Nyakno J. George

Assessment of Textural Variational Pattern and Electrical Conduction of Economic and Accessible Quaternary Hydrolithofacies via Geoelectric and Laboratory Methods in SE Nigeria: A Case Study of Select Locations in Akwa Ibom State

Textural variational pattern of economic and accessible Quaternary aquifer repositories and its conductivity in the south-eastern Nigeria have been assessed through the integration of vertical electrical sounding and laboratory measurements. The results have shown the lithological attributes, pore-water and amount of residual clay minerals in the assumed clean sand; mechanism of charge fixation at the fluid - surface interface; intricate geometry of pores and pore channels; formation’s ability to transmit pore-water and cation exchange capacity.The connections of electrical and hydraulic properties and their distributions have been established. The average interface conductivity contributed by residual clay minerals in assumed clean sands of the aquifer repositories in the study area have been estimated as 30µS/m. Intrinsic average porosity and formation factor have been respectively deduced as 12% and 14.75. Comparing the simulated aquifer formation factor obtained from the observed porosity data with the observed aquifer formation factor, indicates the that study area has 0.5 ≤ a ≤ 0.8 pore geometry factor and 1.5 ≤ m ≤ 2.0 cementation factor as the best fitting values. The interrelations between aquifer parameters have been established through different plots and the aquifer have been empirically proved to be associated with residual clay minerals as the interface conductivity Cq is not equal to zero. The wide ranges of parameters estimated are an indication of variations in grain size. The estimated intrinsic average porosity, formation factor and the average BQv are viable in characterizing the aquifer flow dynamics and contaminant modelling in the associated aquifer sands For low pore geometry factors a (0.2) and low cementation factor m (0.5) the formation factor remains fairly constant. However, marked variability is noticed at higher a (1.0) and m (2.5). Despite the observed variability in formation factors at the indicated porosities, the spatial or geometrical spread of the formation factor remains unchanged in the aquifer units. The Tables for geoelectric and petrophysical parameters and the associated mathematical models generated in this study can be used for groundwater contaminant modelling and simulation of pore space parameters with reasonable accuracy.

Hydrodynamic Implications of Aquifer Quality Index (AQI) and Flow Zone Indicator (FZI) in groundwater abstraction: a case study of coastal hydro-lithofacies in South-eastern Nigeria

We integrated the surface geologically constrained 1-D geoelectric attributes and laboratory analysis of water and hydrogeological cored samples collected from the wells in the coastal area to estimate the effective porosity, permeability and their relations with aquifer quality index (AQI), flow zone indicator (FZI) and normalised porosity index, the ingredients of aquifer dynamics. The cored samples were derived from the economic hydrogeological units and the water associated constants such as density, dynamic viscosity and acceleration due to gravity were utilised to obtain some of the geohydraulic quantities as required in the empirical relations employed. Our main preoccupation is to appraise the hydrodynamic properties which control the pore water abstracted into the wells. The estimated magnitudes of hydrodynamic properties enabled the estimation of two unique hydraulic units called the graded gravelly sands with little or no fines and well graded sands with little or no fines. The area seemed to be predominantly covered by well graded gravelly sand hydraulic unit as about 87% of the groundwater repositories investigated reflects this unit. A specific hydraulic unit has been found to conform to specific flow zone, which are important in groundwater study concerning freshwater-saltwater intrusion. The concept of stratigraphic boundaries in characterising the groundwater repositories do not account for the intra-formational variation of hydrodynamic properties in the study area as evidenced in certain ranges of permeabilities belonging to a unit flow zone indicator. The considered formations, which were all sandy and gravelly in nature, give permeability ranges that conform to the documented ranges in literature and this attests to the workability of the method. The results equally show that highly permeable sands/gravels have low tortuosity and low specific surface area of unit grain volume, which give rise to easy abstraction of water from the geo-pores. The relations between the measured and estimated parameters and the graphic details of hydrodynamic properties can be utilised in contaminant modelling of the hydrogeologic units. Management of coastal groundwater can easily be handled effectively in the single hydraulic unit than geologic units as groundwater flow in hydraulic units can furnish unique and reliable information rather than guessable information about the prime cause of contaminations in a given hydraulic unit. The classification of the hydrolithofacies is significant in the conservation and management of hydrogeologic units in the coastal zone of the study area, which is vulnerable to contaminations.

Prediction of geohydraulic pore pressure gradient differentials for hydrodynamic assessment of hydrogeological units using geophysical and laboratory techniques: a case study of the coastal sector of Akwa Ibom State, Southern Nigeria

We used borehole-constrained geophysical measurements and laboratory analyses of water samples and hydrogeological unit samples to estimate the basic petrophysical parameters required in the Kozeny-Carman-Bear’s equation. This led to the estimation of hydraulic conductivity, permeability and tortuosity. The evaluation of hydraulic pressure gradient differential and the hydraulic pressure differential was possible through Darcy’s law. Our main objective was to assess the effect of hydraulic pressure gradient differential and that of the hydraulic pressure differential on the hydrodynamic coefficients of economically hydrogeologic units of the study area. The specific constants of water such as density, dynamic viscosity and acceleration due to gravity were all employed in estimating some of the parameters as required in the empirical relations used. Graphical relations were used to predict the generic behaviour between permeability and its dependence, and hydraulic pressure gradient and hydraulic pressure differentials respectively. The results of our analyses show that in arenaceous hydrogeologic units like sands characterised by interconnected/communicating pores, hydraulic pressure differential will be high as the thickness of the saturated unit increases—the precondition for high hydrodynamic activity in the saturated medium. Again, in argillaceous materials, the hydraulic pressure gradient differential is high as it is caused by poor geofluid thickness penetration due to little or no communication between pores. This reduces the hydrodynamic coefficients like porosity and permeability in such hydrogeologic units. The observation of these hydraulic energy parameters in hydrogeologic units could be the physical basis for predicting groundwater flow and a guide to designing geofluid flow modelling programmes in saturated subsurface.

A finite integration forward solver and a domain search reconstruction solver for electrical resistivity tomography (ERT)

This paper describes the development of a forward solver and an inverse solver for modelling the resistivity distribution in electrical resistivity tomography. The forward solver is based on the finite integration technique and was developed to simulate and model synthetic resistance data. The inverse solver, the domain search algorithm, reconstructs the resistivity distribution within the subsurface by searching for a model that gives an acceptable fit to the simulated data. To find an optimum model that fits the simulated data, the domain search algorithm searches for the minimum of a multi-variable objective function by using a step-wise refinement approach. The forward and inverse solvers were developed as necessary forward and inversion modelling tools to enable the optimisation of electrode positions in resistivity imaging to be explored. Numerical results from implementing these solvers show that they are successful for simulating and reconstructing the resistivity distribution in electrical resistivity tomography.

Geophysical exploration to estimate the surface conductivity of residual argillaceous bands in the groundwater repositories of coastal sediments of EOLGA, Nigeria

Abstract Electrical geophysical applications exploit a petrophysical relationship governing the electrical properties of rocks/sediments when field data are coupled with laboratory data. Given the robust analytical techniques of electrical method and the interrelationship with laboratory measurements, it seems natural to classify, and hence simplify, the spatially aggregated conductivity information on the basis of rock/sediment lithology. This provides a unique link between lithological sediment/rock parameters and the physical parameters controlling bulk conductivity. In this work vertical electrical sounding (VES) technique employing Schlumberger configuration integrated with sediment and water analysis have been used to determine the conductivity of argillaceous bands of aquifer sands (fine- coarse sands) in Eastern Obolo Local Government Area (EOLGA). The analysis of the data shows that the aquifer systems composing of fine sands, siltstones and coarse sand have bulk and pore-water resistivities ranging from 40.1–2049.4 Ω m (average = 995.18 Ω m) to 2.7–256.9 Ω m (average = 91.2 Ω m) respectively. These ranges respectively correspond to porosity and formation factor of (19.5–40.6%; average = 29.2%) and (7.1–19.7%; average = 12.95%). Within the limit of experimental errors clearly specified in the work, the intrinsic (clay-free) formation factor (Fi) was estimated to be 16.34 while the intrinsic porosity and the conductivity of the pore-scale clay () were respectively estimated to be 20.4% and 3.2679 mS/m. Accounting for this conductivity magnitude of argillaceous bands from bulk conductivity () of aquifer sands makes the aquifer systems in the area to be consistent with Archie’s law that is valid only in clay-free sandy formation. The graphical deductions and contour distribution of parameters realised from data processing could be used to derive input parameters for contaminant migration modelling and to improve the quality of model in the study area.

Estimating the indices of inter-transmissibility magnitude of active surficial hydrogeologic units in Itu, Akwa Ibom State, southern Nigeria

Itu Local Government Area is prone to groundwater contamination occasioned by saltwater due to the geographical location and the aquifer unit inter-transmissibility between the communicating/interconnected pores of the water-bearing units. This inter-transmissibility causes the continuous distribution of contamination among interconnected aquifer repositories. The study integrated the surface electric mapping and laboratory analysis of geologic samples and their corresponding water samples in estimating the transmissibility-dependent petrophysical parameters. The functional relationships between parameters have been determined. Maps of the spatial distribution of the estimated geohydraulic parameters have been drawn. Quantitative links of measured parameters to transmissibility have established. The resulting inferences based on the indices or magnitudes of the parameters measured helped in delineating the directions of water transmission within and across the hydrogeological units under study. The inferred result of permeability serves as a guide in identifying the southern zone of the area under study as having relatively reduced hydraulic pressure gradient differential, while the northern region has on the average, high hydraulic gradient differentials. The results of directions of inter-transmissibility of hydrodynamic properties in aquifer units are promising and capable of increasing the depth of knowledge on groundwater contamination and hence provide substantial input parameters that can enhance groundwater modelling within and near the study location.

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

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 the system 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 aid for 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. The determining 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 resistivity data, mostly when lithostratigraphic data are not available.

Time lapse (4D) and AVO analysis: A case study of Gullfaks field, Northern North Sea

Abstract A 4D seismic or time lapse survey has been used to investigate the amplitude versus offset (AVO) effects on seismic data in order to identify anomalies in the Gullfaks field for three different reservoir intervals namely the Tarbert, Cook and Statfjord reservoirs. Repeatability analysis has shown that the earlier seismic vintages are the most unreliable for amplitude anomaly analysis as normalised root-mean square (NRMS) values are greater than 50%. This is above the threshold of good and medium repeatability. Fluid substitution models show increases in both P-wave velocity and density for increasing water saturations with a maximum change of 7.33% in the P-wave velocity, and this is in line with predictions from previous work using the Biot - Gassman equations. AVO modelling for the top Tarbert Formation interface produced scenarios of increasing amplitudes with offset for the presence of hydrocarbons, which dim out with 100% brine saturation. This correlates to class III gas sands for different situations of varying Poisson’s ratio across an interface, which has been previously modelled. Two anomalies were identified with one being related to increasing pressure due to water injection correlating to poor permeability around injector well 34/10-B-33. The second anomaly is a case of potential unswept hydrocarbons that displayed a consistent bright spot throughout all of the seismic vintages (in-inlines and crosslines). AVO attribute analysis of this event produced a class II anomaly. However, when comparing near and far offset seismic data, dimming effect was observed producing contrasting evidence. The dimming offset is viewed to have been as a result of poor repeatability values at far offsets. The modelling of the fluid contents in the studied formations to conform to existing literatures justifies the efficacy of the method.

Unusual ionospheric variations before the strong Auckland Islands, New Zealand earthquake of 30th September, 2007

Abstract Using the IAP experiment on board, the DEMETER and TEC from GPS data, unusual ionospheric variations have been observed some days before the 7.4 magnitude New Zealand earthquake. Both sets of data recorded perturbations 10 days before the earthquake at about the same time. The total ion density per centimeter cube (cm−3), recorded a variation of 6.94 while the differential total electron content (DTEC) in total electron content unit 1016 electron per metre square gave a value of 2.93TECU. The observed anomalies were screened for false alarm using the geomagnetic indices of Kernnifzer digit (Kp) and disturbance storm time (Dst.) It was however seen that the state of the ionosphere was geomagnetically quiet during this period; hence the observed variations were seismogenic.

Assessment of spatial distrilbution of porosity and aquifer geohydraulic parameters in parts of the Tertiary – Quaternary hydrogeoresource of south-eastern Nigeria

Abstract An integrated attempt exploring information deduced from extensive surface resistivity study in three Local Government Areas of Akwa Ibom State, Nigeria and data from hydrogeological sources obtained from water boreholes have been explored to economically estimate porosity and coefficient of permeability/hydraulic conductivity in parts of the clastic Tertiary – Quaternary sediments of the Niger Delta region. Generally, these parameters are predominantly estimated from empirical analysis of core samples and pumping test data generated from boreholes in the laboratory. However, this analysis is not only costly and time consuming, but also limited in areal coverage. The chosen technique employs surface resistivity data, core samples and pumping test data in order to estimate porosity and aquifer hydraulic parameters (transverse resistance, hydraulic conductivity and transmissivity). In correlating the two sets of results, Porosity and hydraulic conductivity were observed to be more elevated near the riverbanks. Empirical models utilising Archie’s, Waxman-Smits and Kozeny-Carman Bear relations were employed characterising the formation parameters with wonderfully deduced good fits. The effect of surface conduction occasioned by clay usually disregarded or ignored in Archie’s model was estimated to be 2.58 × 10−5 Siemens. This conductance can be used as a corrective factor to the conduction values obtained from Archie’s equation. Interpretation aided measures such as graphs, mathematical models and maps which geared towards realistic conclusions and interrelationship between the porosity and other aquifer parameters were generated. The values of the hydraulic conductivity estimated from Waxman-Smits model was approximately 9.6 × 10−5m/s everywhere. This revelation indicates that there is no pronounced change in the quality of the saturating fluid and the geological formations that serve as aquifers even though the porosities were varying. The deciphered parameter relations can be used to estimate geohydraulic parameters in other locations with little or no borehole data.