SEISMIC REFRACTION INVESTIGATION TO DETERMINE ELASTIC PARAMETERS IN PARTS OF AKAMKPA, CROSS RIVER STATE 

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ABSTRACT

This study is aimed at using seismic seismic refraction method to determine the depth, thickness and elastic parameters in part of Oban-massif basement complex in Akamkpawhich lies between latitude 5015′ – 5018′N and longitude 8020′ – 8023′E. The refraction survey was carried out using a 12- channel signal enhancement seismograph, geophones, a 9kg sledgehammer and baseplate. The data was acquired from two locations; Akamkpa headquarter and Cross River Estate Limited (CREL). The shear wave velocities and primary wave velocities for both locations were obtained from the arrival travel time read from the velocity data. A standard velocity values for various geologic materials and mathematical relationship were used to determine the depth, thickness, lithology and elastic parameters.

CHAPTER ONE

INTRODUCTION

Over the years, a lot of survey has been carried out in determining elastic parameters of rock for different purposes. Carrying out these surveys help to avert severe disaster in the future. The foundation of a structure is in direct contact with the ground and transmit the load of the subsurface to the ground, which can therefore cause upward and outward movement of the soil and result in slight surface heave as a function of the compressibility and relative stiffness of the soil. Since velocity varies with densities and elastic moduli, the elastic properties and the depth to different layers as well as the thickness of the layers can be determined from the knowledge of the P-wave and S-wave velocities. (Kearny and Brook, 1991).

Seismic wave, which obeys the law of travel of light, is employ to give geoscientists an indirect way to see beneath the earth’s surface by sensing physical properties of subsurface rocks through the ray travel path. Measurement of geophysical properties enable physical properties to be determined e.g, elastic moduli from seismic wave velocities. The small-strain elastic moduli are estimated from determinations of S-wave and P-wave velocities taken together with the in-situ bulk density.(Telford, 2001).

Seismic-based techniques are the most sensitive to the physical properties of geo-materials and relatively insensitive to the chemistry of geo-materials and their fluids. The seismic-based techniques shake the ground and produces very small strains (Telford, 1999). Thus, the soil velocities derived from the seismic-based measurements are related to the soil elasticmodulus. The travel times and velocities of waves enables one to reconstruct the paths seismic waves take and hence, the structure of the subsurface (Kerrey and Brooks, 1991, Reynold, 1997). Therefore, the seismic-based techniques can be used to directly derive the geotechnical properties related to the strain including maximum shear modulus, Bulk’s modulus, etc. Due to the elastic nature of the materials in the subsurface, they tend to change their shape when a load is applied to it, most of these materials break off due to the fact that they cannot withstand the effect of the load placed on it.