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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.