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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)
ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue IX September 2025
Seismic Refraction Tomography for Engineering Site
Characterization in Awka, Southeastern Nigeria
Obiajulu, O. O; Olisah, N. C; Nneji, E. G
Department of Physics and Industrial Physics, Nnamdi Azikiwe University, Awka, Nigeria
DOI:
https://doi.org/10.51584/IJRIAS.2025.100900061
Received: 11 September 2025; Accepted: 17 September 2025; Published: 16 October 2025
ABSTRACT
Subsurface characterization is crucial for safe and cost-effective civil engineering design, particularly in regions
underlain by weak or heterogeneous geological formations. This study employs seismic refraction tomography
(SRT) to evaluate the engineering properties of near-surface materials in Awka, the capital of Anambra State,
Southeastern Nigeria. Six seismic profiles were acquired using an ES-3000 seismograph with 24 geophones
spaced at 2 m intervals. Data processing and interpretation were performed using ReflexW software, enabling
delineation of three major subsurface layers with P-wave velocities ranging from 400–745 m/s, 589–1518 m/s,
and 797–2826 m/s, respectively. Engineering parameters including shear modulus, Young’s modulus, bulk
modulus, oedometric modulus, and allowable bearing pressure were derived from the measured P- and S-wave
velocities. Results show that seismic velocity and corresponding engineering parameters increase with depth,
indicating progressive compaction and strength of subsurface materials. The calculated allowable bearing
capacity ranges between 1.36 × 10² and 4.50 × 10² N/m², consistent with regional geotechnical data. Findings
demonstrate that SRT provides a reliable and cost-effective approach for evaluating foundation conditions in
Awka, and can serve as an alternative or complement to conventional laboratory testing. These results are
recommended for application in urban planning, construction projects, and subsurface resource management in
Southeastern Nigeria.
Keywords: Seismic refraction tomography, engineering site characterization, Awka, P-wave velocity
INTRODUCTION
Geophysical prospecting involves the study of subsurface materials by measuring their physical properties with
appropriate instruments. These properties, when properly interpreted, provide information on subsurface
structure and composition (Obiajulu, 2022). Among available techniques, geophysical methods have become
indispensable tools in geotechnical and geo-environmental investigations due to their efficiency, cost-
effectiveness, and non-destructive nature (Riwayat et al., 2017; Ayolabi et al., 2012).
Seismic refraction tomography (SRT) is particularly valuable for engineering site investigations, as it provides
estimates of elastic parameters essential for evaluating foundation conditions. Compared to conventional seismic
refraction, SRT performs well in complex near-surface environments, offering reliable subsurface velocity
models even in areas with lateral heterogeneity, extreme topography, or limited spread lengths (Azwin et al.,
2013). Previous studies in Nigeria have demonstrated the utility of seismic methods for evaluating weathered
layer thickness (Nwosu & Emujakporue, 2016), assessing foundation material strength (Agha et al., 2006), and
determining site vulnerability to structural collapse (Adewoyin et al., 2017).
Despite these advances, there remains a need for detailed seismic-based site characterization in Awka and its
environs, an area underlain by sedimentary formations known for their geotechnical challenges. This study
therefore applies SRT to evaluate subsurface conditions and derive engineering parameters relevant to
construction, with the aim of providing data that can inform safe and sustainable urban development.
Study Area
Awka, the capital of Anambra State, lies within the Anambra Basin of Southeastern Nigeria. The area is situated
between latitude 6°25′N and longitude 7°00′E, with elevations ranging from 150 to 300 m above sea level. The
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topography gently slopes toward the Mamu River, with prominent cuestas trending north–south.The climate is
tropical, marked by a wet season (April–October) and a dry season (November–March), with an average annual
rainfall of ~1500 mm and mean daily temperatures ranging between 22–32 °C (Obiajulu and Okpoko, 2014 :
NIMET, 2012).
Geologically, the study area is underlain predominantly by the Imo Shale Formation, consisting of dark-gray to
bluish-gray clayey shale interbedded with thin sandstone bands (Ehirim & Ebeniro, 2010). The formation
becomes progressively sandier toward the top, transitioning into alternating sandstone–shale sequences. These
lithologies are associated with low bearing strength and high susceptibility to water infiltration, necessitating
geophysical evaluation prior to engineering development.
LITERATURE REVIEW
Traditional refraction analysis used travel-time picks and simple layer models (delay-time, intercept methods).
SRT extends this by inverting first-arrival travel times for continuous 2-D (and increasingly 3-D) velocity fields,
allowing lateral heterogeneity and complex interfaces to be imaged with higher resolution. Modern algorithms
(travel-time tomography, ray-tracing, finite-difference and hybrid forward solvers) plus improvements in
acquisition (multichannel recorders, denser receiver spacing) and inversion regularization have improved
resolution for shallow engineering targets (meters to tens of meters). Practical reviews and evaluation studies
highlight SRT’s advantages—better handling of dipping/irregular interfaces—and its limitations, notably non-
uniqueness, dependence on good first-arrival picks, and difficulty imaging very low-velocity contrasts without
complementary data. Recent methodological trends relevant to engineering applications include: optimized
travel-time forward solvers for speed and accuracy, joint inversion with electrical resistivity tomography (ERT)
and surface-wave methods to reduce ambiguity, and time-lapse SRT for monitoring moisture or seasonal
changes. These advances increase the utility of SRT for site classification, slope monitoring, and locating weak
layers or cavities.
Numerous Nigerian studies demonstrate SRT’s value for engineering and environmental problems. Obiajulu et.
al., 2025 applied SRT specifically in Amawbia (Awka South) used multichannel acquisition and tomographic
inversion to delineate three seismic layers, estimate velocities, and infer bearing capacity zones aimed at
diagnosing building collapse issues and guiding foundation recommendations. This is directly relevant to Awka
engineering practice and demonstrates the feasibility and local value of SRT surveys.
Studies across southeastern and other parts of Nigeria have applied SRT for 2-D geological modelling, depth to
fresh basement, groundwater potential, and characterization of subgrade for highways. For example, SRT
combined with geotechnical tests provided diagnostic information for failed and stable pavement sections on the
Ajaokuta–Anyigba highway. Other work in Akwa Ibom and parts of the basement complex shows consistent
use of SRT to map layer thicknesses and velocities for engineering interpretations. These studies show common
outcomes—identification of weathered zones, estimation of depth to competent material, and derivation of
conservative engineering parameters.
Multiple Nigerian case studies emphasize integrating SRT with ERT, MASW (surface-wave), VES, borehole
logs and CPT/CPTu to overcome non-uniqueness and to translate velocity into engineering metrics (e.g., RQD,
bearing capacity, dynamic shear modulus). Integrated surveys improve subsurface discrimination (e.g.,
separating saturated sands from clays) and better constrain the interpretation for engineering design.
MATERIALS AND METHODS
Theoretical Background
The seismic refraction method is governed by Snell’s law, which describes the refraction of seismic energy at
interfaces of contrasting velocity. At the critical angle of incidence, waves become refracted along subsurface
interfaces and return to the surface as head waves. The first arrivals of these waves, recorded by geophones, are
used to calculate velocity distributions within the subsurface (Dutta, 1984). SRT extends this principle by
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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)
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inverting first-arrival times to generate continuous velocity tomograms.
Data Acquisition
Six seismic profiles were conducted across Awka using an ES-3000 seismograph with 24 geophone channels.
Geophones were deployed at 2 m spacing, while a sledgehammer and striker plate served as the seismic energy
source. Multiple shots were recorded per spread to ensure data redundancy.
Data Processing and Interpretation
Field data were processed with ReflexW software (Sandmeier, 2002). Signal enhancement was achieved through
band-pass and gain filtering. The data collected from the field was subjected to different stages of processing to
enhance the signal-to-noise ratio. The data were first filtered by applying a bandpass filter and then gain filter to
enhance the quality of the real signal. The next step is to pick the first arrival times, this arrival time picks were
used to plot the travel time curves from where the velocity layers can be estimated from the reciprocal of the
slopes obtained. (i.e wave form inversion). Fig 3.1 to Fig 3.6 are different stages involved in the processing of
data
Fig 4.1: Raw data for forward shooting
Fig 4.2: Raw data for reverse shooting
Fig 4.3: Data for forward shooting after gain and band pass filter has being applied
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INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)
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Fig 4.4: Data for reverse shooting after gain and band pass filter has being applied
Fig 4.5: Travel time curve
Fig 4.6: Interpreted data
Time (ms)
Distance (m)
Travel Time Curve for Profile one
Forward Times (ms)
Reverser Times (ms)
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Calculation of Engineering Parameters
Seismic refraction tomography method is in high demand lately because of their relevance in engineering studies.
The fundamental principle of the application of the method in engineering studies is the utilization of the seismic
waves. Many engineering properties can be measured by the propagation of seismic waves. Some of the
engineering properties that can be obtained by this method include Young’s modulus, Modulus of elasticity,
Bulk modulus, Poisson’s ratio and Allowable bearing pressure etc. All these parameters require the knowledge
of both P and S waves to determine their values. Tezcan et al, 2009 derived expressions used to calculate
engineering properties. Table 1 is the summary of the formula developed by Tezcan et al, 2009.
Table 1: Summary of the Formula for Engineering Parameters in terms of V
P
and V
S
(Tezcan et al., 2009: Atat
et al., 2013)
Parameter
Formula
Shear Modulus
Poisson’s ratio
Modulus of Elasticity (Young’s)
Bulk Modulus
Oedometric Modulus
Subgrade Coefficient
Allowable bearing pressure
Where g is the acceleration due to gravity = 9.81m/s, α = (V
p
/ V
s
)
2
, γ is the unit weight of the soil in N/m
3
=γ
0
+
0.002V
p,
γ
0
is the reference unit weight values in N/m
3
, for soil γ
0=
16, n is factor of safety, for soil n =4.0, V
p
is the compressional wave velocity and V
s
is the shear wave velocity
RESULTS AND DISCUSSION
Calculated Engineering Parameters
From the velocity models, P-wave (Vp) and S-wave (Vs) velocities were extracted for calculation of engineering
parameters including shear modulus (G), Young’s modulus (E), bulk modulus (K), Poisson’s ratio (ν),
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oedometric modulus (Ec), subgrade coefficient (ks), and allowable bearing capacity (qa) using established
empirical relations (Tezcan et al., 2009).
Table 2 is the calculated engineering parameters
Profile
Location
V
p
V
s
α
v *10
-1
ϒ* 10
1
G*10
5
E*10
5
K*10
5
E
c
*10
5
K
s
*10
4
q
a
*10
2
(N/m
2
)
(N/m
2
)
(N/m
2
)
(N/m
2
)
(N/m
2
)
(N/m
2
)
(N/m
2
)
1
Comm. Pri. Sch.
556
318
3.06
0.26
1.71
1.76
4.43
3.04
1.31
2.18
1.36
738
436
2.87
2.32
1.75
3.39
8.34
5.19
2.60
3.05
1.90
1011
612
2.73
2.11
1.80
6.88
16.7
9.60
5.43
4.41
2.76
2
Amaenyi Girls
626
356
3.09
2.61
1.73
2.23
5.62
3.92
1.65
2.46
1.54
795
464
2.94
2.42
1.76
3.86
9.59
6.19
2.93
3.26
2.04
1048
625
2.81
2.24
1.81
7.21
17.6
10.7
5.59
4.52
2.83
3
All Saints Church
538
358
2.26
1.03
1.71
2.23
4.92
2.06
2.00
2.45
1.53
641
470
1.86
0.81
1.73
3.89
7.15
2.05
4.21
3.25
2.03
797
626
1.62
3.05
1.76
7.03
9.77
2.02
9.17
4.41
2.75
4
Paul University
854
353
5.85
3.97
1.77
2.25
6.28
10.2
1.36
2.50
1.56
1398
609
5.27
3.83
1.88
7.11
19.7
28.0
4.39
4.58
2.86
2214
865
6.55
4.10
2.04
15.6
43.9
81.3
9.19
7.07
4.42
5
Ukwuorji
833
355
5.51
3.89
1.77
2.27
6.30
9.47
1.39
2.51
1.57
1238
611
4.11
3.39
1.85
7.03
18.8
19.5
4.65
4.52
2.82
1845
866
4.54
3.59
1.97
15.1
40.9
48.3
9.65
6.84
4.26
6
Obunagu
957
358
7.15
4.19
1.79
2.34
6.64
13.6
1.36
2.57
1.60
1524
609
6.26
4.05
1.90
7.20
20.2
35.5
4.28
4.64
2.90
2375
867
7.50
4.23
2.08
15.9
45.3
98.1
9.17
7.20
4.50
Seismic Velocity Structure
The interpreted velocity models delineate three major seismic layers across the study area. P-wave velocities
range from 400–745 m/s in the topsoil, 589–1518 m/s in the intermediate layer, and 797–2826 m/s in the deeper
layer. The progressive increase in velocity with depth reflects compaction and lithological transition from
weathered shale to competent sandstone-shale sequences.
Engineering Parameters
Derived engineering parameters show that shear modulus varies between 1.76 × 10⁵ and 1.59 × 10⁶ N/m², while
Young’s modulus ranges from 4.36 × 10⁵ to 4.53 × 10⁶ N/m². Bulk modulus values lie between 2.02 × 10⁵ and
9.81 × 10⁶ N/m², and the calculated allowable bearing capacity spans from 1.36 × 10² to 4.50 × 10² N/m². These
values indicate that near-surface materials in Awka are generally suitable for shallow foundations, though
variability across locations highlights the importance of site-specific evaluation.
The results are consistent with cone penetration test data obtained from the Anambra State Materials and Testing
Laboratory (2018), reinforcing the reliability of SRT as an alternative to conventional geotechnical methods.
Implications for Engineering Development
The findings demonstrate that SRT can effectively characterize subsurface engineering properties in sedimentary
environments where traditional borehole testing may be costly or spatially limited. In Awka, where rapid
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urbanization increases demand for construction, such data are critical for minimizing risks of foundation failure,
structural collapse, and groundwater contamination.
CONCLUSION
This study applied seismic refraction tomography to evaluate the engineering properties of subsurface materials
in Awka, Southeastern Nigeria. Three seismic layers were identified, with velocities and engineering parameters
increasing with depth, consistent with progressive compaction. The derived shear modulus, Young’s modulus,
bulk modulus, and allowable bearing capacity values show that the area generally supports shallow foundations,
though localized variations warrant detailed site-specific assessments.
Seismic refraction tomography has proven to be a cost-effective, reliable, and non-invasive method for
geotechnical site characterization in Awka. The method is recommended for government agencies, urban
planners, estate developers, and engineers prior to construction projects such as buildings, roads, and boreholes.
Future work should integrate SRT with electrical resistivity and borehole testing to further constrain subsurface
models and improve engineering risk assessment.
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