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The problem of datum correction in seismic surveys and the failures associated with civil engineering structures with specific reference to roads and bui ldings has been attributed to lack of understanding of the soil structure among others. This is why this surve y on downhole seismic refraction was conducted from nine borehole locations in Escravos, Nigeria to det ermine the weathered layer characteristics of the a rea. In carrying out this study, the McSeis 160MX instrument was employed in recording data obtained from a se t of hydrophones arranged in a definite manner in eac h of the boreholes. The data were analysed graphica lly by using the seismic refraction interpretation tech niques. Results of interpretation indicate that the velocity of the weathered layer ranged between 454 and 533 m sec -1 with an average value of 495 m sec -1 . The results also revealed that the depth of the weather ed layer ranged between 2.91 and 4.41 m with an average value of 3.68 m. Analysis of the results revealed t hat the depth and velocity of the weathered layer i ncreased downward towards the southern part of the study are a. This is an indication that the weathered layer s loped downward to the south. It is recommended that shots for seismic survey should be located at a depth of 4.41 m to eliminate the effects associated with the low velocity layer. It is also recommended that constru ction of heavy buildings and roads especially bridges should have their foundations laid to a depth of 4.41 m t o enable the infrastructure sit on the consolidated l ayer.

/pdf/downhole-seismic-refraction-survey-of-weathered-layer-5d0wj0nhr6.pdf

Downhole seismic refraction survey of weathered layer characteristics in escravos, nigeria

Sixteen (16) surface-laid-geophones and fourteen (14) downhole-laid-hydrophones experiments were conducted in Upper Flood Plain and in Mangrove Swamp areas respectively in parts of the Niger Delta. The aim is to compare low-velocity-layer (LVL) characteristics in the two environments. The velocity and depth of the weathered layer and those of the consolidated layers were calculated using depth-time plots. The interpreted data showed a substantial variation of the weathered layer thickness and elevation in the two study areas. In the Upper Flood Plain, LVL thickness varies between 2.8 m and 40 m with an average of 21.46 m. In the Mangrove Swamp, the thickness varies between 2.0 m and 5.5 m with an average of 3.40 m. The weathered layer thickness in the Mangrove Swamp is fairly uniform having an average of 3.4 m. This highly variable LVL thickness indicates the necessity of correcting for this layer during seismic reflection processing. A close study of the results reveals a thickening of the weathered layer northwards accompanying the increase in elevation. Elevations in Upper Flood Plain vary between 3.4 m and 156 m with average of 69.90 m; the elevations in Mangrove Swamp vary between-0.10 m (sub-sea) and 0.4 m with an average -0.0043 m implying that the topography of the Mangrove Swampy area is highly variable and undulated. The weathered layer velocity in the Upper flood Plain varies between 313.67 ms-1 and 984 ms-1 with average of 438.80 ms-1. In the Mangrove Swamp, weathered layer velocity varies between 294.5 ms-1 and 863 ms-1 with average of 524.10 ms-1. The average velocities of the underlying consolidate layer in the Upper Flood and Mangrove Swamp are 1753.28 ms-1 and 1603.51 ms-1 respectively depicting a general increase in the velocity with amount of consolidation of the bedrock in the two areas. Analysis of the velocity spectra suggests that low air-blast velocity, of the order of laboratory velocity of sound in air of 343 ms-1 at 20°C resulting from direct blast waves could have been drawn in. For the dry Upper Flood Plain and moist-laden Mangrove Swamp, the air-blast velocities are 346.80 ms-1 and 539.70 ms-1 respectively at 27°C which is the average atmospheric temperature in the Niger Delta. The results of this work compare closely with those of Uko et al., Eze and Okwueze, Enikanselu, Lazzari et al., Ibogboekwe and Ohaegbuchu, and Ofomola who worked on LVL characteristic in different environments in the Niger Delta. The results of this work can used for static correction in seismic processing, for planning and assessing risk for engineering structures, and for groundwater exploration.

https://www.longdom.org/open-access/comparison-of-the-characteristics-of-low-velocity-layer-lvl-in-the-mangrove-swamp-and-in-the-upper-flood-plain-environments-in-the-2381-8719-1000248.pdf

Comparison of the Characteristics of Low Velocity Layer (LVL) in the Mangrove Swamp and in the Upper Flood Plain Environments in the Niger Delta, using Seismic Refraction Methods

Failures associated with civil engineering structures in the study area with specific reference to roads and buildings have been attributed to lack of proper understanding of the weathered layer characteristics. This study was therefore carried out using downhole seismic refraction data from twenty borehole locations within parts of PortHarcourt, Niger Delta, Nigeria to determine the weathered layer characteristics of the area. The survey was carried out with the Geometrics Stratavisor NZ11 instrument using a set of hydrophones arranged in a definite manner in each of the boreholes. The data was processed using Udysys software and analyzed graphically using the seismic refraction interpretation technique. Several contour and analytical maps were generated for the weathered and consolidated layers. Similarly, four interpretative cross sections namely NW-SE, NE-SW, N-S and E-W were used for the purpose of correlation. Results of the study revealed that the velocity of the weathered layer ranged between 144 and 996m/s with a regional average of 407 m/s. The results also revealed that the thickness of the weathered layer ranged between 3.0 and 9.6m with a regional average of 5.0m. Velocity of the consolidated layer varied between 1449 and 1812m/s with a regional average of 1738m/s which is adjudged sufficiently competent to withstand engineering structures. It is therefore recommended that construction of heavy buildings and roads especially bridges in the area should have their foundations laid to a minimum depth of 9.6m to enable the infrastructure sit on the consolidated layer.

/pdf/weathered-layer-determination-and-its-effects-on-engineering-4srfuh9hbu.pdf

Weathered Layer Determination and Its Effects on Engineering Structures: Case Study of Parts of the Niger Delta

Seismic refraction method has been used to map the near surface geological features in parts of the Oru Area. Ten seismic refraction profiles were surveyed within the Area. The instruments used were a 10 kg sledge hammer, Mark 6 digital 12-channel seismograph, Reflexw software was used to process the seismic data. A GPS system was used to obtain the co-ordinates of the profiles. Surfer 11 software was used to produce contours of the seismic velocities within the study Area. This research has established that the distribution of p-wave velocities within the subsurface of this area show a general increase of velocity with depth and velocity varying from 400 m/s to 770 m/s for the weathered layer. The thickness of the weathered layer is on the average 20.4 m. Field mapping shows that the Northern part of the study area rests on Ogwashi-Asaba formation. The area has mudstone, claystone, gritty claystone, carboniferous mudstone, massive sandstone facies whereas Benin formation is found to the southwest from Mgbidi. And also clay deposits are found within the boundary between the weathered layer and the consolidated layer. This research shows the presence of shallow and deep seated aquifers within the area. There are traces or channels of less dense materials which may be conduits for fluid movement within the consolidated layer. These channels suggest a defect in engineering capacity of this layer. Hence cutting and filling may be adopted even at great depths in the study area for the purpose of infrastructural development. The clay deposits within the Area can be exploited for industrial purposes. Also discovered in this research is a Batholitic structure along Nempi-Ibiasoegbe which permits the possibility of quarrying for crushed rocks along the section.

https://www.longdom.org/open-access/geophysical-study-of-the-weathered-and-near-surface-zone-in-parts-of-oru-area-imo-state-nigeria-using-seismic-refraction-method-2381-8719-1000340.pdf

Geophysical Study of the Weathered and Near Surface Zone in Parts of Oru Area, Imo State Nigeria Using Seismic Refraction Method

This work of correlating incidence rates of typhoid fever and cholera to sewage disposal methods was necessitated by the observed occurrence of typhoid fever and cholera incidences with respect to sewage management approaches. The aim of the work was achieved by observations, interviews and medical records of 25 years (1986 to 2010) from two major health facilities of University of Port Harcourt Teaching Hospital (UPTH) and Braithwaite Memorial Hospital (BMH). The purpose of this work is to highlight the typhoid fever and cholera consequences resulting from improper disposal of sewage. In the first sewage management regime where pit latrines, bucket latrine and pier latrines were used, records show incidence rates of 49% typhoid and 51% cholera for UPTH and 53% typhoid and 47% cholera for BMH. The second sewage management regime where water carriage/closet systems of sewage disposal systems and septic tanks were used, the incidence rate of typhoid fever was 94% and cholera was 6% for both UPTH and BMH. This implies evident correlation of incidences of typhoid fever and cholera with sewage disposal methods in time and space. The increases in the incidences of typhoid fever and cholera are majorly underpinned by poverty, poor food hygiene, poor water sanitation, and generally poor environmental sanitation. Water carriage/closet systems of sewage disposal are here recommended to reduce contamination of food, surface and ground water by sewage.

Correlational analysis of sewage disposal methods and incidence rates of typhoid fever and cholera in Port Harcourt Metropolis, Nigeria

Seismic refraction survey was conducted in the south-east Niger Delta at 27 stations using surface-detectors, and estimated near-surface compressional-wave velocities. At each station, a borehole was drilled with casing down to 60-m depth, and determined downhole compressional-wave velocities. Compressional-wave velocities estimated by surface-detectors were then compared with those estimated by downhole survey. The acquisition system includes 0.2kg dynamite, 12-channel-geophone and 11-channel-hydrophone receiver cables for surface-detectors and downhole surveys respectively. The soil layers in the Niger Delta has 2 distinct layers specified as follows, namely, top weathered layer of loose sand sediments with compressional wave velocity ranging from 213 to 763 ms-1 with an average of 503ms-1. Weathered-layer thickness ranges from 2.7 to 16.50m, with an average of 8.1m. Consolidated-layer compressional wave velocity ranges from 1580 to 1906ms-1, with an average of 737ms-1. A vertical velocity gradient of 30.5s-1 was calculated for the area. There exists a 7-18% difference between the velocities determined by the surface-wave and downhole methods. The surface-wave method gives spatially-averaged velocities along the line of traverse coincident with the geophone spread. In the case of downhole seismic method, velocities are specifically applicable only to the borehole location. The implication is that downhole measurements can be adversely affected by the local borehole factors and conditions resulting from casings and excessive collapse around the flushed-zone of the borehole. The spatially-averaged velocities and depths estimated by surface-detector method are more reliable for the site-specific characterization for reflection seismology, geotechnical engineering and groundwater.

https://archive.aessweb.com/index.php/5007/article/download/2265/3519

Comparison of Compressional-wave Velocity-depth Profiles from Surface and Downhole Detectors in the Near Surface in the Southeast Niger Delta, Nigeria

This work focuses on the estimation of lithological and the mineralogical contents of rocks in parts of the Niger Delta basin, Nigeria. Gamma ray, sonic and density logs from oil exploration wells were used to delineate the different lithologies, transit times, porosities and bulk densities at various depths points for three wells in the same location. The results show that shale is denser than sandstone, and matrix densities are higher than bulk densities. The determined matrix densities range between 2.32 g/cm 3 and 2.87 g/cm 3 which compare closely to established lithological and mineralogical standards of the Niger Delta. The significance of this study is that in the absence of core data, the technique used in work can aid in the identification and establishment of lithological and mineralogical contents of rocks, and in the estimation of petrophysical parameters such as acoustic velocity, compaction factor, porosity, permeability, fluid content. The results of this work can be used in reservoir and basin analysis.

E.D. Uko

Papers

Correlational analysis of sewage disposal methods and incidence rates of typhoid fever and cholera in Port Harcourt Metropolis, Nigeria

Comparison of Compressional-wave Velocity-depth Profiles from Surface and Downhole Detectors in the Near Surface in the Southeast Niger Delta, Nigeria

Estimation of lithological and mineralogical contents of rocks from Matrix density in part of Niger Delta Basin Nigeria, using Well-log Data