depths. This choice was adopted because of complexity of the geology in the area and the need to sample more data points while preserving the spectral peak. All depth estimates were carefully and thoroughly performed and assessed using tectonic framework, geological and geophysical evidence, heat flow, seismicity, and other independent information.

Aeromagnetic and aeroradiometric data covering parts of southern Bida basin, Nigeria and the surrounding basement rocks were processed and interpreted. The research covered both basin and basement rock sections, and was aimed at determining the geothermal heat flow and radioactive heat characteristics of the survey area. The number of data points used for analysis were 2,937, obtained from the digitization of eleven ½ degrees by ½ degrees contour maps. Data processing methods used in the study include determination of heat from radiometric data, regional-residual separation of the total magnetic intensity data, determination of depth-to-top and depth-to-bottom of magnetic sources and estimation of field scaling exponent using the Fractal technique. The research results gave geothermal heat flow values ranging from 69.167 mWm -2 to 124.821 mWm -2 with an average value of 90.959 mWm -2 and radioactive heat values ranging from 0.91 to 4.53 µW/m 3 with an average value of 2.28 µW/m 3 . Deductions made from the survey are, the field scaling exponent varies linearly with depth of downward continuation and Katakwa is a prospect area for geothermal heat.

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