High resolution 3D seismic undershooting acquisition over

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Copyright ©　2020， JAPT

石油技術協会誌　第 85巻　第 1号（令和 2年 1月）44～ 53頁Journal of the Japanese Association for Petroleum Technology

Vol. 85, No. 1（Jan., 2020）pp. 44～53

講　　　演Lecture

1.　Introduction

In recent years, 3D broadband acquisition/processing technology has played an increasingly cr ucial role in delivering high quality seismic data at both the exploration and development stages. This has led to improvements in meaningful geological interpretation, predicting reservoir distribution, reducing volumetric uncertainty and reducing drilling operational risks. One of the many challenges of seismic acquisition in the vicinity of producing fields is the existence of surface platforms. Moreover, a single vessel operation will also create a data gap around the area. To acquire the data beneath the facilities, undershooting （Games et al., 2015） and a dual vessel shooting arrangement （e.g., Barousse et al., 2007; Ebaid et al., 2008; OGP, 2011） are required to avoid processing artefacts.

The survey area is located in the north-western part of of fshore, Sarawak, Malaysia. The water depth in the area varies from around 80 m to 100 m. In brief, there are three discovered gas fields and a number of near-field exploration targets. Two surface platforms, A and B were installed in the block for producing gas （Fig. 1）. The geological setting of the survey area is characterised by a sedimentary sequence,

High resolution 3D seismic undershooting acquisition over platforms and seismic processing challenges in a gas producing �eld＊

Seiya Sano＊＊，†, Tran Quoc Tan＊＊ and Gyuhwan Jo＊＊

（Received August 19, 2019；accepted November 22, 2019）

Abstract：　One of the many challenges of seismic acquisition associated with existing fields is existence of surface platforms. The block where the seismic operation was conducted is located in offshore Sarawak and there are 3 discovered gas �elds and a number of near-�eld exploration targets. The legacy 3D seismic survey covering the block was conducted in 1992 with limited acquisition speci�cations, such as short cable length and low volume airguns, in comparison to the technology of today. Hence new 3D seismic acquisition with broadband processing was commissioned which included the undershooting operation across 2 production facilities to improve the seismic imaging of the deeper exploration targets and to optimize well engineering. Overall, the new 3D data were acquired with a successful undershooting operation followed by survey merging and matching in the processing stage. The new 3D data show better seismic re�ection continuity, a higher signal to noise （S/N） ratio, clearer images, and a more accurate velocity model in the deep section, allowing for a better interpretation of the deep exploration targets.

Keywords： broadband, undershooting, high resolution 3D seismic, survey merge, survey matching

＊令和元年 6月 12日，令和元年度石油技術協会春季講演会地質・探鉱部門シンポジウム「天然ガス探鉱・開発の現状と課題－低炭素社会に向けて」にて講演　This paper was presented at the 2019 JAPT Geology and Exploration Symposium entitled “Natural Gas Exploration & Development - Current State & Challenges Toward Low Carbon Society” held in Tokyo, Japan, June 12, 2019.

＊＊ JXマレーシア石油開発株式会社　JX Nippon Oil & Gas Exploration （Malaysia） Limited

† Corresponding author：E-Mail：[email protected] Fig. 1　Survey location map

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J. Japanese Assoc. Petrol. Technol. Vol. 85, No. 1（2020）

Seiya Sano, Tran Quoc Tan and Gyuhwan Jo 45


which is dominated by siliciclastic sedimentation of Early Miocene to Pliocene. The producing gas reser voirs and deeper exploration potential strata are bound by a series of E-W trending growth faults. These faults form one of the key processing imaging problems in terms of a strong fault shadow anomaly at the main reservoirs and deeper potential strata.

The legacy 3D seismic survey was conducted in 1992 with 3 km streamer length and the seismic data quality, especially the accuracy of the velocity was doubtful. This results in a poor seismic image and difficulty in interpreting the deeper section. Hence, new 3D seismic data have become necessary to improve the seismic imaging of the deeper section to identify hydrocarbon potential in the block. In addition, new 3D seismic velocity data can be utilized for well engineering study to improve the pore pressure and fracture gradient predictions, thereby optimizing the potential well location for the future drilling. Subsequently, a deep tow streamer acquisition and broadband processing, including the

undershooting operation, were conducted.

2.　Undershooting Technique

Undershooting is a technique used to image the sub-surface beneath obstructions （OGP, 2011）. Two vessels are required for this operation. One vessel acts as the streamer and recording vessel while the other as a source vessel. Generally speaking, the reflector points are plotted between source and receivers. Therefore, with a normal shooting geometry, where both the source and streamers are towed by one vessel, imaging beneath the platform will never be achieved, no matter how close the vessel goes to the platform. This data gap creates artefacts, such as edge effects during migration processing, which will lead to unreliable geological interpretations. In undershooting, a dual vessel operation enables data acquisition around the facilities while controlling the data swath by optimizing the vessel positions. Fig. 2 shows the schematic for normal shooting and undershooting.

Considering the fact that the distance between the source

Fig. 2　Schematic for normal shooting（upper）and undershooting （lower）

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石油技術協会誌　85巻 1号（2020）

High resolution 3D seismic undershooting acquisition over platforms and seismic processing challenges in a gas producing �eld46


vessel （airguns） and the main vessel （streamers） is farther than a single vessel operation, undershooting loses the very near offset data, which results in less data in the near offset coverage map. To mitigate this issue, the vessels sail path is planned to be as close as possible to the platforms defined by safety standards. Agreed regulation exists on how close the vessels and their equipment can go to the facilities, which is called the CPA （Closest Point of Approach）. For this project, it was mutually agreed between the acquisition contractor,

person responsible for the facilities and the client such that the CPA would be 150 m and 50 m for main vessel and 2nd source vessel, respectively. The equipment to be considered for CPA for the mother vessel are a vane and tail buoy while the 2nd source vessel is a mini vane （Fig. 3）.

3.　3D Seismic Operation

The new 3D seismic survey with the total full fold area of approximately 425 km2 was conducted in September to

Fig. 3　Agreed CPA for this survey

Table 1　Acquisition parameters for legacy and new 3D surveys

Survey Legacy 3D New 3DStreamer parametersStreamer length 3 km 7 kmStreamer depth 5 m Deep-tow 15.5 mStreamer separation 100 m 75（Near） to 90（Far） mNumber of streamers 2 8Nearest offset 110m 220 mLow cut filter 8 Hz @18 db/Oct 2 Hz @6 dB/OctReceiver group spacing 18.75 m 12.5mSource parameters

Source volume 3360 cu.in.4240 cu.in. （Main vessel）4,100 cu.in. （2nd source vessel）

Source depth 4.5 m 6 m for both vesselsSource separation 50 m 37.5 m for both vesselsShot point interval Flip-flop 18.75m Flip-flop 18.75 m for both vesselsOthersFull fold area 160 km2 425 km2

Shooting Direction E-W （85 deg.） E-W （85 deg.）Nominal fold coverage 40 94CMP spacing （Cross& In） 25＊9.375 m 18.75＊6.25 mRecord length 5 sec 7 sec

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October, 2017. The acquisition parameters are summarized in Table 1. There were two sources from the main vessel and 2nd source vessel and both vessels have similar source specifications. Moreover, the application of flip-flop mode has contributed to fill the data effectively in the undershooting area.

The shooting was proceeded according to the following three steps:

1）normal shooting, 2）close pass approach by single vessel and3）undershooting by dual vessels.

With these steps the coverage map was filled around the facilities （Fig. 4）.

3.1 Undershooting mode

The first data acquisition geometry for the undershooting is called, “standard pass”. This standard pass requires the 2nd

source vessel to be positioned on the other side of the facilities （Fig. 5）. This configuration enables the acquisition of the data

below the facilities, although the coverage of the near offset data tends to be less due to the offset between the vessels. Another benefit of this configuration is reduced operational risk as the two vessels are located on dif ferent sides of facilities and thus have plenty of space for the operation.

The second data acquisition geometry is called, “sandwich pass”. In this configuration, the 2nd vessel is sandwiched between main vessel and the facility. This allows for a shorter offset between the source and receivers and has the aim of collecting more near offset data compared to the standard pass mode （Fig. 5）.

Each mode has different advantages, thus ideally it would be best to conduct with both configurations. However, careful consideration was made when deciding the undershooting plan, especially for the sandwich pass mode which is a more complex operational situation.

3.2 Operational difficulties

In practice, one of the key points in the operation is to control the cable feathering. This is mostly impacted by the ocean currents around the streamers which are towed at a depth of about 15.5 m in this case. Generally, the farther offset cables have a larger feathering effect and are thus more difficult to control the cable position. Since two platforms exist within the survey area, the feathering direction against the facilities also needs to be considered for any close approach or undershooting operation. Other than that, the distance between the two platforms is almost equivalent to the cable

Fig. 4　Shooting plan

Fig. 5　Standard pass（upper） and sandwich pass （lower）

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length, therefore we need to ensure that the safe distance from streamers to both planforms is always maintained whenever the vessels approaching the undershooting area, i.e. when the head of the cables are near platform B, the tails are still positioned around platform A. Thus, if the feathering direction is towards the facilities, the main vessel has to be kept further away from the platforms for safety reasons （Fig. 6）. Therefore, less near offset data could be recorded,

especially around the platform B.Another operational difficulty for this survey was the risk of

the floating debris such as logs and fishing gears. If the debris collided with the source or streamer instruments, controlling the survey configuration could have been challenging.

3.3 Coverage map results

A total of six undershooting lines were completed in the survey including one sandwich pass to platform B. Overall the operation was successfully completed with no obvious data missing in the full offsets （Fig. 7）.

All the offset maps （near, near-mid, far-mid, and far offsets） are shown in Fig. 8 and Table 2 summarizes the definitions for

Fig. 6 Streamer feathering impact to the near offset data coverage

Fig. 7　Before （upper） and after （lower） undershooting comparison coverage maps of the full offsets

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each of the offset range and their coverage percentage. Most of the area was successfully covered without any significant gap in near-mid, far-mid and far offsets.

One of the challenges for the undershooting survey is the near offset coverage as they can be acquired only when the vessels come close to each other around the facilities. In fact, it is not possible to fill in the near offset perfectly due to its configuration. In addition to this, it was not easy to proceed as per plan due to the operational difficulties of handling the streamer feathering and debris around the facilities.

3.4 Undershooting operation

The undershooting operation started with the streamer vessel positioned in the south of the study area. This was

because the feathering direction was opposite to the facilities （from north to south）, and then part of the southern data gap

area was successfully filled （Fig. 9）. However, the feathering became almost straight in the 3rd line and the main vessel was to be positioned more to the south compared to the plan. Subsequently the 2nd source vessel had to be shifted to the north in order to keep the original expected data coverage zone in the coverage map. As a result, the distance between source and receiver became greater than planned and minimal near offset data were acquired （Fig. 10）.

From the 4th to 6th undershooting lines, the main vessel was positioned to the north of the facilities. Unfortunately, the feathering became stronger towards the facilities from north to south, thus the near offset data were limited in the northern area. The 5th line was to fill in the nor thern gap in the coverage and a sandwich pass was carried out. This enabled us to get some near of fset data. With this line, the entire coverage area for far and far-mid offsets was successfully filled with sufficient fold coverage. With the last line, we managed to acquire the near-mid map with no gap around the 2 facilities （Fig. 11）.

Fig. 8　Offset coverage maps （a） near, （b） near-mid, （c） far-mid and （d） far

Table 2　Offset range

Offset Range Near Near-Mid Far-Mid FarRadical Offset Range［m］

100–1,862.5

1,862.5–3,625

3,625–5,387.5

5,387.5–7,150

Required Coverage［%］ 80 90 80 70

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Fig. 9 Near and near-mid offset coverage maps of unflex binning for 1st undershooting line （upper＝ maps before undershooting, lower＝ maps after 3rd undershooting）

Fig. 10 Near and near-mid offset coverage maps of unflex binning for 3rd undershooting line （upper＝ maps before undershooting, lower＝ maps after 3rd undershooting）

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4.　3D Seismic Processing

The Kirchhoff PreSTM （Pre-Stack Time Migration） and PreSDM （Pre-Stack Depth Migration） were carried out for this project. The application of survey merging and matching techniques were required for the 3 different datasets: the 2017 3D normal acquisition, the 2017 3D undershooting and the

legacy 1992 3D data. The matching was conducted based on a cross-correlation method by calculating phase and amplitude matching. The 2017 3D normal acquisition was used as an anchor for the survey matching and mismatch calculations. The initial survey matching was done for demultiple modelling purpose （Fig. 12）. The residual survey matching was then performed prior to binning and regularisation.

5.　Conclusion

The binning coverage map around the facilities after a total of six undershooting lines shows an obvious missing zone related to the near of fset data. However, the other offsets were successfully filled with the sufficient coverage percentages. The legacy 3D and new 3D data of the producing reservoirs and deep target intervals are compared in Fig. 13. In summary, the new 3D data acquired by the undershooting technique has generated the broadband seismic section that shows better seismic reflection continuity, higher signal to noise （S/N） ratio and clearer images, especially in the deeper section, allowing for a better seismic interpretation of the deeper exploration targets （Fig. 13） and optimizing well engineering. The frequency spectrum is greatly improved, especially in the low frequency range. This will provide better low frequency content enhancing the seismic inversion

Fig. 11 Near and near-mid offset coverage maps of unflex binning for 5th undershooting line （upper＝ maps after 3rd undershooting, lower＝ maps after 6th undershooting）

Fig. 12　Survey matching before （left） and after （right）

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results. However, it is noted that the fault shadow issue in highly faulted areas remains a further challenge to be resolved, which is currently being addressed.

Acknowledgements

We would like to gratefully thank PETRONAS for allowing us to publish this work.

SI unit conversion factor

　　cu.in. × 1.6387064 E － 05 ＝ m³

References

Barousse, C., Herron, D., Stanley, D., Kaldy, J., Flynn D. and H. Ebaid, 2007 : 4D repeatability using dual vessel acquisition: Holstein Field, Gulf of Mexico, SEG Expanded Abstract, 2852−2856.

Ebaid, H., Tura, A., Nasser, M., Hatchell, P., Smit, F., Payne, N., Herron, D., Stanley, D., Kaldy J. and Barousse, C., 2008 : First dual-vessel high-repeat GoM 4D survey shows development options at Holstein Field. The Leading Edge, 27（12）, 1622−1625.

Games, K. P., Pretty, A. J., Stennett, I. A., Wakefiled N. D. and Mann, D. S., 2015 : High resolution seismic undershooting of platforms – stretching the limits. First break, 33（3）, 39−45.

OGP, 2011 : An Overview of Marine Seismic Operations: Report 448, OGP Publications, 50p.

ガス生産フィールドにおける

高分解能三次元地震探査アンダーシュート

収録作業と処理上の課題

佐野　成哉・トランクオックタンジョウギュハン

既存油ガス田を含むエリアにおいて地震探鉱データの収

録を実施する場合，課題の 1つに生産施設の存在が挙げられる。今般，JX Nippon Oil & Gas Exploration（Malaysia）Limitedがオペレーターとして保有するマレーシアサラワク沖鉱区において，新たに三次元地震探鉱データの収録を

実施した。本調査域には 3つのガス田および複数の探鉱プロスペクトが存在し，加えて海上には 2つのガス生産施設が配置されている。本鉱区の既存三次元地震探鉱データ

（1992年収録）では，ケーブル長やエアガン容量などといった収録パラメーターが昨今の収録スペックに比べて劣るた

め，特に深部のイメージングや速度構造の不確実性が課題

となっていた。そこで，2つの生産施設に対するアンダーシューティングオペレーションを含む新規三次元地震探鉱

データ収録，およびブロードバンド処理を実施することで

これらの課題の解決に向けて取り組むこととした。本稿で

は，アンダーシューティング技術とその有効性，ならびに収

録作業上の課題や制限などについて紹介する。

Fig. 13　Seismic section of legacy 3D survey （left） and new 3D survey （right）

53




実際の収録作業においては，結果として計 6本のアンダーシューティング収録を行うことにより，安全上の問題

もなく，生産施設周辺エリアにおける十分なデータを補填

することができた。また，その後の処理では，既存データ，

新規通常データ，新規アンダーシューティングデータの 3つのサーベイデータを適切にマージ・マッチングするこ

とにより，マイグレーション時のエッジ効果などのアーチ

ファクトが抑制されたシームレスな探査記録成果物を作成

することができた。この新規収録作業および処理によって，

特に深部の探鉱目的層準に対しては，イベントの連続性や

イメージング品質が飛躍的に改善され，より正確な構造解

釈および掘削リスク低減の助けとなった。また，低周波数

帯域の拡張により，今後のインバージョン処理においても，

低周波数モデル構築の際に大きな強みになるものと期待さ

れる。

Documents

High resolution 3D seismic undershooting acquisition over