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Shallow subsurface imaging using high resolution seismic reflection
methods
Ghunaim T. Al-Anezi
King Abdulaziz City for Science & Technology
Riyadh, Saudi Arabia
[email protected]
SUMMARY
In order to evaluate the use of shallow seismic technique
to map the bedrock up to the depth of 20-25 meters, three
high resolution seismic reflection profiles were carried
out. The data were acquired using a Strata Visor with 48channel, 40 Hz geophones and a weight drop system as
seismic source. Seismic reflection data were recorded
using a CMP (common mid-point) acquisition method.
The results show that the bedrock lies at about 18-25
meters depth. The bedrock related horizon observed here
is of low frequency, its depth is almost similar in all three
seismic lines and thus giving us the enough confidence in
results and also following the subsurface structure.
Reflection line 3 is been crossed by reflection line 1 and
reflection line 2. To confirm the structure and same
statics, I did tie these lines to confirm and the reflectors
are exactly matching, hence no need to give any shifting.
There is high frequency loss due to high attenuation in
near surface. A good structural image of subsurface is
visible from the seismic sections and for interpreter it’s
easy to mark structure and integrate it with other
methods. With the proper equipment, field parameters
and particularly great care in data collection and
processing, we can image reflections from layers as
shallow as 25 meters.
Key words: High-resolution seismic reflection; bedrock.
reflection efficiency up to the depth of 20-25 m. using the
output results; we were able to interpret the area for required
objective. The processing stage of seismic data showed
consistency which suggesting reasonable confidence in the
results. Fig. 1 shows the locations of seismic reflection lines in
the study area.
Figure 1. Diagram of geophone array development and
shot locations used for the acquisition of seismic CMP line.
The geophone spread consists of 48 receivers spaced 2 m
apart. 2 receiver lines spread and three source lines shoot.
Source lines 1 and 2 shoot at 4m offset from receiver line 1
either side and source line 3 shoots at 4 m offset from
receiver line 2.
INTRODUCTION
Data acquisition and processing
Seismic reflection techniques have been widely utilized to
detect and map subsurface features, especially the layered
sedimentary sequences in search of oil and gas reservoirs
(Burger et al., 1992). Advantages of seismic techniques over
other geophysical methods are due to their high accuracy,
high-resolution and deeper penetration (Sheriff et al., 1999).
Recently, the seismic methods, which include the highresolution seismic reflection method, have been applied to
characterize shallow subsurface structures, depth of water
tables and identification of engineering related problems
(Kearey and Brooks, 1984). Since, all the engineering and
environmental aspects are located at shallow depths (near
surface); seismic reflection techniques are an excellent choice
to achieve high-resolution images from that domain. The
survey covered 3 lines of high-resolution seismic reflection
with a total length of 282 m in Salboukh, about 50 km towards
north from Riyadh city. The objective of the survey was to
map the bedrock of the area and high-resolution seismic
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The instruments used here are the most advanced and up-todate sold commercially for high- resolution investigation. The
system consists of the source which is a weight drop, sensors,
and the acquisition system. The sample rate kept was 1ms and
the record length of 2000 ms at each point the data is stacked
automatically to improve the signal to noise ratio. i.e., 6 shots
for seismic reflection at each point stacked to enhance the
signal to noise ratio.
The acquired high-resolution seismic reflection data was
processed to enhance signal to noise (S/N) ratio, for which
Landmark’s ProMax Software Package has been used. Fig. 2
shows an example of shot gather after SWNA (surface wave
noise attenuation). In general terms, processing steps of the
shallow seismic reflection data is similar to that of
conventional seismic reflection data (Steeples et al., 1990;
Feroci et al., 2000). In general, the near surface layers have a
low velocity value that varies abruptly with lateral extension,
23 International Geophysical Conference and Exhibition, 11-14 August 2013 - Melbourne, Australia
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Shallow Subsurface Imaging Using High Resolution Seismic Reflection Methods
Al-Anezi
which often make seismic reflections subtle and noisy. Hence,
as compared to conventional processing techniques, more
attention must be paid when we process the high-resolution
data e.g., ground roll, removing of air-blast noise, spatial
aliasing and refraction muting (Steeples et al., 1998; Steeples,
2000).
Figure 3. Final stack of reflection line-1 in depth (time to
depth converted with final velocity model).
Reflection Line-2
Figure 2. Shot gather after SWNA (surface wave noise
attenuation) application of reflection line-1.
Results
First the time migrated images and depth conversion of time
migrated stacks by using appropriate velocity models are
discussed. Like any geophysical investigation the results are
approximate and uncertainty is part of the equation. Figs. 6, 7
and 8 present the final stack of reflection line 1, 2 and 3 in
time scale.
Reflection Line-2 length is 94 m; and its direction is NE-SW
and is running parallel to Reflection line-1. This line is just 8
m away from Reflection Line-1 so it will follow almost same
subsurface structure. In this line, the bedrock depth is ranging
from 18 - 25 m Fig. 4. A big fault noticed on the right portion
where the portion of bedrock is uplifted to be at 18 m depth.
The frequency of the data is a bit on the lower side that’s
maybe due to the energy penetration problem or attenuation in
the complexity or heterogeneity of the near surface. The signal
to noise ratio is not that good here as it was in Reflection
Line-1 but it is conforming the same structure. Reflection
Line-2 was also crossed by Reflection Line-3 shows the same
time of bedrock at crossing.
Reflection Line-1
Reflection Line-1 length is 94 m; its direction is SW-NE. In
this line, the bedrock depth is ranging from 18 - 25 m Fig. 3.
A big fault noticed on the right portion where the portion of
bedrock is uplifted to be at 18 m depth. The frequency of the
data is a bit on the lower side that’s maybe due to the energy
penetration problem or attenuation in the complexity or
heterogeneity of the near surface. Reflection Line-3 cross
Reflection Line-1 and the time of the bedrock layer matched
which gives enough confidence in saying that this is the true
image of bedrock.
Figure 4. Final stack of reflection line-2 in depth (time to
depth converted with final velocity model).
Reflection Line-3
Reflection Line-3 also shows the same depth range of
bedrock, that is 18 to 25 m Fig. 5. Reflection Line-3 length is
also 94 m; its direction is NW-SE and is crossed by Reflection
line-1 and Reflection Line-2. The Fault is not obvious in this
line but it is following a synform shape. Again the frequency
content looks lower but as long as it is mapping the bedrock,
the worries remain away. The maximum offset was very small,
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23 International Geophysical Conference and Exhibition, 11-14 August 2013 - Melbourne, Australia
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Shallow Subsurface Imaging Using High Resolution Seismic Reflection Methods
Al-Anezi
so we cannot expect anything else at deeper depth. The high
frequencies attenuate very fast in near surface due to its
heterogeneity. The signal to noise ratio is good up to the
desired objective.
Figure 8. Final stack of reflection line-3 in time scale.
CONCLUSIONS
Figure 5. Final stack of reflection line-3 in depth (time to
depth converted with final velocity model).
The high-resolution seismic reflection technique has been
shown to be useful for mapping the bedrock. This study was
initiated with acquiring 2D high-resolution seismic reflection
data over about 282 m line length consisting of 3 seismic
lines. The results show that the bedrock lies at about 18-25 m
depth and some faulting observed in seismic reflection data. It
seems that imaging depths shallower than 25 m is still a great
challenge.
ACKNOWLEDGMENTS
We are thankful to King Abdul Aziz City for Science and
Technology (KACST) for the accomplishment of this project.
We are also thankful to staff members of the Seismic Analysis
Center (KACST) for their support in data processing.
REFERENCES
Figure 6. Final stack of reflection line-1 in time scale.
Burger, H. R., Sheehan, A. F., Jones, C. H, 1992.
Introduction to applied geophysics: exploration of the shallow
subsurface. Norton, New York, p 65.
Feroci, M., Orlando, L., Balia, R., Bosman, C., Cardarelli, E.,
Deidda, G, 2000. Some considerations on shallow seismic
reflection surveys. J Appl Geophys 45:127–139.
Kearey, P., Brooks, M, 1984.An introduction to geophysical
exploration. Scientific Publications, Osney Mead, Oxford, p
296.
Sheriff, R. E., and Geldart, L. P., 1999, Exploration
Seismology, 2nd edn.: Cambridge University Press, 592 p.
Steeples, D. W, 2000. A review of shallow seismic methods.
Annali di Geofisica 43 (6):1021–1044.
Figure 7. Final stack of reflection line-2 in time scale.
Steeples, D, W., Miller, R. D, 1990. Seismic reflection
methods applied to engineering, environmental, and
groundwater problems. In: Ward SH (ed) Geotechnical and
environmental geophysics: volume l— review and tutorial.
Society of exploration geophysicists, Tulsa, Oklahoma, pp 1–
30.
Steeples, D. W., Miller, R. D, 1998. Avoiding pitfalls in
shallow seismic reflection surveys. Geophysics 63:1213–1224.
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