Near-surface structural model was geostatistically developed from a sample density of 36 Uphole/LVL survey points to determine the properties of the weathering layer geological materials. Results from the generated Isopachs and Isovels show a dominant 2-weathered layer model with 1-weathered layer model in only five locations, mainly along the escarpment. The average thickness of the weathering layer to the first refractor consolidated layer is 18.84 m with an average velocity of 1791.40 ms -1 . Weathering thicknesses ranged from 1.3 to 4.7 m, subweathering thicknesses from 11.4 to 35.7 m; weathering velocities from 119 to 941 ms -1 ; subweathering velocities from 425 to 1665 ms -1 and consolidated layer velocities from 1610 to 2208 ms -1 . Proper application of these results will prove very useful and reliable for the enhancement of optimization processes during stages of seismic data processing and serve as baseline data for future 4D seismic data acquisition for accurate mapping of the deep underlying structures for oil and gas exploration in the North-Central part of the Niger Delta.

/pdf/near-surface-structural-model-for-enhanced-seismic-data-yfwmqa26p4.pdf

Near-surface structural model for enhanced seismic data acquisition and processing in North-Central Niger Delta

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

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

The near-surface anisotropic properties within the South-Western Niger Delta were investigated to determine their impacts on geotechnical constructions. Uphole data analyses reveal a weathered layer that is thickest in the NW, E and SE with an average of 4.2 m and thinnest in the North-Central with a value of 3.8 m; a sub-weathered layer that is thickest in the NW and SW with an average of 32.1 m and thinnest in the SE with a value of 19.0 m and an underlying consolidated layer with great thickness. Vp/Vs values of 1.16265, 1.16279 and 1.16278 were computed for the layers, respectively. Similarly, Poisson’s ratio of -0.9214, -0.9201 and -0.9202 were obtained for the layers respectively. Shear modulus has an average of 4.01 × 108 Nm-2 in the weathered layer, 26.0 × 108 Nm-2 in the sub-weathered layer, and 49.98 × 108 Nm-2 in the consolidated layer. Young’s modulus has an average of 0.63 × 108, 4.11 × 108 and 7.98 × 108 Nm-2 in the layers, respectively. Bulk modulus has an average of 0.74 × 108 Nm-2, 4.62 × 108 Nm-2 and 0.94 × 108 Nm-2 in the layers, respectively. The ultimate bearing capacity, qf has an average of 804.9, 2241.0 and 3349.8 kNm-2 in the layers, respectively. The allowable bearing pressure, qa of the soil has an average of 201.2, 560.2 and 837.4 kNm-2 in the layers, respectively. Core samples collected to a depth of 66 m from 12 wells revealed an admixture of sand, sandy-clay, clayey-sand and dominant clay units. These quantitative results correlate with the standard geotechnical values for clay. The large deposit of about 30 m of clay delineated in this research may act as expansive soils which portends danger for foundations of buildings located in this area and must be avoided.

	Key words:  Anisotropy, uphole, near-surface, clay, geotechnical, Niger Delta.

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Anisotropic properties of the near-surface in South-Western Niger Delta: Implications on geotechnical constructions

Downhole seismic survey was conducted over the lower flood plain of the Central Niger Delta within the Kaiama prospect (OML 28) in an attempt to compute and determine the static correction parameters for the area. Analysis and interpretation of data from 14 downhole locations using NESURVELANA software shows that the thickness of the weathered (low velocity) layer is laterally variable from about 2.6m to 5.8 m with an average of 4.3 m. The weathering velocity ranges from 383 m/s to 985 m/s with an average value of 546 m/s. The consolidated layer below the weathered layer has a velocity ranging between 1646 and 1893 m/s with an average value of 1763 m/s. Knowledge of these computed parameters can be applied in oil and gas exploration and interpretation, also in civil engineering, where the determined LVL provides an estimate of the foundation depth for massive construction works, another area of application is in the exploration and exploitation of groundwater.

/pdf/static-correction-parameters-in-the-lower-flood-plain-of-the-f3ae7ubbbz.pdf

Static correction parameters in the lower flood plain of the central Niger delta, Nigeria

A total of 14 upholes were shot in the Mangrove Swamp of the Niger Delta of Nigeria. The aim of the survey was to determine the thickness and velocity of the low velocity layer (LVL). The velocities and thickness of the layers were computed from the reciprocals of the slopes of the straight-line segments using the FACE Static computer program. Results obtained showed a variation in the thickness of the weathering layer from 2.0m to 5.7m with an average of 3.40m. The variation was random and did not show any trend. The velocity of the low velocity layer ranges between 295ms-1 and 727ms-1 with an average of 562.7 ms-1 while the velocity of the sub-weathering layer ranges between 1502 ms-1 and 1918 ms-1 with an average of 1716 ms-1. These data are important in seismic static corrections and in geotechnical engineering. 
Key words: Thickness, Velocity, Low velocity layer, Uphole 
[Global Jnl Pure & Appl. Sci. Vol.9(3) 2003: 369-374]

https://www.ajol.info/index.php/gjpas/article/download/15942/3294

C. L. Eze

Papers

The velocity-thickness characteristics of the mangrove swamp low velocity layer (LVL), South Central Niger Delta, Nigeria