It was not an unusual situation when Apache was awarded two new exploration licenses in blocks adjacent to existing Apache production areas, nor was it unusual that the two areas were separated from the old by a simple wire fence; what made this situation unusual was that the fence did not just mark a boundary between two landowners or between two blocks — it marked the international boundary between two countries.
This was the case in late 2007 in Tierra del Fuego, close to the southernmost tip of South America, with the award to Apache of exploration rights in the Lenga and Russfin blocks, which lie immediately west of Apache’s Los Chorrillos and Uribe blocks; the problem was that the Los Chorrillos and Uribe blocks lie in Argentina, but Lenga and Russfin lie in Chile. Geological trends defined on new 3-D data in Argentina showed promise across the border in Chile based on mapping of legacy 2-D data, so a 3-D dataset was needed that spanned the international frontier without the detrimental edge-effects that surround every migrated 3-D data volume. We needed to undershoot that fence.
Seismic acquisition in Argentina had been in progress for eight months and was nearly complete at the time of the Chile award, so Apache had a good understanding of the issues of working in the area and acquiring quality data, but the challenge faced was how to economically tie two adjacent surveys in two countries without infringing upon laws of either nation that govern cross-border movement of personnel, equipment, or data.
The perfect solution would have been to mobilize two complete 3-D acquisition crews and acquire full receiver spreads for every shot, regardless of which side of the frontier it lay, by slaving one recorder to the other. However, acquisition was nearly complete on the Argentina side of the frontier, and retaining a full crew to re-record a narrow strip east of the fence was not economically attractive; the Argentina crew needed to be small, light, and mobile to be cost-effective.
This left two options: to drill and load shots in Argentina and record them into the receiver spread laid out as part of the primary Chile survey, or to lay out receiver tails in Argentina to record the shots fired in Chile. Both solutions had advantages and disadvantages, and neither was perfect, but both were believed sufficient to address the problem at hand — achieving continuity across the border.
At that time Apache still had drills and explosives in Argentina sufficient to redrill the necessary points, but this would have required detonation of explosives in Argentina from a remote location in Chile, and aside from the potential challenge of finding, licensing, and maintaining a common, quiet, VHF radio frequency in both countries throughout the project, there was a fundamental unease with the concept of cross-border detonation of explosives — especially in an area where in 1978 border tensions were so high that mines were laid. Looking at the options for receivers in Argentina, there was also concern about the connecting of line cables across the fence marking the frontier. Once connected, which country was the cable now in? When disconnecting, it would be critical to know which country this cable was imported to in order to keep equipment counts in order. Wireless transmission of data across the border using laser or microwave links to virtually connect cable based equipment in each country was also considered, but as was mentioned earlier, a minimal lightweight crew was wanted, which a full cable telemetry crew is not.
Finding the right solution
The solution was found in the then-recently released OYO Geospace GSR seismic recorder. Made up of a set of autonomous, continuous-recording nodes, the GSR allowed the Argentina receiver tail-spreads to be deployed and retrieved by a very small crew of 12 using only light vehicles and a small base camp, and as each unit records a continuous data series into memory, there was no need for any cross-border data transmission, which also alleviated any concern about the legality of such transmissions. Integration with the Sercel recorder was also straightforward; the addition of a timing device to the Chile shot firing equipment that captured the precise GPS timestamp of each shot was the only modification necessary to the Chile recording system. Using the Argentina-side receiver stations, tail-spreads could be recorded for the necessary Chile shots, thus achieving continuous sampling under the frontier.
The Chile survey was laid out as a westward extension of the Argentina survey using the same geodetic datum and projection to remove a potential source of positioning error and with the same line and point spacing and binning grid, both to enable easier synchronization of the two crews and to minimize problems with the survey merge in processing. Receiver line numbering was maintained, but not enough space existed in the station numbering scheme to extend all the way to the west of the Chile blocks — when initially numbering the Argentina survey, it was not anticipated that the survey could be extended west of the fence marking the border.
Communication between the crews was extremely simple: once a line was released from the spread in Chile, a message would be sent to the crew in Argentina to retrieve that line and return the units to base camp for data harvesting and battery charging. Once downloaded and recharged, the units would then be returned to the field to become the new front line of the spread. At periodic intervals the timestamp file from Chile would be sent to the crew in Argentina, and when combined with the day-long time series recorded by each unit, the Argentina crew was able to create the shot gathers associated with each Chile shot within a half-spread length of the block limit.
In processing, the new Argentina gathers were combined with the Chile gathers to create complete and seamless shot records for each Chile shot and then re-sorted along with existing Argentina prestack data from the overlap zone prior to prestack migration. Phase matching was achieved using a model-based wavelet process that removed the known characteristics such as sensor and instrument filter responses for the different recording systems and data vintages to create a stable and consistent wavelet free of those differences prior to migration.
The cross-border project was a success, both operationally and geophysically. No time was lost on the primary Chile crew due to issues or delays on the secondary Argentina crew and vice versa, and a data recovery rate over 98% was achieved in Argentina, a statistic that answered the fears of some about the reliability of shooting blind. By removing the migration edge-effects from the frontier side of both surveys, the use of the node-based system added hundreds of fully imaged square kilometers at a cost per kilometer lower than either primary 3-D survey, enabling the production of a seamless 3-D volume that covered the Lenga and Russfin blocks in Chile as well as the Los Chorrillos blocks in Argentina, a combined 3-D volume covering 1,030 sq miles (2,666 sq km) with no seam under the frontier.
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