Cost Reduction and Quality Improvement in 3D Marine Seismic Surveys using local tidal conditions
Authors: Kapotas Sotiris , Bordenave Claude 
Published: October 2008

Abstract
In the recent surge of hydrocarbon exploration in marine environments via 3D seismic methods that require more sofisticated equipment associated costs, especially in remote areas of the world like Tierra del Fuego, have to be analyzed and optimized using all possible constraints. The most cost effective of these constraints is the use of natural phenomena to plan the seismic acquisition methods accordingly. We will present here a methodology called "tidal prediction and matching" that, using the characteristics of the strong and predictable tidal streams offshore Tierra del Fuego, in conjunction with a series of feasibility studies using existing data, not only reduced our acquisition costs substantially but also helped overcome limitations of subsurface imaging techniques.

INTRODUCTION
The growing use or 3D seismic data in both the Development and Exploration Phases of Oil and Gas Exploration has increased the relative weight of seismic expenditure within the overall budget of a project. It is therefore essential that the Explorationist has an accurate and reliable estimate of the overall cost of a survey as well as the expected data quality. For a typical Marine 3D survey, acquisition accounts for approximately 60% of the total cost of a project. During the 3D marine seismic acquisition phase, the seismic boat pulls simultaneously a number of cables, which include constantly spaced hydrophones, and air gun sources, that generate seismic waves to be registered (listened to) by the hydrophones (Fig 1). Typically the distance between these 337 cables is 100m spread over an area of 400 to 500m and the boat follows successive sail lines in opposite directions over the zone of interest. At each passage the boat registers data, within the extent of the cables in both lateral directions that corresponds, in depth, to a narrower band of subsurface imaged information. However, during the acquisition stage, holes in the subsurface image distribution occur due to a variety of factors, cable feathering being the major cause. Cable feathering is a term used to indicate the deviation of cables due to currents (Fig 2). These holes require infill (re-sailing of the boat) which is typically between 15 to 30% of the total kilometers of a survey, resulting to a proportional increase in costs and problem creation for imaging techniques. We will show here how these costs and problems can be reduced using the predictability of tidal streams.

PROPOSED METHODOLOGY
In order to reduce the above mentioned infill passes, two majors approaches have to be taken: Use of 3D feasibility studies as a means of recommending acquisition parameters, predicting the expected quality of the data and estimating the financial impact of key stages in acquisition and processing. In particular, the effects of feathering angle and resulting infill requirements. Use of the strong and predictable tidal streams to orientate the seismic lines in order to minimize feathering and match adjacent lines.

I) Feasibility study
This first approach can be done before project initiation, when the specific objectives of the study should be clearly defined and understood by all parties involved. The geological targets should be clearly stated and all available information including previous seismic, well and geological data should be made available.