SIMULATING PRE AND POST STACK MIGRATION FOR 2D SEISMIC LINE IN BALAD-SAMARRA AREA, CENTRAL IRAQ: A SEISMIC UNIX IMPLEMENTATION

Migration is a process by which reflectors are re-positioned to their true subsurface locations. The accuracy of the migrated velocity model and time determines the subsurface imaging quality. For complex structures, the reflecting and/or diffracting energy from source points will not vertically be oriented beneath the common mid-point (CMP). However, an acceptable migrated velocity model is usually performed by several parameters. Stolt migration method is applied to a 2D seismic line within Salah Al-Din province (Balad-Samarra area), central Iraq. To understand the effect of Stolt migration method and parameters used in the migration, a simulated synthetic model has been made with the same field parameters for the studied seismic line and with the same vertical cross-section of the geological subsurface layers. This model is constructed to generate raw seismic data, which was reprocessed using Seismic UNIX (SU) package in the post and pre-stack migration situation. Stolt migration is applied in the time domain for the modeled processed data to test the effects of changing the values of each parameter. A suitable migrated velocity values deduced from the analyzed model is tested for real surveyed line and the images results for the subsurface structures are accurate without diffraction. The results indicate that pre-stack time migration is best due to the lateral velocity variation and structural complexity and velocity analysis confirm that pre-stack is better than post-stack time migration.


INTRODUCTION
Seismic imaging techniques are accustomed to produce an image, or section, that equivalent the geological structure in the subsurface.Migration is a term that used in seismic reflection survey which is defined as moving the dipping reflectors events to their true subsurface positions and decreases the diffractions (Gardner, 1985), thus increasing spatial resolution, as well as increase the signal-to-noise ratio on the time sections and, at the same time, provide a velocity distribution profile of the geological subsurface will create a perfect seismic image of the subsurface (Gray et al., 2001).Due to the source and receiver are sets on the ground as Omnidirectional, the reflecting or diffracting energy from source points will not vertically beneath the common mid-point (CMP), which it can be ambiguous to the interpretations, here, the migration will be applied to reposition the subsurface structural events to their true position (Fehler and Huang, 2002).Migration principle is Huygens's secondary source principle.This is a state that any point on a wave front can be as a secondary source which is producing circular wave fronts when viewed as two dimensions.The process of migration is to collapse the apparent secondary wave fronts to their points of origin.(Reynolds, 1997).
In a complex geological structure with high dips, the migration process needs to test several parameters to get the final reasonable geological image.The 2D seismic section of Ajeel field Balad-Samarra (BS-92) along a seismic line, which is located in Salah Al-Din province, between Balad-Samarra area, central Iraq, reveals bad data and complex geological structure.In this work, a simulated synthetic model is constructed for the same field with the same parameters that are used to acquire BS-92 line in the Iraqi Oil Exploration Company.The simulated model is constructed to get raw seismic data and processed to get the final stacked section.Later on, a Stolt migration method was tested to endeavor the geological image.All simulation, acquiring raw data, processing and migration is implemented using Seismic Unix which is an open-source seismic utility package that was supported by the Center for Wave Phenomena (CWP) at the Colorado School of Mines (CSM).

THEORETICAL BACKGROUND
Stolt migration method, also called (F-K) Direct Fourier Transform Migration.It was presented by Stolt in 1978.The term (F − K) is derived from the Fourier transform of time to frequency(F), and distance to wave-number(K).This method is idyllic when the velocity is constant for all the stacke section and provides correctly migrated section up ninety degrees (Stolt, 1978).

Al-Rahim and Obaid
Vol.52, No.2, 2019 80 Stolt described his method mathematically as think through the earth to be a twodimensional, half-space and assume sound travel as a scalar field with a velocity at point (X, Z) of C (X, Z).Each point in the earth has the facility to transform down-going sound waves into upgoing sound waves this property is characterized by reflection strength [R(X, Z)], (Fig. 1) (Stolt, 1985;Claerbout, 1971).The mathematical details are left and the final equation is (Yilmaz, 2001): The output of the Stolt migrated is scaled and defined by the quantity(S) as follow:

𝟐
Where P (K F , Z = 0, w) is the zero-offset section, P (K H , K I , t = 0) is the migrated section in the is the vertical wavenumber and (w ) is the temporal frequency.The horizontal direction of (F − K) algorithms has restricted the ability in processing velocity variations.Stolt method includes the coordinate transformation from frequency to vertical wavenumber axis (K − X), while the horizontal axis of the wavenumber is kept without any adjustment.Stolt improved his method by presenting expansibility in the time direction to handle the forms of velocity variations for which time migration is suitable to combine theory with practice in the field of migration with an assurance on the frequency-wavenumber methods (Yilmaz, 2001;Stolt and Wegleint 1978).A time-domain section can be transformed to the frequency domain to yield the corresponding (F − K) plane.In the process, all dip components having a common angle on the time domain seismic section are consolidated into a single dip vector in the frequency domain (Lindseth and Geoph, 1982;Forel et al., 2005;Bleistein et al., 2013;Lorenzo, 2018;Prado et al., 2019).

Fig. 1 :
Fig. 1: Migration may be observed as a prediction of alterations in the seismic field as sources and receivers' locations are moved into the earth.With reverence to small changes in receiver or source location(Stolt, 1978)