Download a06 hybrid source-receiver geometry for short

A06
HYBRID SOURCE-RECEIVER GEOMETRY FOR SHORTSPREAD LAYOUT IN 2D SEISMIC
REFLECTION
ACQUISITION
Khairul Arifin Mohd Noh1, Zuhar Zahir Tuan Harith2, Abdul Halim Abdul Latif3,
Hazwan Syahmi Hashim4
1Faculty of Geosciences and Petroleum, Engineering Universiti Teknologi PETRONAS
[email protected]
2School of Energy, Geoscience, Infrastructure, and Sustainability Heriot Watt University
Malaysia
[email protected]
3Faculty of Geosciences and Petroleum Engineering Universiti Teknologi PETRONAS
[email protected]
4Faculty of Geosciences and Petroleum Engineering Universiti Teknologi PETRONAS
[email protected]
A seismic spread refers to the location of the source with respect to the receivers
associated with that specific source. In land 2D seismic reflection acquisitions, survey
coverage (i.e. distance and depth)is the main issue since the numbers of active geophone are
limited. This issue become critical when dealing with steeply dipping geologic features such
as dipping planes, fault planes and over-turned layers, as useful reflections may arrive at
relatively far offset. For land data, the near- offset portion of seismic data is often strongly
contaminated by ground-rolls, on the other hand the long-offset reflections could arrive
beyond the ground-rolls. In both cases good signal to noise ratio data is very hard to be
obtained. This paper is trying to address this issue. The data was acquired using ABEM
MK8 Seismograph with 48 active geophones (10 Hz) spaced 5 m apart and 12 gauge (shell)
seismic guns as the source. The data was collected using hybrid source-receiver geometry and
Common-Mid-Point (CMP) gathers. As shown in Figure 1.a, geologically the sedimentary
rocks that underlie much of the study area (i.e. Seri Iskandar, Perak) consists of alternating
beds of sandstone, shale, clay or mudstone and subordinate siltstone. The clastic sequence in
this area is most likely equivalent to Kati Beds, as Carboniferous to Permian age (Wong,
1991).
Acquisition was done by firing the source within 475 m straight line survey layout into
48 receivers simultaneously (Figure 1.b).The general idea of the CMP gather is to acquire a
series of traces which reflect from the same depth point (creating multi-fold reflection) and
the traces are then stacked so that superior signal-to-noise ratio to that of the single-fold stack
results. The hybrid source-receiver geometry consists of; i) in-line offset spread, ii) split
spread and iii) zero offset spread. For the in-line offset spread, the source is moved away
from the nearest receiver because the seismic energy level maybe high enough for useful
recording by nearby geophones. Moreover, in-line offset spread in the up-dip direction is
likely to increase the recorded reflected signal amplitude and improve the signal•to•noise
ratio in areas of dipping reflectors (Avasthi and Agrawal, 1974). For the split spread, the
source is at the centre and geophones placed on either side of it. In this study, symmetrical
and asymmetrical split spread geometry has been executed. Records from split spread
reflection survey are used to detect the dipping reflectors and to find the amount of their
dip (Upadhyay, 2013). Here, the zero offset spread is being implemented simultaneously with
CMP acquisition using a coincident source and receiver pair during in-line shooting (source
and receiver are at the same location). Using zero offset trace, a minimum two-way travel
time (normal- incidence wave) and reflection coefficient can be obtained.
Through post-stack time migration data processing, results obtained from this hybrid
acquisition are illustrated in Figure 2. For this paper, only three signal characters observed
from seismic results will be discussed, i.e. marked as A, B and C (Figure 2). Most of
discussion emphasised the seismic amplitude recovery, signal to noise (S/N) ratio and
resolutions (i.e. vertical and horizontal) especially in areas of dipping reflectors. Moreover,
a better-quality of seismic section obtained from merging both geometries also discussed in
this paper. Finally, the design of such seismic acquisition geometry has been developed in
this paper, and a field method using this configuration is suggested for short-spread
survey layout with limited numbers of active geophone.
References
Avasthi, D. N. and Agrawal, M. C. (1974).A short-spread configuration for mapping dipping
horizons in reflection seismic surveys. Pure and Applied Geophysics, Volume 112,
Issue 5, pp 845-854.
Upadhyay, S.K. (2013). eismic Reflection Processing: With Special Reference to Anisotropy.
Springer
Science & Business Media, ISBN: 3642074146.
WongT.W. (1991). Geology and mineral resources of the Lumut-Teluk Intan area,
Perak Darul
Ridzuan. Geological Survey of Malaysia Map Report 3, Geological Survey Laboratory,
Ipoh, Perak, 1991, pp. 96.
S1 / S192
475m
S96 /
S97
Outcrop 1
Outcrop 2
(a) 1st source-receiver
geometry
5
m
2nd source-receiver
geometry
5
m
Inline offset shots
(interval: 5m)
Inline shots
(interval: 5m)
b)
Figure 1: (a) Survey locality (yellow line representing the 475 m survey line and
black line representing the geological strike and dipdirection for clastic rocks from
nearby outcrops) and (b) source-receiver geometries for 475m seismic reflection
survey spread.
survey spread
Distance (m)
0
Distance (m)
475
0
0
400
475
0
A
400
A
800
B
B
TWT (ms)
TWT (ms)
800
120
0
1600
1200
1600
C
C
2000
2000
240
0
2400
1st Geometry
2nd Geometry
Figure 2: Post-stack time migration results for 1st and 2nd geometry based on Figure
1.b.