• A E Akpan

      Articles written in Journal of Earth System Science

    • Approximate relationship between frequency-dependent skin depth resolved from geoelectromagnetic pedotransfer function and depth of investigation resolved from geoelectrical measurements: A case study of coastal formation, southern Nigeria

      N J George D N Obiora A M Ekanem A E Akpan

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      The task involved in the interpretation of Vertical Electrical Sounding (VES) data is how to get unique results in the absence/limited number of borehole information, which is usually limited to information on the spot. Geological and geochemical mapping of electrical properties are usually limited to direct observations on the surface and therefore, conclusions and extrapolations that can be drawn about thesystem electrical characteristics and possible underlying structures may be masked as geology changes with positions. The electrical resistivity study pedotransfer functions (PTFs) have been linked with the electromagnetic (EM) resolved PTFs at chosen frequencies of skin/penetration depth corresponding to the VES resolved investigation depth in order to determine the local geological attributes of hydrogeological repository in the coastal formation dominated with fine sand. The illustrative application of effective skin depth depicts that effective skin depth has direct relation with the EM response of the local source over the layered earth and thus, can be linked to the direct current earth response functions as an aidfor estimating the optimum depth and electrical parameters through comparative analysis. Though the VES and EM resolved depths of investigation at appropriate effective and theoretical frequencies have wide gaps, diagnostic relations characterising the subsurface depth of interest have been established. Thedetermining factors of skin effect have been found to include frequency/period, resistivity/conductivity, absorption/attenuation coefficient and energy loss factor. The novel diagnostic relations and their corresponding constants between 1-D resistivity data and EM skin depth are robust PTFs necessary for checking the accuracy associated with the non-unique interpretations that characterise the 1-D resistivitydata, mostly when lithostratigraphic data are not available.

    • Electron temperature and ion density perturbations prior to the M6.8 Eastern Honshu, Japan earthquake of July 23, 2008

      J E THOMAS A M EKANEM N J GEORGE A E AKPAN

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      The Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) microsatellite-monitored data (IAP and ISL) were employed in investigating pre- (30 days) and post- (10 days) perturbations in ionospheric parameters associated with the M6.8 Eastern Honshu (Japan) earthquake (EQ) of July 23, 2008. The results constrained by synchronously monitoring geomagnetic indices data: Kernnifzer digit and disturbance storm time (Dst), revealed strong seismic event-induced disturbances three weeks to 5 days before the seismic event. The geomagnetic indices data were used in filtering normal geomagnetic disturbances from the seismic counterparts, thereby constraining the interpretations. The total ion density measured in per cubic centimeter (cm$^{-3}$) recorded variations of 7.90, 4.51, and 5.92 on days-20, -19, and -16, respectively, from the earthquake day during the night time half orbit observations. Contemporaneously, perturbations of 8.81 were observed for electron temperature measured in Kelvin (K) five days afore the earthquake. The geomagnetically quiet state of the ionosphere during the pre-seismic days suggests that the observed disturbances are seismogenic. More researches should be encouraged in this area to deepen their applications in earthquake monitoring and prediction.

      $\bf{Highlights}$

      $\bullet$ Use of DEMETER data for earthquake prediction.

      $\bullet$ Ionospheric plasma parameters used in detecting seismo-genic induced perturbations.

      $\bullet$ Quiet atmospheric geomagnetic perturbations before the earthquake is seismogenically induced.

      $\bullet$ Seismo-genic anomalies are promising for short-term earthquake prediction.

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