MSc. Research Proposal:

Seismic Illumination Study Using Focal Beam Incorporating Multiple Data

Author:Zulfadhli Bin Mohd Zaki

Propose Main Supervisor:A.P. Wan Ismail Wan Yusoff

Propose Co-Supervisor:Abdul Halim Abdul Latiff

Universiti Teknologi Petronas,Bandar Seri Iskandar,31750, Tronoh,Perak Darul Ridzuan.

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INTRODUCTION

Seismic illumination study has becomes a new approach by the industry to improve the

seismic data quality in recent years. The complexity of geology has resulting in improper

illumination on the reflector surface, thus giving rise to the problem of poor data quality on a

certain area with low illumination. Given the fact that seismic illumination study can be

conducted in small amount of time with relatively cheaper cost than acquiring 3D seismic

survey, it is an new industry approach to solve the poor data quality problem in complex

geology. The ability to 'guest' the seismic data quality of the survey area before and during the

acquisition thus give an advantage to acquisition survey be conducted economically and

effectively by changing the array geometry of source and receiver to give the best illumination

on the whole area or seismic infill for the local area with low quality image.

The objective of this study is to conduct a seismic illumination study using a local area

(propose target area is zone of poor image quality resulting from shallow gas cloud) solely on

focal beam method to a multiply scattered wave. Conventional seismic processing eliminated the

multiple as geophysicist regards the multiples as 'noise'. However, if multiple are consider as

scattered wave which later recorded by the detector which reflected on a surface reflector, it

increase the illumination of seismic wave especially in the zone which are poorly illuminated (i.e

shallow gas cloud) by the primary wave. The idea is to use the multiple as the illumination from

below of the target zone. Using the double focusing beam, it does not required to change the

migration algorithm but rearranging source and receiver array distribution at the designed target

area with minimal cost possible.

The poor image quality of the shallow gas cloud would be a good study area for a focal

beam method as the gas cloud effect seismic imaging is localized. A gas cloud, as defined by

Sheriff (2001), is an overburden region of low-concentration gas, escaping and migrating

upward from a gas accumulation. The reflected events in this region appear with lower

amplitude and frequency content. In addition, strong reflection amplitude anomalies, phase

variations along seismic reflections and areas of ‘acoustic blanking’ where no reflectors can be

seen below the gas. The effects are probably caused by incoherent scattering, absorption and

poor stacking because of non-hyperbolic normal moveout. Example in figure 1 shows the effect

of gas cloud resulting in misinterpreted of fault due to absorption and internal scattering of wave.

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FIGURE 1. Shallow gas cloud anomalies resulting in misinterpreted fault. The wave is subjected to internal scattering and absorption due to properties of gas cloud..

An illumination studies prove to give the optimum acquisition for the poor quality

region and be compensated through seismic infill. One of the ways is to prove it is the

illumination could increase if the shooting direction is change as shown in figure 2. The studies

focusing on the use of focal beam method for illumination studies on the shallow gas region

using multiply scattered wave which reflected on horizon reflector beneath the target area as the

primary wave to image the subsurface from above and below the target area.

FIGURE 2. Seismic illumination of the same location with different sail line. There is significance difference in seismic illumination of the target area.

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LITERATURE REVIEW

Illumination in seismic term is the "seismic wave energy falling on a

reflector and be reflected" (Laurain et al, 2004). It is importance to note that in

real case not all of the seismic waves illuminating the subsurface will be

recorded by the receiver. As example, in the complex structure, the Common

Midpoint Gather (CMP) could no longer be equal to Common Reflection Point

(CRP). These create a different intensity of energy or hit-count on a reflector

surface as shown by the figure 3. Seismic illumination method can be

classified into two categories; global method and local method. The global

methods are based on the information on the whole area while local methods

are based on the information on a target area.

FIGURE 3. (a) CMP represents the CRP in a horizontal layer with same elevation of source and receiver (b) The CRP shifted due to different elevation of receiver elevation (c) CRP shifted due to dipping of the

reflecting bed. (Lauraun et al, 2014)

One of the local method mentions mention by Laurain, et al. is focal

beam analysis as introduced by Berkhout et al. (2001a) and Berkhout et al

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(2001b). This method required a full prestack migration of the target zone and

it will give results on image quality based on the acquisition geometry. A

WRW model introduced by Berkhout (1982) is the key for framework for

focal beam analysis. It explain the wavefield simulation as double step of

extrapolation, an extrapolation of downward propagation of wave from source

to target surface and a downward propagation extrapolation from receiver to

target surface. It is formulated as:

Pn ( z0 , z0 )=D [n ] ( z0 )¿ (1)

where, Pn is the reflection response, as the results of four matrix

multiplication. The D matrix does represent the detector array. W represents

the wavefield propagation. The positive or negative sign tells the direction of

the propagation of the wave, where negative sign representing the downward

propagation of wave from source to a depth level of zm, and positive sign

representing upward propagation of wave from depth level of zm to detector.

The S matrix represents the source array. The R matrix represents the angle

dependent reflection coefficient at the depth level zm. The reflectivity matrix,

R could have positive and negative sign. The up going wavefield at depth level

zm are transformed into additional down going wavefield thus creating multiple

which latter will be discuss. These directly explained that there are two type of

focusing operator, which is focusing in the detecting

∆ P j† ( zm, z0 )=F j

†( zm , z0)∆ P(z0 , z0), (2)

where row vector ∆ P j† ( zm, z0 ) is CFP gather of single frequency component.

Row vector F j†(zm , z0) represents focusing operator. For focusing in emitting

∆ P jj ( zm , zm )=[F j† ( zm , z0 ) P ( z0 , z0 )] F j(z0 , zm), (3)

where scalar ∆ P jj ( zm , zm ) gridpoint trace of one frequency component and

column vector F j(z0 , zm) is the focusing operator. The focusing operator is

directly be relates by the source and detector geometry with the wavefield

propagation to a certain target point of zm. Based on equation (2) and equation

(3), the analysis of source and receiver geometry can be done in two steps

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manner. The expressions later translated for two focal beams: the focal

detector beam

D j† ( zm , zk )=D(z0)W (z0 , zm)

And the focal source beam,

S j ( zk , zm )=W (zk , z0) S(z0 , zm)

Where k = 1,2,….., M

A special situation where the detector matrix is the same for all shots

arise when all the shot are fired in the same receiver spread. The situation

could describe stationary acquisition geometry. The data matrix will contain

non zero elements. In order to cover the whole survey area with high fold data

to suppress noise, the roll along technique had been introduced (Shock, 1963).

The systematic or random “noise” characters at various offset are well

recognizable, significantly increase the signal to noise ratio. Figure 4

illustrates the data matrix due to roll along technique. The diagonal of the

matrix will contain the geometry information while elsewhere is zero.

FIGURE 1. Data matrix schematic representation. Berkhout (2001)

The relationship between acquisition design and seismic migration

clearly describe by Berkhout (1997a) and Berkhout (1997b). The two steps

migration operation is introduced in these paper which according to the WRW

framework. The technique describe by Berkhout required to eliminate the

acquisition and propagation effect and left the reflectivity information as the

end results. If the expression are translated into WRW framework, the DW-

and W+S are removed and leaving R which contain angle dependence

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reflection coefficient information. By applying focusing operator, the focusing

in emission transform the shot gather into common focus point (CFP). The

second focusing step, focusing in detection, transforms the CFP gather into

prestack migrated section. The migration algorithm is shows in the following

expression

P jj ( zm , zm )=I j† ( zm ) R ( zm ) I j ( zm )=R jj (zm)

Where

I j ( zm )=W +¿(zm , z0 )S (z0) F j(z 0 ,z m)¿

I j† ( zm )=F j

†(z0 , zm) D(z0)W−¿(zm , z0 )¿

Where F j† and F j is the focusing operator.

Illumination assessment of the 3D acquisition geometry with known

macro velocity model for the prestack migrated target zm can be done using the

source and receiver beam as expression (4) and (5). The evaluation is done

separately from the source geometry and receiver geometry. By using these

double focusing beams, the migrated data is use for further analysis by

resolution function and AVP function. The resolution can be modelled by

multiplication of source beam and receiver beam in the space – frequency

domain. The computation of AVP function can be done by transforming the

source and receiver beam into radon domain. Latter, the source and receiver

beam are multiplied. The assessment of focal function is useful for designing

optimum acquisition geometry.

Van Veldhuizen and Blacquiere (2003) previously has done an

illumination studies on subsalt survey configuration by investigating the

acquisition geometry influence on illumination angle and angle dependent

reflection amplitude. Abdul Latiff, et al. (2013) and Abdul Latiff, et al. (2014)

has using the illumination analysis to incorporate the poor seismic quality data

resulting from a shallow gas cloud. Although the acquisition geometry propose

are not realistic, though the given studies can improves the seismic quality

beneath the gas cloud. The illumination studies are focusing on one of the local

method which is focal beam method.

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Interestingly, this model framework could extend for multiple

reflection (Berkhout, 1982; Verschuur, 1991) based on WRW model. The

wavefield extrapolation method for WRW framework is determined by

multiples from the surface (z0) to depth level (zm) with pre-set n-th order of

multiple.

(−1)n¿¿

Based on this expression, it is possible for the migration the multiple

(Berkhout and Verschuur, 2004) to a primary reflector based on so called focal

transform method. The idea of using multiple to image the low illumination

zone has been done by Berkhout & Vershuur (2006), Malcolm, Ursin, & de

Hoop (2009), and Kumar, Blacquiere, & Verschuur, (2014). Malcolm, Ursin,

& de Hoop (2009) converting the multiples wave into a primary wave through

interferometry with condition of having a smooth reflector beneath the target

area. The multiply scattered wave still treated as one way wave equation which

requires multipass approach of generalized Bremmer series. Kumar,

Blacquiere, & Verschuur, (2014) had use multiple to image a zone beneath a

high velocity zone relative to surrounding area model. The results from poor

data quality beneath the area are compensated by the multiple waves

illuminates from below the target area using modified WRW model by

Berkhout, et al. (2001).

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METHODOLOGY

The illumination study will require a poor quality subsurface image specifically a

shallow gas cloud problem anywhere in the Malay Basin. The component of data needed is:

3D Volume of Post-Stack Migrated Unscale Seismic Data

Navigation Data

Acquisition Report

Velocity Data, Check Shots, Sonic & Dipole, Density & Lithology Information

The model building will involve using previously seismic interpretation and velocity model from

migration data. Selection of interest horizon must be beneath the gas cloud and it must be a

smooth reflector (possibly no dipping).

In order to determine the optimum acquisition geometry, illumination studies on the

conventional acquisition will be conducted by using full-offset ray tracing by the given

coordinate. The ray tracing must be following the WRW model modified after Kumar and

Blacquiere to convert the multiple waves as the primary wave.

Several of acquisition geometry is simulated using ray tracing method by the appropriate

software. Later, the results will be compared to the conventional acquisition illumination results.

Several parameters will be added to ensure the studies are as realistic as possible.

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CONCLUSION

Seismic illumination studies require special attention to reduce the acquisition expenditure

resulting from current technique; CMP does not full represent CFP. Different acquisition design

could have impact on the illumination of the subsurface. The focal beam analysis is used in the

studies because the WRW model explained that wavefield extrapolation could directly relate the

detector and receiver array geometry with propagation of the wave. The focal function such as

resolution function and AVP function are used as the tools to evaluate optimum acquisition

design. The geological model proposed to use is a complex geology with the presents of gas

cloud, which the geology beneath the gas chimney recently are poorly been illuminated due to

the properties of the gas inside the subsurface. The studies could provide an optimum acquisition

design to reduce the cost fot the operating companies such as PETRONAS.

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Reference

Abdul Latiff, A. A., Ghosh, D. P., & Tuan Harith, Z. Z. (2013). Seismic illumination analysis of different shallow gas cloud velocities by focal beam method. IOGSE 2013.

Abdul Latiff, A. A., Ghosh, D. P., & Tuan Harith, Z. Z. (2014). An Integrated Method of Subsurface Illumination Analysis for Shallow Gas Anomaly Data. International Petroleum Technology Conference. doi:10.2523/17356-MS.

Berkhout, A. J., 1982, Seismic migration, imaging of acoustic energy by wavefield extrapolation, A: Theoretical aspects: Elsevier

Berkhout, A. J., & Vershuur, D. J. (2006). Imaging of multiple reflections. Geophysics , 71 (4).

Berkhout, J. A., Ongkiehong, Volker, O., A, W. F., & Blacquiere, G. (2001). Comprehensive assessment of seismic acquisition geometries by focal beams—Part I: Theoretical considerations. Geophysics, 66 , 911-917.

Volker, A. W. F., G. Blacquière, A. J. Berkhout, and L. Ongkiehong, 2001, Comprehensive assessment of seismic acquisition geometries by focal beams — Part II: Practical aspects and examples: Geophysics, 66, 918–931, http://dx.doi.org/10.1190/1.1444982.

Kumar, A., Blacquiere, G., & Verschuur, D. J. (2014). Optimizing Illumination Using Multiples - Application to Seismic Acquisition Analysis. 76th EAGE Conference and Exhibition 2014 . EAGE.

Laurain, R., Gelius, J. L., Vetle, V., & Lecomte, I. (2004). A Review of 3D Illumination Studies.Journal of Seismic Exploration, 13 , 17-37.

Malcolm, A. E., Ursin, B., & de Hoop, M. V. (2009). Seismic imaging and illumination with internal multiples. Geophysical Journal International 3 , 847-864.

Sheriff, R. E. (2001). Encyclopedic dictionary of applied geophysics (4th ed.). Society of Exploration.

van Veldhuizen, E. J., & Blacquière, G. (2003). Acquisition Geometry Analysis Using Quantitative Measures For Image Quality. 2003 SEG Annual Meeting. Dallas, Texas: Society of Exploration Geophysicists.

Berkhout, A., & Verschuur, D. (2004). Imaging multiple reflections, the concept. SEG Technical Program Expanded Abstracts 2004

Shock, L. (1963). Roll‐Along And Drop‐Along Seismic Techniques. Geophysics, XXVIII(5, Part II), 831-841.

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Berkhout, A. (1997a). Pushing the limits of seismic imaging, Part I: Prestack migration in terms of double dynamic focusing. Geophysics, 62, 937-953.

Berkhout, A. (1997b). Pushing the limits of seismic imaging, Part II: Integration of prestack migration, velocity estimation, and AVO analysis. Geophysics, 954-969.

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Appendix A: Workflow of Research

Velocity Data Migrated Seismic Data Lithology Data

Build velocity model with shallow gas anomaly

Design acquisition survey area

Ray Trace from Shot toReceivers of Processed

Data Simulate acquisition design

Calculate Amplitude Ray Tracing Finite DifferenceDistribution

Illumination comparison of real and synthetic data

Develop optimum survey method

Propose Solution /Recommendation

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Thesis Writing

Appendix B: Gantt chart429

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Key Milestones

December 2015 : Research Proposal Defence

July 2016 – March 17 : Two Paper Submission / Presentation in any available conference

July 2017 : Submission of Thesis and VIVA

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