Geophysical Investigations of the South-Western Wairarapa Region of New Zealand

<p>A model of the sub-surface structure of the south-western part of the Wairarapa sedimentary basin in central New Zealand was constructed from geophysical data obtained by the author during the period 1990-1991. A total of 122 new gravity observations, eight magnetotelluric (MT) stations and approximately two kilometres of seismic refraction lines were established in the study area. All methods complemented each other in the determination of the final result. The gravity data provided the depth for the basement layer of the basin, and determined the position of a fossil fault in the region of the Wharepapa River channel. The MT and seismic data confirmed the fossil nature of the fault. The results for the boundaries determined by the MT surveys were also shown to agree with the upper layers of the gravity model. The final model presented is of a sedimentary basin with thickness of approximately 2km, with a fossil fault dividing the basin near the vicinity of the Wharepapa River. To the west of the study area is a highly anomalous region which could not be fully modelled in this study because of lack of data in this particular area.</p>

Geophysical Investigations of the South-Western Wairarapa Region of New Zealand

<p>A model of the sub-surface structure of the south-western part of the Wairarapa sedimentary basin in central New Zealand was constructed from geophysical data obtained by the author during the period 1990-1991. A total of 122 new gravity observations, eight magnetotelluric (MT) stations and approximately two kilometres of seismic refraction lines were established in the study area. All methods complemented each other in the determination of the final result. The gravity data provided the depth for the basement layer of the basin, and determined the position of a fossil fault in the region of the Wharepapa River channel. The MT and seismic data confirmed the fossil nature of the fault. The results for the boundaries determined by the MT surveys were also shown to agree with the upper layers of the gravity model. The final model presented is of a sedimentary basin with thickness of approximately 2km, with a fossil fault dividing the basin near the vicinity of the Wharepapa River. To the west of the study area is a highly anomalous region which could not be fully modelled in this study because of lack of data in this particular area.</p>

Hydro-Geoelectrical Study for Groundwater Potential of Lassa And Environs, North Eastern Nigeria

Twenty-two Vertical Electrical Sounding (VES) were carried out to evaluate groundwater potential and aquifer protective capacity of the overburden units using Schlumberger configuration. It was observed that H-curve is the dominant curve type in the study area. The Geo-electro stratigraphic section revealed that the geologic sequence beneath the study area is composed of topsoil, highly weather basement, partly fractured basement and fresh basement. The first layer has an average thickness and resistivity of 1 m and 130 Ωm, respectively. The second layer has an average thickness of 14 m and an average resistivity of 53 Ωm. The third layer is partly fracture basement with an average resistivity of 747 Ωm while at some VES point represents fresh basement. The highly weathered basement and partly fracture basement layer make up the water bearing formation of the area. Dar-Zarrouk parameter revealed that the area under study has protective capacity range from weak to good capacity of the overburden material. Areas that are classified as poor and weak are indicative areas are thus vulnerable to infiltration of leachate and other surface contaminations. The groundwater potential of the area ranges from low to moderate potential; the moderate zone constitutes 86% while the low potential constitutes 14% of the study area. The transverse resistance within the study area ranges from 119.6 to 6983.7 ohms-m2 with an average value of 1024.59 ohms-m2. Hydraulic conductivity values determined from geoelectrical technique range from 3.05 to 38.04 m/day with an average value of 14.86 m/day.

A Comparative Histological Study of Skin in Clarias gariepinus and Oreochromis niloticus

This study aimed to evaluate the comparative of histological figures in Clarias gariepinus and Oreochromis niloticus skin. A total of 18 samples were observed and collected the skin preservation. The hematoxylin-eosin staining performed to determine the histological structure. Results showed that there were different structures in the epidermis and dermis layer between C. gariepinus and O. niloticus skin. Club cells showed dominated in the epidermis layer of C. gariepinus. The pigmented cell showed clearly in the basement layer of C. gariepinus compared to O. niloticus. Stratum compactum in O. niloticus also showed adequate compared to C. gariepinus.

Identification of surface-basement layer distribution of Wangi-Wangi Island based on HVSR method of microtremor data

<span class="jlqj4b"><span>To find out the distribution of the surface-basement layer of Wangi-Wangi Island, it can be done by calculating the surface layer thickness of each measurement point using microtremor data. The surface layer thickness (<em>h</em>) also indicates the position of the basement to the surface. The microtremor recording data used in this study were 47 data. Furthermore, the data were processed using the HVSR method to obtained the soil predominant frequencies which together with the <em>V</em>_s30 values were used to calculated the <em>h </em>values. The results showed that the thickness of the surface layer or sediment layer of Wangi-Wangi Island was in the range of 5.7339 to 241.2557 meters. Based on its distribution, the areas with a thick surface layer are in the areas of Sombu, North Wandoka, Wandoka, South Wandoka, Pada Raya Makmur and Wanci. In the case of a disaster, these areas are thought to have the most potential to experience damage if an earthquake shakes.</span></span>

The Use of Combined Geophysical Survey Methods for Groundwater Investigation in a Typical Basement Complex Terrain: Case Study of Erunmu, Ibadan, Southwest Nigeria

A combined Survey involving the very low frequency electromagnetic (VLF – EM) and Electrical resistivity surveys were carried out in order to appraise the groundwater potential, and locate appropriate positions for sighting boreholes in Erunmu Community, Egbeda local government area, Oyo State, Nigeria. VLF data were obtained along five traverses as the first step in order to locate suitable vertical electrical sounding (VES) stations. Vertical Electrical Soundings using Schlumberger array were thereafter carried out at twenty 20) locations. The integrated interpretation of both data confirms the presence of aquifers, which includes, weathered zone and basement transition/fractures beneath the area, which prior to this investigation have a history of failed boreholes and wells. The resistivity curve types obtained includes H and A which revealed the presence of 3 to 4 subsurface layers consisting of topsoil, the clay, the sandy clay, fractured zone and the highly resistive bedrock. The resulting geo-electric section from the interpretation revealed the Reflection coefficient which ranges from 0.45 – 0.98. The dominated curve type in the area investigated is the H which is typical of basement complex while the A-type is about 20% of the total curves. Hydrogeological, the topsoil is not important because the degree of water saturation in this layer is very low and cannot be utilized for groundwater. The fractured basement layer (which is present in less than 15% of the study area is very relevant in groundwater prospecting; when it is thick enough the layer could support borehole drilling. Areas identified as geological interfaces in the VLF anomaly charts were also confirmed by the interpreted VES data as poor and intermediate zones for groundwater potential in the study area. The significance of this study is such that it will serve as a useful reference for future research efforts in the aspect of basement complex groundwater studies.

TECTONOSTRATIGRAPHIC CHARACTERIZATION OF THE ADJACENT PORTION TO THE SÃO VICENTE EARTHQUAKE EPICENTER IN THE SANTOS BASIN, BRAZIL

ABSTRACT. The São Vicente earthquake, which had occurred in the Santos Basin, approximately 200 km from São Vicente city, São Paulo state in 2008, was well detected in the global network, allowing a good determination of their focal mechanisms and associated nodal planes, sub-horizontal (strike 82◦) and subvertical (strike 341◦). Understanding seismic, geological and geophysical data have been necessary to determine which nodal plane is the earthquake generating fault. Gravity anomaly maps and structural contour maps allowed defining structures on regional scale. It has been observed that the epicenter is located in the high portion of the slope next to a canyon feature and the aligned graben occurrence, the Merluza Graben, coincident with Capricorn Lineament occurrence. The seismic sections have shown that in the direction of the sub-horizontal plane, less significant faults have occurred when compared to the direction of subvertical plane where there have been faults that intersect from the basement layer to the Cenozoic layers, indicating that they were activated or reactivated on neotectonic events. The mapping of a normal antithetic fault to the NS direction quite significant along the data, has suggested that a horizontal trend of this fault plane might bind to the earthquake hypocenter.Keywords: intraplate seismicity, seismic, passive margin. RESUMO. O sismo São Vicente, que ocorreu na Bacia de Santos, a aproximadamente 200 km da cidade de São Vicente (SP) em 2008, foi bem detectado na rede mundial sismográfica, o que possibilitou uma boa determinação de seus mecanismos focais e planos nodais associados, sub-horizontal (strike 82◦) e subvertical (strike 341◦). Para determinar qual plano nodal representa a falha geradora do sismo é necess´ario o entendimento de dados sismológicos, geológicos e geofísicos. Mapas de anomalia gravimétrica e mapas de contorno estrutural permitiram definir estruturas em escala regional. Observa-se que o epicentro está localizado em um alto topográfico na porção do talude próximo a uma feição de cânion e à ocorrência de grabens alinhados, o Graben Merluza, coincidente com a ocorrência do Lineamento Capricórnio. As seções sísmicas mostram que na direção do plano sub-horizontal ocorrem falhas menos expressivas se comparadas à direção do plano subvertical onde ocorrem falhas que interceptam desde o horizonte do embasamento até os horizontes cenozoicos, o que indica que as mesmas foram ativadas ou reativadas em eventos neotectônicos. O mapeamento de uma falha normal antitética de direção NS, bastante expressiva ao longo dos dados, permite supor que uma tendência horizontal desse plano de falha poderia se vincular ao hipocentro do sismo.Palavras-chave: sismicidade intraplaca, sísmica, margem passiva.

VLF-EM and VES: an application to groundwater exploration in a Precambrian basement terrain SW Nigeria

<p>In a hard rock environment with surface or near surface expression of outcrops, the use of VES only becomes difficult due to its associated problems and limitations. However, the use of VLF-EM becomes imperative in mapping basement structures relevant to groundwater development. A total of 2100 VLF-EM data plus selected VES points were covered over 7 traverses with length varying from 3000 to 4500 m. The VLF-EM result mapped basement structures relevant in groundwater development. This is further studied using VES. Results of the investigation indicate that a HKH sounding curve is obtained with a maximum of five subsurface layers comprising: the top soil, clay/sandy clay unit, the fresh basement, fractured basement and the basement bedrock. The fractured basement layer constitutes the main aquifer unit with thickness of 20-25 m. The layer resistivity and thickness ranges are 60-100 ohm-m; 40-80 ohm-m, 1600-2100 ohm-m, 250-420 ohm-m, and 2200-7500 ohm-m; and 0.3-1.2 m, 1.9-2.5 m, 16.5-22.5 m and 22-30 m for the five layers respectively. The geoelectric results compared favorably well with drilling information to above 90%. The yield of the well is 2 L/s with good recharge capability.</p>