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Study of near-surface layer effects in reflection seismic exploration from the dynamics point of
view
Congde Lu*, Yun Ling, Jun Gao, Desheng Sun and Jixiang Lin, BGP, CNPC.
Summary
The effects of near-surface layers in land seismic exploration had been known long before. However, previous studies mainly
focused on statics brought by propagation through near-surface layers (from the kinematics point of view), and only some
surface-consistent processing methods were applied to compensation. Until now, few studies discussed the effects of nearsurface layers on the amplitude, wave shape, phase, frequency, etc., of reflection seismic data (from the dynamics point of view).
The near-surface effects are described through a 3-D seismic exploration case history in this paper. Based on the 3-D velocity
field of the near-surface layers built by 3-D tomography, the spatial structure of near-surface layers gained by uphole and the
monitoring results of amplitude, wave shape, and frequency of reflection data, the near-surface effects on reflection seismic data
is evaluated. These experimental results show that near-surface layers greatly impact on reflection data.
Introduction
Research on the near-surface started in 1940 when Zirbel (1940) gave the explorative result of near-surface layers in the
Michigan basin and named a layer of unconsolidated topsoil above the water table as weathering profile. In the research that
followed, Sheriff (1991) presented the definition of a low-velocity layer of the near-surface, i.e., the unconsolidated topsoil or
the aerated sediments that have very low velocity. Moreover, he also pointed out that the velocity of a weathering profile
generally varies from 500 to 800 m/s and the interval velocity of stratum below weathering is commonly greater than 1500 m/s.
Cox (1999) gave a classification of near-surface layers as follows: (1) sand dune, (2) highly anomalistic weathering layer, (3)
youthful topography, (4) mature topography, (5) ever-frozen layer topography, (6) mountain front, etc. Yamanaka et al. (1980)
not only measured the attenuation of weathering layer by absorption, but gave the energy attenuation and the waveform
distortion of reflected wave arising from the weathering layer. In addition, Adriansyah and McMechan (1997) thought that the
effects of the near-surface layers in land seismic exploration is an important issue, worthy of study, because their experimental
results about the effects of near-surface layers on AVO indicated that the near-surface layers directly influenced the shooting
and receiving in data acquisition. So, how to monitor and analyze the effects of near-surface layers in land seismic exploration
has become an important subject for researchers and practitioners.
Figure 1: (a) Elevation of the earth’s surface; (b) velocity slice of 15m below surface
Estimation of near-surface velocity by 3-D tomography
The research area, located in an oil field in northeast China, covers a width of 12 km and a length of 45 km, giving an area of
540 km, where the elevation of the earth’s surface varies from 0 to 15 m, as shown in Figure 1a. Moreover, a 3-D geometry, of
12 lines, 12 shots, 84 folds, 25× 25 of bin, and a total of 28670 shots, is adopted in this research area.
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Study of near-surface layer effects
We obtained some velocity slices of near-surface layers using a 3-D seismic tomography technique (Dines and Lytle, 1979;
Chon and Dillon, 1986), where Figure 1b is the horizontal velocity slice at 15m below the earth’s surface. By comparison of the
velocity slices and the elevation variation of the earth’s surface, it may be deduced that the current channel is not situated in the
low-velocity zone of the near-surface layers, whereas there exist two low-velocity strips and one low-velocity zone at the west
part of current channel. Waveforms of single shots are shown in Figure 2, where Figures 2a presents the 2 shot gather, i.e., A, B
from low-velocity region and 2b shows shot C1, C2 from high-velocity region (Figure 1b for the corresponding shot location).
From Figure 2a, we find that the seismic waveform has a low frequency feature which seems to be the result of strong
absorption attenuation. It may be deduced that the first arrivals of the low-velocity region have the apparent property of lowvelocity. From Figures 2b, we have a contrary conclusion that the reflection data from the high-velocity region have higher
frequency, stronger energy and higher velocity, in comparison with those in the low-velocity region.
Figure 2: (a) waveform of shot from low-velocity region; and (b) waveform of shot from high-velocity region.
Velocity and structure of near-surface layers gained via uphole
Figure 3: Waveform of single uphole: (a) and (b) from low-velocity region; (c) and (d) from high-velocity region.
There are 732 uphole surveys designed in the research region, thus, there is one uphole per square kilometer on average.
Moreover, downhole sources and ground receivers are adopted in the uphole surveys. Waveforms of some upholes are displayed
in Figure 3, where 3a and 3b are from low-velocity region while 3c and 3d are from high-velocity region. From Figure 3, we
may deduce that low-velocity, low-frequency, and weak energy are the characteristics of those upholes from low-velocity region
and vice versa for those upholes from high-velocity region. For accounting for the absorption attenuation characteristics of near-
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Study of near-surface layer effects
surface layer, Figure 4 presents the spectrum analysis of 4 upholes in different shot depth, where 4a is from 0 to 5 meter while
shot depth of 4b varies from 11 to 18 meter. In addition, blue and yellow mark 2 upholes from high-velocity region while 2
upholes from low-velocity region are marked red and pink. From Figure 4a, we find that the energy difference between those
upholes from high-velocity region and low-velocity region, respectively, is not very obvious, because the transmission distance
of seismic wave is too short. From Figure 4b, we also find that the energy difference between those upholes is distinct and the
maximal difference reaches 5 dB at 100 Hz and 13dB at 150 Hz. By spectrum analysis shown in Figure 4, it can be deduced that
under the same conditions, only the transmission distance over 10 meters can make a maximal difference of 13 dB on the
seismic wave energy. This result shows that the impact of near-surface can not be ignored.
Figure 4: Spectrum analysis of upholes from Figure 3: (a) shot depth: 0~5m; (b) shot depth: 11-18m.
The structure of the near-surface layers are obtained via refined interpretation of these upholes. The maximal thickness
difference of low-velocity layers from different regions may reach 15 m, and the velocity of low-velocity layers vary from 350
m/s to 1600 m/s (Figure 5a) for single uphole interpretation from low-velocity region, and 3b for those from high-velocity
region. Moreover, the interpretation results of these upholes also show that the corresponding low-velocity strips of Figure 1b
may be ancient channels because of its low-velocity characteristics and the velocity variation of the near-surface layers in the
vertical direction (Figure 5a). The region situated in the current channel, however, has the feature of high-velocity and a simple
structure of near-surface layers (Figure 5b). Using upholes and 3-D tomography, we obtain the structure and velocity
distribution of the near surface which is a basis for the detection of reflection data variation and the compensation of the nearsurface.
Figure 5: Uphole interpretation of the near-surface: (a) the interpretation of single uphole from
low-velocity region, and (b) the interpretation of single uphole from high-velocity region.
Detection of reflected wave and analysis of effects of near-surface layres
From the results of 3-D tomography and of upholes shown above, it is clear that the near-surface structure and the velocity
change rapidly in the spatial direction though the elevation changes very little. Therefore, some problems may arise. For
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example, would the spatial variation (including velocity and thickness) lead to a change in amplitude, waveform or frequency of
reflection data? How much is the range of spatial variations? These problems in land seismic exploration deserve further study.
The solutions of the above problems can help us in seismic processing quality control, selecting the correct compensation
processing method and the real AVO information of the reservoir as well as the collection of inversion results. The reflection
energy of 4 shot gathers, i.e., A, B, C1, C2 (Figure 1b), are displayed in Figure 6, where A, B are from low-velocity region while
C1 and C2 are located in high-velocity region of near-surface layers, by using the statistical computation method (Wu et al.,
2003). From the comparison of the spatial variation of reflection energy and the velocity variation of the near-surface (Figures
1b and 6), it is apparent that the reflection energy in the low-velocity region is weaker than that in the high-velocity region.
From the reflection energy of the 4 shot gathers, it is also clear that there are distinct differences in absorption attenuation
between those shots from the low-velocity region and those from the high-velocity region. The frequency spectrum of shot
gathers (Figure 6) shows that the difference of reflection energy may reach 28 dB at 50Hz. It can be deduced that the difference
of velocity and of structure of near-surface layers may lead to the difference of spatial distribution of reflection energy and this
difference is greater than the spatial variation of the reservoir.
On the other hand, the effects of near-surface layers may also be detected by analysis of waveform variation of the reflection
wavelet. The spatial variation of the reflection wavelet (Figure 7) can be obtained by the statistical autocorrelation of shot
gathers (Wu et al., 2003). Comparison between the spatial variation of the reflection wavelet and the velocity slices of the nearsurface also verified the difference between the reflection wavelet in low-velocity region and that in high-velocity region. A
monitory line L of the reflection wavelet (Figures 1b and 7) shows apparent spatial variation in the statistical autocorrelation of
reflection wavelet. Again, the frequency of the reflection wavelet in low-velocity region is obviously lower than that in highvelocity region, and the side lobe cycle of the reflection wavelet from the low-velocity region is greater than that from the highvelocity region. Based on the results above, it may be inferred that the structural difference of near-surface layers may lead to
the spatial variation of the reflection wavelet, and the velocity difference of the near-surface layers brings the difference to
absorption attenuation which may result in the difference of the dominant frequency of reflection wavelet. Apparently, we can
not obtain the right AVO attribution of the reservoir and inversion results if no appropriate processing method or workflow is
adopted to compensate the effects of the near-surface layers.
Figure 6: The energy difference of reflection wave
brought by near-surface layers.
Figure 7: The difference of reflection wavelet
(of the monitoring line L in Figure 1(b) )
brought by near-surface layers.
Conclusions
Based on the analysis of near-surface effects via 3-D tomography, upholes, reflection energy and reflection wavelet, some
conclusions could be obtained as follows. Firstly, for near-surface layers, the thickness variations of the low-velocity zone range
from 0 to 15m while the velocities of the corresponding low-velocity zone vary from 1600 to 350 m/s. Secondly, the spatial
variation in structure and velocity of near-surface layers bring the spatial amplitude variation of reflection wavelet. In addition,
by frequency analysis, the energy difference of reflection data reaches 28 dB at 50 Hz. Thirdly, the effects of near-surface layers
are greater than those of reservoir. As a result, the spatial variation of near-surface greatly influences the AVO information of
reservoir and the precision of inversion, especially in lithologic or thin-bed exploration. In conclusion, the proposed method,
which analyzes the structure and velocity of near-surface layers and monitors their effects on reflection data, may be considered
a basis for selecting an appropriate processing method or workflow and attenuating the near-surface effects.
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EDITED REFERENCES
Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2009
SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for
each paper will achieve a high degree of linking to cited sources that appear on the Web.
REFERENCES
Adriansyah, A., and G. A. McMechan, 1997, Effects of near-surface structure, scattering and Q on AVO measurements: 67th
Annual International Meeting, SEG, Expanded Abstracts, 146–149.
Chon, Y.-T., and T. J. Dillon, 1986, Tomographic mapping of the weathered layer: 56th Annual International Meeting, SEG,
Expanded Abstracts, 593–595.
Cox, M., 1999, Static corrections for seismic reflection surveys: Society of Exploration Geophysicists.
Dines, K. A., and R. J. Lytle, 1979, Computerized geophysical tomography: Institute of Electrical and Electronics Engineers, 67,
1065–1073.
Sheriff, R. E., 1991, Encyclopedic dictionary of exploration geophysics, 3rd ed: Society of Exploration Geophysicists.
Wu, L., Y. Ling, and X.-Y. Guo, 2003, 3-D seismic data monitoring and evaluation: 73rd Annual International Meeting, SEG,
Expanded Abstracts, 2140–2143.
Yamanaka, H., K. Seo, and T. Samano, 1989, Effects of sedimentary layers on surface-wave propagation: Bulletin of the
Seismological Society of America, 79, 631–644.
Zirbel, N. N., 1940, Michigan weathering: Geophysics, 5, 382–384.
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