Open Journal of Geology, 2013, 3, 17-21
doi:10.4236/ojg.2013.32B004 Published Online April 2013 (http://www.scirp.org/journal/ojg)
Determining the Basaltic Sequence Using Seismic
Reflection and Resistivity Methods
A. Alanezi, A. Qadrouh
Seismic Analyses Center, King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia
Received 2013
ABSTRACT
This study was carried out in Harat Rahat (south of Almadinah Almonwarah) using seismic reflection and resistivity
methods. The main objectives of this study are to determine the extent of the basaltic layer and to define the subsurface
faults and fractures that could affect and control the groundwater movement in the study area. A 2D seismic profile was
acquired and the result shows that the subsurface in the study area has a major fault. We obtained a well match when the
seismic result was compared with drilled wells. As a complementary tool, the resistivity method was applied in order to
detect the groundwater level. The results of the resistivity method showed that six distinct layers have been identified.
The interpretation of these six layers show that the first three layers, the fourth layer, the fifth layer and the bottom of
the section indicated various subsurface structures and lithologies; various basaltic layers, fractured basalt, weathered
basement and fresh basaltic layers, respectively. It is obvious that the eventual success of geophysical surveys depend
on the combination with other subsurface data sources in order to produce accurate maps.
Keywords: Seismic Method; Vertical Electrical Sounding (VES); Water Table
1. Introduction
The high-resolution seismic reflection method has per-
formed effectively to delineate the subsurface structures
of the earth. Many geophysicists such as ]10[, ]15[,
]14[, ]11[ have detected coal using this method. Other
investigators have utilized the high-resolution seismic
reflection method for a variety of applications. For ex-
ample, ]12[ applied it to determine unconsolidated
sedimentary structures in the Netherlands. ]8[ applied
the method to detect unconsolidated aquifers in Australia.
]6[ used it to assist in mine planning and future
horizontal drilling for coal-seam methane extraction.
]16[ applied it to image a thin, diamondiferous
kimberlite dyke. ]5[ used it to delineate growth folding
and shallow faults beneath the southern Puget lowland in
Washington state. Electrical resistivity methods have been successfully
applied in ground investigations for several purposes.
Their applications in geotechnical and engineering site
investigations were achieved by many authors ]9[, ]1[,
]3 [. Soil and groundwater were mapped by other re-
searchers such as ]4[. ]13[ utilized electrical resistivity
methods to locate subsurface structures. Mapping the
extent of the basaltic layer and the subsurface structures
were the main objectives of this study, which was carried
out by using seismic reflection, drilled wells and resistiv-
ity methods. The seismic results show that the subsurface
in the study area has a major fault as well as the effi-
ciency of the seismic method to identify the water table
level. The results of resistivity methods show that six
distinct layers have been identified. Therefore, the results
of seismic reflection and resistivity surveys are quite
similar to the drilled wells.
1.1. Field Procedures and Data Processing
Seismic Reflection Method
A 2D seismic profile was acquired using equipment and
selected parameters based on the nature geological phe-
nomena and area accessibility. Uncorrelated seismic data
was recorded with 112 channels, where the geophone
group interval and the shotpoint interval were 5m, and
the offset between geophone and shotpoint was 10m, as
shown in Figure 1. In addition, the selected field pa-
rameters in this study are shown in Table 1.
The primary objectives of seismic data processing are
to produce high resolution images of the subsurface,
achieved by enhancing signal to noise ratio and migrat-
ing the reflected waves to their correct position. The
conventional processing sequence of seismic reflection
data include, but are not limited to: filtering, statics ap-
plication, deconvolution, CMP sorting, velocity analysis,
NMO correction, stacking and migration. Figure 2 and
Figure 3 show the stacked seismic section in time scale
and the appropriate processing sequencing, in order to
obtain better signal enhanced results respectively.

A. ALANEZI, A. QADROUH
18
Figure 1. Shows the geometry of this study.
Table 1. The selected field parameters in this study.
SPREAD
Number of Traces 112 traces
Receiver interval 5 m
Source interval 5 m
Near offset 10 m
Max.offset 555 m
CDP fold 111 fold
SOURCE
Type Vibrosies
Model Mini IVI
No. of Vibrator One
Sweep type Linear upsweep
Band width 20-300 Hz
Number of Sweep 1
RECEIVER
Type Geophone Flat base
Model GS – 20 DH
Response 365 ohm, 40 Hz, 0.70 Damping
INSTRUMENTS
Type Geometrics, Strata Visor NZ
Sampling interval 0.5 ms
Gain constant 36 dB
Sweep length 5 s
Record length 2.5 s
Filter out
Figure 2. The stacked seismic section in time scale.
Figure 3. The processing steps in this research.
1.2. Electrical Resistivity Methods
The conventional equipment for resistivity surveys, such
as an ammeter, voltmeter, power source, electrodes, and
connecting wire were used in this research. In addition,
we performed five vertical electrical soundings (VES)
alone the seismic line in order to study the variation of
resistivity with depth. Figure 4 illustrates that this
sounding was taken with the Schlumberger array, having
a maximum separation of 1000 m between the current
electrodes, and the separation of the half current elec-
trodes being gradually increased from 3 to 1000 m. The
maximum separation between potential currents was 120
m, with increments started from 0.6.
The major steps in processing resistivity data consist
of the following. First, producing a sounding curve
which displayed the apparent resisitivities against the
electrodes spacing, as illustrated in Figure 5 Next, the
forward model in bars was created to show the curve of
theoretical sounding corresponding to the model of the
current earth as shown in Figure 6. After that, building
an inverse model which represented the curve of theo-
retical sounding corresponding to the model of the initial
earth and every subsequent trial model as convergence

A. ALANEZI, A. QADROUH 19
proceeded. Finally, the equivalence analysis was applied
to indicate the earth models range that corresponded to
the acquired data for the final inverted model, as revealed
in Figure 7. Note that the processing of the first VES is
represented here. This is due to the fact that the process-
ing sequence for the others was quite similar to steps for
the first VES.
2. Interpretation of the Results
The seismic section was interpreted in order to identify
the water table layer across the survey area. The inter-
pretation began by transforming the stacked section from
time (ms) to depth (m), where the accuracy of such depth
conversion depends on how accurate the processing of
seismic data is; that is to say, accurate depth conversion
depends on the accuracy of velocities and times recorded.
We applied the interval velocity approach to convert the
data from time to depth scale. The stacked section shows
that the subsurface in the study area has a major fault. In
addition, the results of the seismic section show that
there are two seismic reflectors, where the second re-
flector is associated with a water table at a depth range of
125 m to 230 m. This reflector is considered the surface
of the third seismic layer that represents groundwater
saturated fractured basalts. The abrupt and remarkable
increase in the depth of the second layer is due to faulting
Figure 4. Sketch of the field setup for VES in Schlumberge r
array.
Figure 5. Sounding curve.
Figure 6. The forward model in comparison with data.
Figure 7. The inversed model in comparison with data.
in the area between 40 and 260 m distant from the shot
point. In order to reach a more accurate depth interpreta-
tion than that presented only by seismic section, we tied
five drilled wells (W-1,W-2,W-7,W-11, and W-12) to the

A. ALANEZI, A. QADROUH
20
seismic data as shown in Figure 8. Table 2 demonstrates
the detail information about the five drilled wells inte-
grated with stacked section to mark the water table layer
related horizon. W-1 and W-2 are dry wells because they
are at a shallow depth of 135m, while the water table
layer is located at a depth of 230 m. The water layer is
found in W-7, W-11 and W-12 at depth 125 m, 150 m,
and 150 m respectively.
Resistivity Data Interpretation
The interpretations of the five VES stations show that the
area under study has six layers as shown in Figure 9.
The first layer is a thin layer of very dry weathered basalt,
with an average value of ρ = 8430 Ohm.m, and at a depth
ranging from 1 to 7.5 m. The value of apparent resistivity
in the second layer is about ρ > 4000 Ohm.m and at
depth ranging from 7.5 to 20 m, which might be com-
prised of fresh basalt. The third layer has an apparent
resistivity value between 1000 to 1200 Ohm.m and at a
depth range of 20 to 60 m. This layer was interpreted as
fresh water saturated fractured basalt, intercalated with
some gravelly sand. The analysis of the fourth layer
showed that the values of apparent resistivity are be-
tween 150 and 200 Ohm.m and with a depth range of 60
to 155 m. This layer was characterized as fractured basalt
with clay saturated with salt-water. The values of the
apparent resistivity in the fifth layer were between 100
and 120 Ohm.m and the depth range was 126 to 184 m.
This layer was interpreted as a weathered basement. The
sixth layer had an apparent resistivity ρ > 300 Ohm.m,
and which was interpreted as compact basement.
Figure 8. Interpreted Stack (Depth) .
Table 2. The information of the five drilled wells.
Wells information W-7W-1 W-2 W-11W-12
Distance from 1st Geophone
(m) 117342 530 8001155
Water table depth (m) 125135(dry) 135(dry) 150150
Figure 9. This composite diagram show data from seismic
and VES.
3. Conclusions
The geophysical methods used allow us to identify the
subsurface structures, to obtain lithological information,
and to characterize the conditions of the underground
flow in the studied area. The interpretation of seismic
data was agreed with the available drilled wells to locate
the water table depths which varied generally from 120
m to 150 m, as well as the effectiveness of the seismic
method to detect the surface fault. In addition, the resis-
tivity method located the various lithologies in the sub-
surface, such as different basaltic layers, fractured basalt,
weathered basement and fresh basaltic layers respec-
tively. It is clear that the ultimate success of geophysical
surveys depends on the inclusion other subsurface data
sources in order to produce precise maps.
4. Acknowledgments
The authors wish to thank KACST for their wealth of
knowledge and valuable data for this research.
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