Seventeen Schlumberger vertical electrical soundings (VES) were carried out in parts of the Umuahia area of Nigeria, using a maximum current electrode separation of 1 km. The data were interpreted using a conventional partial curve-matching method to obtain initial model parameters, which were used in a computer program to obtain final parameters. Three soundings were made at existing boreholes for comparison. Aquifer parameters of hydraulic conductivity and transmissivity were obtained by analyzing pumping test data from existing boreholes.The results of the interpretation revealed three distinct geoelectric layers overlying a conductive geoelectric basement. Based on the model obtained, aquifer hydraulicconductivity and transmissivity were calculated. The hydraulic conductivity calculated agreed closely with that determined from pumping test data. The results of the present study also indicate that the entire area is divided into two zones, each of which is homogeneous in hydraulic properties and water quality, but distinct from the other zone. A local groundwater divide between the lmo and Kwalbo drainage systems was also defined.

We conducted a large-offset seismic experiment in the northwestern Gulf of Mexico using largecapacity air guns and digital ocean-bottom seismographs to determine the velocity structure of the sediments, crust, and upper mantle. Five multiple-detector split-profile lines were shot over an area extending from the shallow midshelf south of Galveston to the continental rise just south of the Sigsbee escarpment. The data were of adequate spatial density and quality to allow combined use of near-vertical reflections, wide-angle reflections, and refractions for interpretation. Several techniques were used to obtain velocity-depth functions: (1) conventional constant-velocity-layer analysis, (2) interval velocities from moveouts of the precritical reflections, (3) analysis in the tan-p domain to determine extremal depth bounds, (4) estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions, and (5) forward modeling using two-dimensional (2-D) ray tracing. A sedimentary sequence of nearly constant thickness (13 to 15 km) covers the study area. Various mobilized salt features exist within the sediments in the slope area, ranging from deeply buried layers and diapirs under the inner slope to shallow, thin, allochthonous bodies under the outer slope. In contrast to the sediments, the crust shows considerable variation in thickness, from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf. The transition under the slope, however, is not monotonic but includes thinning to nearly oceanic thickness under the midslope, possibly suggesting an incipient rift that failed to materialize during the opening of the gulf. 2Now at Paper number 6B6309. 0148-0227/88/006B-63 09505.00traces enable us to adopt modem processing and analysis techniques for interpretation. In this paper we will use the term "crust" to refer to the material beneath the Mesozoic and younger sediments and above the mantle. Basement here means the top of the crust. GEOLOGICAL SETrINGThe Gulf of Mexico (Figure 1) is a relatively small sea enclosed by continental and insular land masses except for the Straits of Florida and the Yucatan Channel. The West Florida and the Yucatan platforms rise steeply from the abyssal depths to the east and south, respectively. To the north and west are relatively gentle dips of the Texas-Louisiana slope and the undulating Mexican Ridges. In the central Gulf is the relatively flat basin, the Sigsbee plain, where water depths reach about 3.6 km. The central Gulf basin is underlain by a thick sequence of Jurassic-to-Recent sedimentary rocks overlying oceanic crust [Ewing et al., 1960, 1962; Antoine, 1972]. This sequence of sedimentary rocks can conveniently be subdivided into two major depositional sequences by the middle Cretaceous unconformity (MCU), a prominent erosional surface which represents a major period of nondeposition. The pre-MCU sequence reveals a complex history of the early evolution of the Gulf basin [Buffler, 1984,...

Refraction experiments were conducted over parts of the east‐central Niger Delta in an attempt to provide an interpretation of the region’s weathering structure. The velocity and depth of the weathered layer and the velocity of the refractor were calculated from critically refracted arrivals using flat‐layer models. An interpretation of the data, carried out using the time‐intercept method, shows that the thickness of the low velocity weathered layer in the region is highly variable, from between about 2.9 m and 45.5 m, with a regional average of about 20.0 m. The weathered layer and the refractor beneath it have an average compressional‐wave velocity of about 500.0 m/s and 1732.0 m/s, respectively. The knowledge of this weathering structure can be applied in oil and ground water exploration in the area of the study. It is also expected that this structure could be used by groups interested in civil engineering.

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