ABSTRACT

The seismic refraction technique was used to characterise the topsoil of Nine Local Government Areas in Akwa Ibom State. The survey area lies between latitudes 4^o 45’ and 5^o 15’ N and between longitudes 7^o.45’ and 8^o 30’ E. The study area covered an area of about 1110.1km². The study area lies within the tertiary units of the continental plain sands built on the silicic deposit of hard energy arcuate Niger Delta. Shootings covered 27 forward and 27 reverse P-wave shootings carried out using 12- channel seismograph. P-wave velocities were obtained from seismic refraction survey covering a spread line of 50 m, with 2 m geophones spacing in the topsoil of the study area. Due to the unavailability of S-wave geophones, the S-wave velocities were estimated using modified Castagna equation characterised with constants a = 0.4563 and b = -1.2. The study area covered Ibiono Ibom, Itu, Uyo, Uruan, Ibesikpo, Etinan, Nsit Ubium, Nsit Ibom and Nsit Atai Local Government Areas of Akwa Ibom State, Southern Nigeria. The arrival times of recorded signal (seismograms) were picked and plotted against the offset distance using IX Refrax and Pickwin software. This software  generated T-X graphs and the seismic models, which generally showed the two velocity layers serving as foundation in the study area from the generated forward and reverse seismic refraction shootings. Result show that the  ratios with average 2.2108 and range 2.2071 – 2.2183 for layer 1 and average 2.202 and range 2.199 -2.205 for layer 2 were used to determine the topsoil geoelastic parameters such as Poisson’s ratio (s) ranging from 0.37085 – 0.37248 and averaging 0.37139 for layer 1 and with an average and range of 0.370 and 0.370 – 0.371 respectively for layer 2. Other parameters determined were shear modulus with an average of 4.28 x 10⁷N/m² and ranged between 2.12 x 10⁷ – 6.27 x 10⁷ N/m² for layer 1, while layer 2 had an average of 3.84 x 10⁸ N/m²   and a range of 2.15 x 10⁸ – 2.55 x 10⁸ N/m².  Young modulus had an average of 1.17 x 10⁸ N/m² for layer 1 while it range was 5.8 x 10⁷ – 1.72 x 10⁸ N/m²  for layer 1 and the average for layer 2  was 3.85 x 10⁸ N/m² while 2.15 x 10⁸ – 6.995 x 10⁸ N/m²   was the range. For Bulk modulus, its average for layer 1 was 1.52 x 10⁸ N/m²  while it ranged from 7.62 x 10⁷ – 2.22 x 10⁸ N/m²   for layer 2, it average was 4.93 x 10⁸ N/m²  and it range was 2.77 x 10⁸ - 8.95 x 10⁸ N/m². Lame’s constant (l) had an average of 3.89 x 10⁷ N/m² and  ranged  from 1.98 x 10⁷ – 5.81 x 10⁷ N/m²   for layer 1 while the average for layer 2 was 1.297 x 10⁸ N/m²  and a range of 7.27 x 10⁷ – 2.36 x 10⁸ N/m². For  ratio, the average for layer 1 was 2.7428 while it ranged from 2.7417 – 2.7450. For layer 2, its average was 2.740 and with a range of 2.7417 – 2.7450. For  ratio, 3.5543 was the average for layer 1 with a range of 3.5381 – 3.5875. The average for layer 2 was 3.157 and the range was from 3.503 – 3.531. 

The area under consideration was found to be gently dipped as the dip angles were greater than 5^o in some locations. This variation in the depth of topsoil formation is responsible for the variations noticed in the depths of formations measured in different positions. From the above averages and ranges of geoelastic parameters, it can be summarily inferred that there is no much disparity in the geologic facies and hence in the presence of adequate compaction and compression, durable and sustainable engineering structures can be constructed.