The use of F-K filter in the elimination of artefacts from a shallow seismic reflection data in Zaria, Nigeria

Near surface high resolution 2-D seismic reflection survey was carried out along the Zaria batholith. Zaria area forms part of the Precambrian basement complex of north central Nigeria which comprises Precambrian rocks made up of granite, gneiss and Low-grade metasediments. The granitic Zaria batholith intruded the gneiss and metasediments which forms the country rock. Reflection signals from shallow seismic survey in this area are often marred by surface waves, refraction and multiple reflections. The processing of the seismic data if not well handled often leads to migration artefacts which could be misconstrued as a seismic event. The field procedure employed for the geophysical survey was the split spread mode. The geophones were fixed at an interval of 1m, and a constant offset of 0.5 m. The common depth point method (CDP) with 12 fold coverage was adopted for the survey.Several artefacts were noticed on the result of the seismic migrated section, when the fk filter was initially applied only on the pre-stacked seismic data. The filtering and the common midpoint stacking could not completely eliminate the seismic noise on the pre-stacked seismic data. However, when an fk filter was applied on the post-stacked seismic section, the remaining seismic noise that survives the initial processing flow was eliminated. When migration was carried out on the filtered post-stack seismic section, the resulting seismic section was free from artefacts, andshowed clear seismic events.KEYWORDS: f-k filter, artefacts, shallow seismic, reflection, migration. Zari

2-D Seismic Refraction Tomography Investigation of a Sewage Treatment Site

Geophysical investigation involving seismic refraction tomography (SRT) was conducted at a sewage treatment site located at Ahmadu Bello University Zaria with the objectives of determining the overburden thickness and velocity of the subsurface layers in the area, and delineating any geological structures, such as fracture zone, that may pose a threat to the safe running of the Sewage lagoon system in the area.. Five seismic profiles, with three running N-S and two running E-W, were acquired over the earthen-dikes separating the lagoons with the aid of a 24-channel digital Seismograph and a 5 kg sledge hammer striking a rubber plate. Tomography models were generated for each of the profiles with the aid of ray tracing through a wave-front inversion method that is based on finite difference approximation of the eikonal equation. Geological artifacts, as a result of noise observed in the tomography models of some of the profiles, were removed by filtrations that include manual editing of model’s physical parameters and iterative model updating through simultaneous iterative reconstruction techniques (SIRT). The result of the investigation shows that the area consists of three subsurface layers; an undifferentiated overburden layer with a P-wave velocity range of 891 m/s to 1421 m/s, a partially weathered zone with P-wave velocity range of 3010 m/s to 5129 m/s and underlying fresh granitic basement with P-wave velocity range of 5704 m/s to 7762 m/s, while the overburden thickness in the area varies from an average of about 18 m in the northern part to about 55 m in the southern part. The investigation also revealed a fracture zone within the underlying granitic basement, which occurs at about 60 m below the earthen-dike located at the centre of the study area. Keywords: 2-D Seismic, Refraction, Tomography, Sewage Lagoo

Frequency dependence of magnetic susceptibility of weakly contaminated soil sediments

This paper reports the frequency dependence of magnetic susceptibility of weakly contaminated soil sediments. Low field magnetic susceptibility measurements were carried out on soil sediments at Bomo irrigation dam, Samaru College of Agriculture, Ahmadu Bello University, (ABU), Zaria, using the MS2D field loop. Soil samples were also collected for laboratory analyses. The volumetric percentage composition of magnetite (Fe) in the soil was found to be about 0.02%. Frequency dependent magnetic susceptibility measured at two frequencies (0.465 kHz and 4.65 kHz) using MS2B sensor reveals a slight difference between magnetic susceptibility values with the higher frequency indicating lower values. The coefficient of frequency dependence (%) values varying between 2.37% and 7.39%, indicating the presence of a mixture of ultrafine superparamagnetic and coarse nonsuperparamagnetic grains or SP grains < 0.005 m ferromagnetic minerals occurring as crystals in the top soil. The soil formation process was as a result of secondary ferromagnetic minerals occurring as a result of burning, biochemical and authigenic processes in the area under investigation