Adetayo Folorunso

ABSTRACT Mapping hydrocarbon reservoirs with sufficient resistivity contrasts between them and the surrounding layers has been demonstrated using marine Controlled Source Electromagnetic (mCSEM) technique in this study. The methodology was applied to the Niger Delta hydrocarbon province where resistive targets are located in a wide range of depths beneath variable seawater depths in the presence of heterogeneous overburden. An efficient 2.5D adaptive finite element (FE) forward modeling code was used to delineate the characteristics of the mCSEM responses on geological models; and to establish the suitable transmis- sion and detectable frequencies for targets with variable seawater and burial depths. The models consist of three resistive hydrocarbon layers of 100 O m resistivity, two of which overlain each other. This pre- sents an opportunity to study and understand the 2.5D marine CSEM responses such as the transmission frequency, transmitter–receiver-target geometry, seawater depth and burial depth of the resistive hydrocarbon layers that is characteristics of the region. We found that mCSEM response to two verti- cally-placed thin resistors is higher than that of the individual resistive layer, which could be a veritable tool to identify the two reservoirs, which would have been previously identified by seismic, as possible hydrocarbon layers. For the seawater depths model, detectability of the resistive hydrocarbon increases for the deeper models but decreases for the shallow anomalous depths (305-m and 500-m subsea). This is noticeable for all offsets in the electric filed amplitude responses. The responses are obvious and distinct for the long range electric fields models. The modeling results also indicates that lower frequencies pro- duce high E-field amplitude though higher frequencies generate higher anomaly measured as normalized amplitude ratio (NAR). Generally, it was deduced that expanded frequency spectrum will be needed to significantly resolve thin resistive layers owing to the wide range of burial depths and sharply variable seawater depths in the region.

Saltwater intrusion into the coastal aquifer, a phenomenon brought by the flow of seawater into freshwater aquifers originally caused by groundwater extraction near the coast, has long been recognised as a major concern around the world. In this study, we employed geophysical and geochemical techniques to map and provide evidences that the coastal aquifers in the study area have been intruded by saltwater from the adjacent Lagos lagoon. The resistivity data were acquired with an electrode spacing (a) that vary between 1.6 to 8 m, and expansion factor n of 30. The depth inverted models obtained from inversion of the fifteen resistivity data obtained in the area revealed significant impact of the lagoon water on the aquifers indicated as low resistivity usually below
7 Ωm. A combination of four different electrode arrays – Schlumberger, Wenner, Dipole-dipole and pole–dipole, with at least three deployed at each site ( except for three traverses – traverses 13, 14 and 15), yield better horizontal and vertical resolution, having depth range of 36–226 m with 1.6–8 m electrode spacing used. The delineated geoelectric layers were juxtaposed with logs from both boreholes located within the campus. Evidence from geochemical study of borehole and the lagoon water samples corroborated the ERT result. Progressive decrease in total dissolved solute (TDS) and electrical conductivity (EC) from the lagoon to the coastal aquifer
buttresses gradual encroachment of the inland aquifers by the intruding lagoon water. In addition, similar trend was observed in heavy metal distribution Pollution Index (PI) plot suggesting possible underground flow of water from the lagoon to the aquifers. From this study, we deduced that excessive groundwater extraction and possibly the reduction of groundwater gradients which allows saline-water to displace fresh water in the aquifer of the investigated area are responsible for the saline water intrusion observed.

Electrical  resistivity  sounding was  employed  to delineate  different water bearing  layers  and  their  architectural
parameters.  Sixty Vertical  Electrical  Soundings (VES) were obtained  in the built-up  area  of  Olabisi Onabanjo
University  campus  to  map  out  variations  in  subsurface  resistivity,  which  was  used  to  determine  aquifer
parameters  favourable  for  groundwater  development.  Interpretation  of  electrical  sounding  data  helps  in
determining  the  resistivity  and  thickness  of  aquifers,  aquifer  and  overburden  thicknesses  and  depth  to  the
basement. We employ a lithology-based hydrogeological model with definite boundary to classify the apparent
resistivity.  In  this  model,  range  of  electrical  resistivity  values  were  assigned  to  different  layers  based  on  field
observation and knowledge of the geology of the area. In all, five litho-units were delineated besides the topsoil,
which are: clay with resistivity range of 25–53 Ωm; clayey sand/sandy clay with resistivity range of 122–440
Ωm;  weathered  layer  with  62–119  Ωm;  fractured  basement  with  208–667  Ωm  and  resistivity  above  720  Ωm
were  taken  as  fresh  basement.  From  the  initial  lithologic-based  model,  we  developed  2D  model  that
conceptualized aquifer architecture and bedrock topography along the major traverses obtained in the area. Also,
Isopac  map  reveals  that  the  overburden  is  thicker  in  the  western  half  while  the  2D  model  and  structural  map
clearly show undulating bedrock topography made up of bedrock ridge with elevations of 127.0–170.0 m and
bedrock depressions with elevation of 97.0–123 m. Derived Geo-electric parameters revealed that aquifer quality
increases  as  we  move  northward,  though  recommended  aquifers  cut  across  all  the  area  survey.  Based  on  the
study,  we  recommended  VES  stations  with  good  overall  qualities  to  be  developed  to  productive  water-supply
tube wells.

An electrical resistivity tomography supported with Vertical Electrical sounding (VEs) survey was carried out within the precincts of a Faculty building at Olabisi Onabanjo University Main campus, Ago-Iwoye, Nigeria. The building is intensively affected
by dangerous cracks that cause structural instability  a  few  years  after  its  construction.  The  survey aimed to image the shallow subsurface structures and determine the possible cause of the structural instability.  2D  Resistivity  imaging  using  Wenner
array with smallest electrode spacing of 10m and VES  using  schlumberger  electrode  configuration were  carried  out  along  four  traverses  extending about  130  meters  at  the  precinct  of  the  building complex.  The  acquired  data  were  processed  and
interpreted  spatially  to  expose  the  shallow  structural  setting  of  the  site.  Integrated  interpretation led  to  the  delineation  of  hazard  zone  in  the  area. This zone is interpreted as fault with vertical and sub-vertical linear features having moved both vertically  and  horizontally  with  associated  resistivity values as low as 5Ωm. This feature is suggested as the main reason of the rock instability that resulted in potentially dangerous cracking of the buildings. Inferred lithologic units which include silty/clayey
soil, sandy clay and the basement were mapped out at  the  site.  compacted  lateritic  (hardpan)  topsoil along traverse 4 (backside of the building), though not  a  rock  unit,  constitutes  an  important  factor to  the  re-construction  of  geologic  unit  from  ERT model. The possible cause of failure of the building was  proposed  to  result  from  the  underlying  fault on which the whole weight of the building rests. It was also discovered that the pre-construction geo-technical  investigations  possibly  failed  to  capture the zones because they are point investigations that lack continuous imaging of subsurface condition. Key  words:  2-D  electrical  resistivity  tomography, structural  instability,  Fault  zone,  synclinal  features, compressible soil and springs.

The  phenomenon  of  building  collapse  in recent  times  has  become  a  source  of  national concern. Particularly alarming is the situation in Lagos,  the  commercial  nerve  center  of  Nigeria.
Several  attempts  made  to  proffer  solution  to this  excluded  geophysical  investigations,  as general  opinion  did  not  support  subsurface geology as being responsible. However, 2-D and
3-D  electrical  resistivity  surveys  were  carried out    using    wenner    electrode    configuration with  64  electrodes  connected  to  a  multi-core cable,  along  three  (3)  profiles  of  260  m  length at  an  inter-electrode  spacing  of  4m.  Resistivity
measurements were    taken    alongside    with Induced  Polarization  measurements  using  the same  electrode  configuration.  The  survey  was conducted  round  to  cover  all  the  areas  where distress in building structures is noticeable. The
2-D and 3-D electrical resistivity and IP images for  the  three  profiles  revealed  that  the  main cause  of  structural  defect  in  buildings  around the  area  is  the  subsurface  geology,  contrary  to general opinion which favours insufficiency and/or lack of genuine building materials. From the results,  it  is  quite  obvious  that  the  entire  land mass of the study area is underlain by materials of  very  low  resistivity  values  below  30  Ωm  at
the  near-surface  depth  of  5  m  down  to  above 30  m.  Local  high  resistivity  in  the  near  surface material  is  due  to  either  the  effect  of  sand-filling  for  road  network  and/or  the  presence  of exotic  highly  resistive  concrete  materials  used
to reclaim land. The IP inversion models help to delineate  clay  formation  from  sandy  formation filled  with  saline  water.  The  high  IP  signals  in the  subsurface  mainly  confirm  the  presence  of clayey formation.

Vertical electrical sounding (VES) and Induced Polarisation (IP) methods of geophysical survey were incorporated with
physiochemical analysis of well water samples to determine vertical extent of petroleum-product contamination in subsurface
soils and groundwater from bulk-fuel storage and distribution terminals in Iganmu area of Lagos. Interpreted results of VES
and IP revealed four geoelectric layers. Clay with resistivity and IP values ranging from 1.5 – 14 m and 50 – 400mV/V re-
spectively was encountered at the last layer penetrated by the survey except in four VES stations where the clay horizon was
delineated at the third layer. This implies that subsurface aquifer is sealed by impervious layer which possibly prevents it from
being contaminated by hydrocarbon and other refuse materials from the surface. Borehole log and electrical resistivity survey
from a control site within the area were also incorporated with the geophysical measurements and these confirm lithologic sim-
ilarity and the presence of a sealant above the aquifer layer. In addition to this, the results of the physical and geochemical anal-
yses carried out on groundwater samples from shallow wells within the pack show very negligible level of hydrocarbon
contamination which has no serious environmental implications on subsurface water in the area. However, electrical conduc-
tivity, salinity and TDS values obtained show high level of dissolved minerals (salts) making the water highly saline and un-
suitable for drinking being far above recommended values for drinking water. We thus inferred that Lagos lagoon must have
invaded the aquifer in some places leading to high salinity observed.