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Dr. Richard Stocker, PhD, MSc. Senior Geoscience Consultant
Pablo Alvarez, MSc., BSAT. Senior HSE Advisor
VISION PROJECT SERVICES (UK) LTD., Dorset, England.

Introduction
Any client contemplating a marine seismic program should be cognizant of the general principles for seismic acquisition. The details are also very important but the brevity of this article precludes their discussion. If the client adheres to the general principles, the right consultants and contractors will attend to the details.

Exploration Objectives
All seismic acquisition should flow from well-defined exploration objectives. These are geological objectives translated into seismological objectives, that is, the characteristics and attributes of the finally processed seismic data. They dictate the acquisition design and the technical specifications.

The Contract - Stand-by Time
The best contract is turnkey with chargeable stand-by time. The turnkey provision puts the responsibility on the seismic contractor to be logistically efficient when the crew can work but acknowledges situations out of the contractor's control when the crew cannot. In such circumstances, the contractor should be compensated for the operating expenses. If extensive standby time occurs and the cont

Necessary Consultants
An instrument engineer should conduct a pre-production audit and act as a resource throughout the entire program. Consultants to perform the dockside verifications and calibrations of the positioning equipment and to examine the positioning data from the first acquired data are the norm.

Client representatives for real time verification to ensure the work complies with the technical and HSE specifications. Extensive QC seismic processing or final data pre-processing on board may require a seismic processing client representative.

For marine acquisition the standard is two sets of rotating client representatives, one for seismic and one for navigation-positioning. Commonly the client only requires an initial HSE audit before the vessel leaves port. At sea, the seismic and navigation-positioning client reps have responsibility for overseeing HSE. Some clients may also require a technical audit, which is conducted partly at the dockside, and at sea during the initial deployment of the equipment (energy source and streamer(s))

Selecting Consultants and Contractors
The simple answer is to select personnel who have done a good job previously but this begs the question. For individuals, recommendations from trusted and knowledgeable colleagues who have worked with the people are the best method. Given the gravity of this decision, a telephone interview is warranted.

Selecting a seismic contractor is much more difficult than selecting individuals. Many, if not most seismic contractors have many crews. Not all are of equal quality. The composition of crews change with time. What was a good crew may become less so and vice-versa. Many seismic contractors divide the world into different administrative regions and the regions may not have the same attitude toward HSE and data quality. Every seismic contractor, to hear them speak, is dedicated to world-class HSE procedures and data quality. However, seismic contractors are not charitable organizations. In effect, seismic contractors sell the time of their personnel and rent their equipment to the client. The better the HSE procedures and the higher the data quality, the more time is required to complete the program and often more equipment is necessary. World class HSE and high-resolution data cost more – as a client do not expect something for nothing from your seismic contractor.

The technical and HSE specifications must be part of the package sent to the seismic contractors bidding on the work. How else can the seismic contractors bid appropriately? The client should involve the consultants in writing said specifications. The instrument engineer should comment on the appropriateness and the reliability of the instruments and sensors proposed by the bidders. One seismic contractor may propose equipment with a significant advantage. Remember that the specifications are the "rules of the game" that the "referees", i.e. the client representatives enforce. Poor rules leave the client representatives powerless.

The Acquisition Design
The mantra of all seismic acquisition is acquiring data which meets the exploration objectives at minimum cost. Clearly, this goal cannot be accomplished with imprecise exploration objectives. Even with precise exploration objectives the acquisition design can overdesign or under-design the program. Overdesigning the program means the exploration objectives will be met but the cost will be unnecessarily high. Under-designing the program means the data will at best only partially meet the exploration objectives. The client needs two independent designs to consider.

The seismic industry has made remarkable technological and methodological progress over time. The issue for the client is, do you need the "latest and greatest equipment and/or methodology"? For marine acquisition do you need MAZ or WAZ methodology? If you do, be thankful it is available. If not, it represents overdesign. For an expert and objective discussion of these issues read the books and papers by Gijs Vermeer.

The Technical Specifications
The technical specifications, despite their importance, are the bete-noir of acquisition. Learned treatises, papers and meeting presentations consider acquisition design. Very little public discussion of technical specifications is available. Seismic contractors have their internal specifications but generally are loath to disclose them

Since the technical specifications concern what constitutes lost data and the percentage of lost data that is acceptable, the seismic processor should be the ultimate decider. For example, it is not the signal-to-noise ratio on the raw records that is important but the signal-to-noise on the final processed data. How seriously does a channel failing a particular test degrade the trace? What tests can be failed and still have acceptable data? The instrument engineer knows the effect of a test failure on the amplitude and phase characteristics of the response but only the seismic processor knows if incorporating such data would do more harm than good.

In the field, the observers have to make the judgment whether to record or stand-by in real time. They require numbers for the maximum permissible ambient random noise and how many channels can be above the limit. They need to know what tests to run on the instruments and sensors and what are the tolerances. The instrument engineer has the expertise to advise the client on the tests and tolerances. The seismic processors, given they have at least a sample of the seismic data, know the maximum permissible ambient random noise and how many channels can be above the limit. The recommendations of the seismic processors can then be translated into the numbers required by the observers.

Extensive and Comprehensive Start-up Meeting
An extensive and comprehensive start-up meeting should be conducted after the seismic crew has completely mobilized and just before the parameter testing, Attendees should include the client, client representatives, the instrument engineer if possible, the heads of department from the seismic crew, the party chief, the country manager and the seismic contractor regional HSE and technical gurus if possible. All technical and HSE specifications should be discussed deliberately and thoroughly, and then agreed to as appropriate and doable by all parties.

Parameter Testing - Production Testing
To reduce the expense of parameter testing all relevant data should be analyzed prior to generating an acquisition design. Existing seismic and well data are the most valuable. Clients need to do their homework.

The ultimate parameter testing is production testing. A production test acquires an appropriate interval of fullfold seismic data along a production line and then evaluates the parameters from the fully processed seismic data. The client has either to have the data fully processed on-board or have fast-track seismic processing on shore.

The final concept is that of seismological areas. The optimal values of acquisition parameters for a given seismological area differ from those of adjacent seismological areas. Differences in bathymetry, near-surface and subsurface geology create seismological areas. The larger the program area the more likely there is more than one seismological area. Each seismological area requires a separate parameter and production testing.

Approaches to Seismic - Rational versus Budget-Constrained
The principles stated form the basis of rational seismic exploration in which data meeting the exploration objectives is the independent variable and cost the dependent variable. Hopefully the budget for the project is adequate. Cost is the independent variable in the opposite approach, which may be termed budget-constrained seismic or you-get-what-you-get seismic or hope-and-pray seismic. This approach is quite common, in fact probably the more common. Again, one hopes the budget is adequate, otherwise the data will only partially met the exploration objectives or may not meet them at all. One may contemplate the aphorism that the most expensive seismic data is data that fails to meet the exploration objectives.

Health, Safety and the Environment
For the HSE professional, marine seismic offers the advantage that Safety is an integral aspect of all marine operations following IMO (International Maritime Organization) regulations as well as IAGC and OGP directives on this subject.

All personnel and crew in a boat can only board after following minimum training, which ensures that HSE is part of the marine mind set.

Still, HSE in marine operations is a demanding task. Many sets of regulations must be followed to ensure compliance with "best practices". One example is MARPOL, the International Convention for the Prevention of Pollution From Ships (1973, modified by the Protocol of 1978). Guidelines and regulations are clearly explained there, so the role of the HSE personnel is to manage a complex system, as opposed to implementing one.

Training and drills are also clearly stated and scheduled. That leaves the HSE personnel the freedom to concentrate on the quality and effectiveness of the training program, as opposed to whether these drills are performed (or which ones should).

Still, high seas operations are complex scenarios. In streamer marine jobs, the handling of the heavy equipment poses clear dangers. Proper planning, from fully fledged operations (those that are not routine) to the daily "Tool Box Meetings" must be carried out, and documented. The TBM's can be used specifically for planning and reviewing procedures, which may have a higher level of risk if the crew begins to see them as routine and "easy". It is the role of the HSE professional and Heads of Departments to foresee problems that break with this routine and can lead to incidents.

In Ocean Bottom Cable (OBC) jobs, the key issue is the use of several different types of crafts. HSE must ensure that different regulations and levels of seaworthiness mesh together, and that maintenance and safe practices cover all procedures. OBC also calls for land bases, mixing both land aspects with marine. This usually leads to duplicate or "extended" HSE departments.

These must ensure that emergencies in both land and sea are taken care of while not relying on the same personnel and services. The intrinsic spreading of personnel, crafts and equipment over both sea and land make these operations more complex than the Streamer jobs, calling for a more detailed planning of the initial aspects of the operation, as well as the day to day after the job has started.

Labels: Exploration, marine seismic, offshore acquisition, Russian oil gas

posted by The Rogtec Team @ 17:02 

2 Comments:

 Marine Surveying said...

Great report on Marine Seismic Surveys, a resource and primer for those interested in the field!

11 September 2009 09:15  

 The Rogtec Team said...

Doug,

I am a bit of a fan of format standards and it's interesting that these are not mentioned in the article. This is a very important aspect of data acquisition and with the advent of SEGD 3.0 it will become extremely important for data owners to specify how they wish to record data. SEGD 3.0 if used correctly becomes part of the whole life-cycle of data. The format is not only multi component and has taken away many historic limitations but has recommendations on how to write a table of contents and positioning data so that the dataset is a complete entity. The SEGD 3.0 format is due to be ratified at this year's SEG meeting in Houston so now is the last time for those interested to comment.

For further information on the current draft please go to:

Index of ftp://ftp.troika-int.com/segfield/SEG_Technical_Standards/SEGD_3.0/Drafts/

Regards

Jill Lewis
MD - Troika

28 September 2009 15:25  

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