Offshore seismic surveys are expensive to acquire, so companies that pay for these surveys want the data to have as long a “shelf life” as possible.

Figure 1. Two sets of crosslines. On the left, (1), seismic is merged with original navigation data (labeled Orig Nav). On the right (2), seismic is merged with reprocessed navigation data (labeled New Nav). (All images courtesy of AS Geograf)
With advances made in reprocessing, these surveys can often be useful 20 years or more after their original acquisition if the original data were of good quality. But it’s not just the seismic data that matter — navigational data are absolutely critical to accurately locate areas of interest. Good-quality seismic data can become useless without accurate navigation data.

Historically, when seismic surveys were reprocessed, the navigation information was left out of the process and used in its original processed version. In the last few years there has been more emphasis on ensuring the quality of the navigation data. This is in part related to the increased use of 4-D seismic where it is critical that the navigation data is correct for both the base survey and the repeat survey, but also because wells have been drilled in the wrong place due to incorrect navigation. Many believe that the navigation information should be quality-controlled in conjunction with every reprocessing project.

AS Geograf has been built around this understanding of the importance of navigation data and quality control (QC). The company was founded in 1996 as an independent navigation and survey consultancy company with the primary aim of serving the oil industry as well as Norwegian mapping authorities. Geograf has experience in offshore exploration and field development projects as well as the processing of seismic and navigation data, and it has undertaken the QC of both corporate and central positioning databases, the processing of bathymetric data, and the evaluation and recalculation of geodetic networks.

Case study
A client wished to carry out a traditional navigation QC of a North Sea 3-D survey from the 1980s before reprocessing the seismic data.

The original source/receiver navigation data were stored on UKOOA P1/90 format, and the raw navigation data were stored on WISLON format. The WISLON data were converted to a UKOOA P2/91 format as the first step in navigation QC. This was carried out on a selection of lines representing about 10% of the total, using Geograf’s ResProg navigation QC system.

The QC indicated possible errors of 50 to 65 ft (15 to 20 m), mostly in the inline direction.
Figure 2. Time slices at 500ms: Left timeslice is from seismic dataset where original navigation data is applied. Right timeslice is from seismic dataset where reprocessed navigation data is applied. Footprint caused by errors in the navigation data has been removed.
To verify or invalidate a positioning error, navigation data for 16 lines were selected to be reprocessed. The methodology included several steps:
• Selecting a sub-volume where problems were evident;
• Stacking the sub-volume using existing navigation data;
• Selecting new parameters for complete reprocessing of the navigation data within the sub-volume;
• Applying the new dataset to the seismic and stacking the sub-volume again to verify the results of the new navigation dataset; and
• Repeating this process in instances where the problem is particularly complex.

The stack, after applying the new navigation data, showed great improvement in both resolution and in the continuity of the reflectors (Figures 1 and 2).

Time-lapse surveys
In time-lapse, or 4-D, seismic surveys, repeatability is a key factor to determining whether or not differences between two surveys over the same area can be attributed to fluid movement in the reservoir. Therefore, the first step in planning a 4-D survey should be to ensure that the baseline survey is correctly positioned. Due to the increased interest in 4-D surveys, Geograf has become more involved in the planning of these surveys.

Ideally, all of the shot points, both receivers and sources, would be in exactly the same position during repeat surveys as they were in the baseline survey. In the real world, ocean currents, new infrastructure and other issues make this goal difficult to achieve. But recent emphasis has been to reproduce the source position as closely as possible.

Geograf conducted a pilot study to produce a pre-plot for a 4-D survey based on reproducing the source position for two adjacent surveys. The specifications differed between the two surveys, and the client also wanted to extend the new survey in three directions.
For the northern part of the survey, the shot points for the new pre-plot needed to be positioned in the middle between a nominal pre-plot and the position of the actual shot point. This was to ensure that subsequent 4-D surveys would not move further away from the original pre-plot.

For the southern part of the survey, a pre-plot did not exist. The survey was acquired based on fold coverage. The source position path for this survey was smoothed to prevent amplification of its sinuosity.

Several issues had to be resolved, including the handling of re-shoot lines, the merge zone in the area where the lines were extended, infill lines, etc.

The task clearly called for the use of a geographic information system (GIS) to collect,
Figure 3. The northern part of the survey with shot points colored according to the process that was applied to them.
manage and analyze the large volumes of spatially referenced information (position of 200,000 shot points). GIS software includes spatial indexing techniques that make the storage and manipulation of large volumes of spatial data fast and secure.

First, the navigation data was converted from its UKOOA format to a comma-separated text file (.csv), a format that’s easily imported into a GIS system. In addition to transferring selected attributes for every seismic shot, the import tool also calculated pseudo source positions and the position between the active and the passive source as well as the feather for every shot.

The data were then imported from .csv into a geodatabase, ideal for collecting several loosely coupled datasets and also for creating a geometric line for every seismic line. This enabled the team to build lines between parallel shot points and then create new shot points at the center point of those lines.

The advantage to using GIS for this type of geographic planning is the visualization options available. During the process, all shot points were tagged with information about what kind of process, was applied to them (Figure 3). Such “metadata” information can be used to visualize the planning process.

Geograf plans to extend the use of GIS to manage and analyze all kinds of positioning data and other data relevant to a seismic survey such as receiver positions, compasses and tail buoys, cable feather, water depth and acoustic measurements.