Wireless vs. Wireline Land 3D Seismic in North Italy M. Pellegrino*, N. Pajola, G. Tortini, A. Esposito, P. Follino (eni e&p), A. Di Lecce (Geotec SpA) Summary Land seismic acquisition in densely populated areas is often affected by severe operative problems, mainly due to difficulties to access the survey area caused by different permitting issues, and more in general by the presence of obstacles. As a consequence, it is usual to acquire 3D seismic surveys characterized by important discrepancies between the planned and the actual lay out. These variations can typically produce negative effects in the final seismic data quality. The use of cable-free acquisition systems can help to solve this problem. We present here the results of a recent case, where both acquisition systems (wireline and cableless) were deployed at the same time, recording simultaneously two independent datasets over an area of 22.5 sqkm for a total of 4707 cableless receiver points and 2770 shots. Introduction A conventional 3D land seismic survey was designed to be carried out in the Po Valley (Italy), during Q4 2011. Taking advantage of this project, we decided to test the field performances and the data quality of a wireless 3D seismic volume acquisition, through a comparison between two independent datasets acquired simultaneously. Both receiver spreads were laid out in the field with the same theoretical source-receiver geometries. The main scope of the test was:

assess the operative advantages and disadvantages of the wireless acquisition, when compared to the conventional wireline in a highly urbanized and extensive farming environment;

evaluate the entire workflow of the wireless system and the related performances, from initial channel setup to receivers lay out and retrieval, data harvesting and download.

Characteristics of the survey The area, where the survey was carried out, is a plain ground, densely inhabited and characterized by the presence of little towns, several small rivers and watercourses. Just in the middle of the area there is a huge gas storage plant, together with farms, agricultural products warehouses and cattle sheds. Moreover, different networks of underground pipelines (water, gas, electric line) are present over the whole operations field.

The very same 3D theoretical parameters defined by the survey design were used for both the wireline and wireless campaigns. The active spread was made of 960 channels, deployed over eight receiver lines, spaced out by 300m each other. Receiver interval was 30m, shot lines were perpendicular to geophone ones and the shotpoint interval was 60m. The distance between shot lines was 180 m, and the nominal fold to be achieved was 40th. The seismic source used for the acquisition was dynamite, and after the initial set of tests a combination of 9/12 m depth and 1or 2Kg of dynamite per shot-hole was selected. The presence of the different obstacles and the safety distances to be kept from sensitive buildings and premises forced to significantly modify the theoretical source-receiver geometry. In particular, shotpoints positions were quite often put in offset, due to the security limitation established for dynamite management.

Fig. 1 - Source-Receiver final actual lay-out. The wireless project The on field operations were designed to have two independent crews for wireless and wireline spreads, each one fully dedicated to the specific activities related with seismic channels management. The conventional 3D survey took place from 7th November 2011 to 24th January 2012 (57 working days), while the wireless dataset was recorded between 16th November 2011 and 16th January 2012 (42 working days). The initial project area was split into eight swaths and five out of eight were recorded in duplicated mode. This means a nominal full fold area of 30.4 sqkm acquired in wireline

Wireless vs. Wireline 3D Land Seismic in North Italy

mode, 22.45 sqkm of which recorded using also the wireless spread. The total number of recorded receiver locations was 4707, and the related shotpoints were 2770. Exactly the same (spike type) geophones were used for both the spreads: SM4 (12 geophones per single channel). Aim of the project Aim of the test was the evaluation of the performances of a totally wireless spread, in terms of system reliability, advantages and drawbacks verified on field, necessary personnel effort. Six parameters were considered for a global assessment of the system and a complete comparison with the conventional survey operations: fold, killed traces, on field seismic crew personnel involved, system weight, time management, data quality. Fold Unexpectedly, the use of cable-less system produced a slightly higher fold of coverage in some areas. This was due to the possibility, mainly in the more densely inhabited areas, to better match the required active channels theoretical positions with wireless system rather than with the conventional wireline. This feature applied in particular for 145 out of 4707 total receiver points.

Fig. 2 - Difference between the wireline and the wireless spread laid out into a small town present in the area. The lateral offset distribution, measured comparing the actual on field position of wireless and wireline channels, showed that the average displacement was respectively about 30 and 65 m, with a maximum deviation of 206 and 139 m.

Fig. 3– Wireline Channels lateral offset distribution.

Fig. 4– Wireless Channels lateral offset distribution. Killed traces The percentage of killed traces referred to the two different active spreads in both cases was always under 1.5%. We also noted a progressive reduction of killed traces in the wireless dataset. This can be explained by the progressive enhanced familiarity of the equipment gained during the work by the seismic crew operators. Therefore, we infer that the percentage of faulty traces could be further reduced as long as the seismic crew is getting more and more acquainted in optimizing the troubleshooting operations and the whole data management. This feature should not be underestimated, since many seismic crews all around the world are not yet completely used to work with wireless systems. From the statistical point of view, especially in remote areas, it is likely to be forced to use, at least for a part of the survey, a wireless spread managed by a not perfectly acquainted seismic crew.

Wireless vs. Wireline 3D Land Seismic in North Italy

Personnel involved A significant reduction of the personnel dedicated to manage the seismic channels was achieved. In fact, all the activities not directly related to the spread management, such as permitting, surveying, drilling, shooting and recording were led by the same personnel. However, two completely independent crews were set up for the wireline and wireless spread management. After just a few days it was possible to reduce the personnel of the wireless crew by one third less than the wireline one. System weight The weight of the used wireless external batteries (12V 18Ah) is the most important factor affecting the total equipment weight. Indeed, we noted that the total weight of the wireless system exceeded the weight of the wireline by approximately 29%. This was mainly due to the weight of the batteries, which was 7488 Kg vs. 504Kg necessary for the wireline spread. Specific tests on battery life were also carried out to define the best trade-off between the on field charge duration and its related weight. In particular, three different sets of batteries with different capacities were used. 7Ah, 9Ah, and 15Ah batteries were tested. 18Ah batteries, typically used during this acquisition, guaranteed the wireless channels operations for about 20 days. The lighter 7Ah and 9Ah batteries were able to last for 5-7 operation days, but since they have to be recharged more often, they look more attractive and suitable for seismic surveys with a high daily production (minor time presence of wireless seismic device on the same station) or in places where manpower costs are not excessive. Conversely, 15Ah and 18Ah batteries, although heavier, need a less frequent rotation, and therefore they are best suitable for surveys with a low daily production rate and high manpower costs. Time savings Fig. 5 shows the spreads layout time efforts. The time saving during the layout operations using the wireless system, estimated at 50%, is quite evident. This is mainly due to the presence of watercourses and rivers and the lack of bridges and crossing roads in the area. This forced the front (lay-out) and back (pick-up) dedicated personnel to long detours for the conventional spread management, which is obviously not needed for the wireless spread.

Fig. 5 – Wireline vs. wireless spread layout time efforts. Data Quality As expected, the raw field data (wireline and wireless) do not show any significant difference in the signal content. The differences are just noise and they are likely related to geophone coupling effects. The wireless dataset was processed separately through the same sequence and parameters of the benchmark wireline. The final image stacks are absolutely equivalent; the difference sections show noise mainly related to repositioned receivers and slight variations in the near offset coverage.

Fig. 6 – Difference between wireline and wireless data. Raw shot gather (left) and final migrated stack (right). HSE The most difficult links for allowing the continuity of the wireline spread are those necessary to cross roads, watercourses, premises and different obstacles. Since they can be in general avoided or at least much reduced using a

Wireless vs. Wireline 3D Land Seismic in North Italy

wireless system, the potential risks for the seismic crew personnel are definitively decreased. Conclusions The results of the test carried out in the Po Valley proved the high reliability of the wireless system and highlight the opportunity to exploit a hybrid wireline and wireless spread to optimize the recording operations, both in terms of field efficiency and data quality. As expected, no significant differences in the quality of both raw and processed data acquired with the two different technologies were observed. We should also mention that a full integration between any recording systems and any wireless spread would be highly recommended. Indeed, a complete consistency in data format, electronic routines and workflows in general is essential to manage the seismic acquisition in the best and safest way. From the operational point of view, the weight and duration of the batteries is still one of the most critical issues related to the wireless system. The batteries weight reduction expected in the next future and the possibility to choose different battery types according to the project environment conditions will allow decreasing all the costs related with warehousing, management and manpower devoted to this activity. The experimented on field high reliability of the system make it definitively useful in projects to be carried out in harsh environment or difficult terrain morphology, where laying down connection cables between seismic channel would be challenging or even dangerous. Moreover, the use of cableless system in densely populated areas can significantly reduce the potential HSE risks related to the management of the seismic channels spread out over the survey area. Nevertheless, we must consider that seismic acquisitions in densely inhabited area are often affected by electromagnetic interference phenomena, as well as many different kinds of external noise sources. With regard to the first problem, data transmission protocols should be improved in order not to be excessively sensitive apart from the seismic information flow itself. The second issue can currently be mitigated using hybrid spreads, with a combination of wireline and wireless channels: in fact, totally blind acquisition mode does not allow the real time monitoring of external noise sources, and the possibility to acquire data not compliant with technical specification and difficult to be later processed in not to be overlooked. Real time ambient noise monitoring is a desired development, which could potentially lead to a dramatic increase in the wireless technology use and distribution. Finally, we propose that the future system development efforts are directed to achieve the complete removal of any

cable, wire-link with the seismic channels geophones strings included. Acknowledgments Authors wish like to thank eni e&p for the permission to publish this work, and all colleagues involved in the work. Geotec, Mitcham Industries and Sercel are also acknowledged for their important support.