recent advances in nodal land seismic acquisition …recent advances in nodal land seismic...

AEGC 2019: From Data to Discovery – Perth, Australia 1

Recent advances in nodal land seismic acquisition systems Tim Dean* Denis Sweeney Curtin University – Exploration Geophysics SuperSeis Pty Ltd Bentley, West Australia Queensland [email protected] [email protected]

INTRODUCTION

Early on in the history of seismic acquisition the advantages of being able to acquire data without the impediment of cables was identified (Burg 1941). Often cabled systems suffer down time due to cable problems (e.g. connectors becoming unplugged, line boxes losing power, cables being cut) so the removal of the cables promised to improve the reliability of the system. Unfortunately, cables are also the method by which data is transferred from the sensors to the recording system so early systems required a wireless method to transfer data. Early wireless or nodal systems therefore relied on radios to transfer data. Initially, data was transferred sequentially from each recording unit after acquisition of a record, but later systems could transfer data from multiple units in real-time. Due to their bulk, limited battery life, and limited channel count, the use of such systems was typically limited to transition zone surveys where conventional, cabled, land systems were unsuitable and the water was too shallow for a marine streamer survey. The nodes were typically mounted on floats attached to anchors with the sensors themselves placed in the water (Figure 1). The first system extensively used for land acquisition was the Seismic Group Recorder or SGR introduced in the early 1980s (Shave 1982). The SGR differed from other systems in that the data was recorded internally on tape. Development of radio systems continued through the 1990s, such systems either sent the full dataset in real-time or sent limited QC data with the full data being downloaded manually later. The next major innovation occurred with the introduction of the Ultra G5, which was the first to utilise continuous data

recording (rather than radio triggered recording). Although it continuously recorded data it still required time synchronisation messages to be sent via radio. The first non-radio real-time data system introduced was the VibTech (later Sercel) Unite, which used a mesh Wi-Fi system to transmit data. The first ‘blind’ (i.e. no real-time data or QC status) system was the Geospace GSR launched in 2007. This approach was enabled by the introduction of low-cost GPS chips that enabled time synchronisation across multiple disconnected units. As with the Ultra G5, it incorporated continuous recording with the shot records being extracted or ‘combed’ based on source GPS times after the data had been manually downloaded. The system utilised a digitiser unit with separate geophone and battery (Figure 2) and this was the approach taken with other systems introduced at this time including AutoSeis, iSeis Sigma, and INOVA Hawk.

Figure 1. Diagram showing the use of a node to acquire data as part of a transition zone survey. Reproduced from Olofin and Will (1989).

Figure 2. Photo of the Geospace GSR (courtesy of Geospace).

SUMMARY Early on in the history of seismic acquisition the advantages of being able to acquire data without the impediment of cables was identified. Despite the introduction of numerous systems since the 1970s nodal land acquisition systems have only relatively recently become popular. We begin this paper by looking at the history of nodal land seismic acquisition systems. We then look at the rapid developments that have occurred over the last 10 years, including reductions in weight, increases in recording duration, and the increasing popularity of integrated systems. The question of which nodal system to use depends to a large extent on the survey location and requirements. Overall, we see a trend towards using larger numbers of lighter nodes. Key words: Nodal acquisition, land seismic.

Recent advances in nodal land seismic acquisition systems Dean and Sweeney


Downloadable (or blind) nodes all work in a similar way. The nodes are deployed into the field programmed to record continuously during the standard acquisition times. The GPS times of the sources are then recorded by the source control system. Once back in camp the data from the nodes is downloaded or harvested, usually by placing them into racks, and, if required, the batteries recharged. Individual records are then separated or combed from the continuous time datasets using the source initiation times. The resulting common-receiver gathers are then sorted into the more familiar common-shot gathers for further processing. The next major advancement came with the introduction of the Fairfield ZLand. This system incorporated the digitiser, geophone, and battery into a single integrated unit (Figure 3 left). This had the disadvantages of being limited to recording a single geophone type and being unable to change the battery to keep the digitiser in use (it takes a lot longer to charge a battery than download the data) but the simplicity of the approach made it highly successful at the time.

Figure 3. Photograph of the original ZLand node (left) and the latest version (right).

MODERN SYSTEMS Since 2014 ten new nodal acquisition systems have been introduced. In this section I will briefly describe each system and its peculiarities. We begin by looking at the five new integrated systems. The first of these is an upgraded ZLand which has an increased battery life (12 to 40 days), lighter weight (2.2 to 1.8 kg), and smaller case (Figure 3 right). The SmartSolo system differs from the others in that it splits into two sections as part of regular operations (Figure 5). The lower section contains the battery and the spike which also serves as a screw to secure the two sections together, while the upper section contains the digitiser and geophone. This has the advantage, similar to external battery systems, in that the battery can be changed allowing the digitiser to be used more efficiently, but obviously requires the unit to be disassembled (although this gives an added advantage in that there are no exposed download/recharge contacts). An additional advantage is that the battery section can be replaced with a higher capacity (although larger) battery and/or the sensor section with a 3-component unit (Figure 6).

Figure 4. Photograph of the five new integrated nodal systems. From left: ZLand, SmartSolo, Quantum, GCL, and NuSeis.

Figure 5. SmartSolo split into its two sections, the section on the left contains the battery and the one on the right the geophone and digitiser.

Figure 6. Photo of the different SmartSolo battery and sensor configurations (from left), single component with standard battery, 3-component with extended battery, 3- component with standard battery, single component with extended battery. The INOVA Quantum node (Figure 4) is the lightest commercially available node at 650 g and also has the longest battery life (100 days). It has a unique method of switching on, with the unit recording data whenever it is placed vertically. Similar to SmartSolo, the GeoSpace GCL node (Figure 4) also dispenses with external connector pins but in this case, it is charged and downloaded via inductive coupling. The final integrated node is the GTI NuSeis (Figure 4). This node clearly has a different form factor to the others and has been specifically designed to ensure that the node is well coupled to the ground (the node is inserted into the ground up to the metal collar, although it can also be fully buried). To enable the node to be planted special tools are used to create an appropriately sized hole in the ground into which the node is placed.



Three new eternal sensor systems have also been introduced (Figure 7). These include the Geometrics Atom, which has been designed for small-scale surveys and thus has limited battery life (3 days) but does allow data to be harvested via WiFi and charging via USB. The Geospace GSB is an evolution of the GSR that includes an internal battery (although the use of an external battery is still an option). Finally, the INOVA HawkHD is the latest version of the Hawk system and has a lighter weight (0.9 vs. 1.7 kg) and lower battery consumption (155 vs. 309 mW for a single channel unit). The RT3 system also has the option to connect an external sensor but this is unlikely to be the preferred configuration.

Figure 7. Photograph of the three new external sensor/sensor & battery systems. From left: Geometrics Atom, Geospace GSB, and INOVA HawkHD. Finally, real-time data and QC nodes are limited to the Wireless Seismic RT3 and the Sercel WTU508 respectively (Figure 8). The RT3 is an evolution of the RT2 system, which was fully radio-based, that uses WiFi to move data from sets of receivers to a radio backbone. The WTU508 system also uses WiFi but utilises a network hoping architecture between units so doesn’t require a backbone. The WTU508 has an additional advantage in that it offers the ability to wirelessly harvest the data from the nodes. The Hawk system offers the ability to wirelessly harvest QC data and most of the other systems offer some form of wireless system check (typically obtained using Bluetooth).

Figure 8. Photographs of (left) the RT3 real-time data system and the Sercel WTU508 real-time QC system. Note that unlike Figure 4 and Figure 7 these units are not shown to scale.

THE FUTURE One of the stated advantages of nodal systems has always been a reduction in weight, but as the weight of nodes has come down so has the weight of cabled systems. Figure 9 is an adaption of a graph given in Lansley, Laurin, and Ronen (2008) that shows the current weight of a modern cabled system (Sercel 508XT) compared with that of modern nodes depending on the station interval. The station spacing at which nodes are lighter has decreased, previously it was more than 50 m, but it is still between 8 and 30 m, i.e. at small station intervals cabled systems are lighter.

Figure 9. Weight per channel comparison for cabled and nodal acquisition systems. The red and green lines are taken from Lansley, Laurin, and Ronen (2008). The light blue line is a modern cabled system and the dark blue box current nodal systems. Reproduced from Dean, Tulett, and Barnwell (2018). The Sercel 508XT cabled system also overcomes issues with cable breaks that have affected previous cabled systems by having memory inside the line boxes that buffers data. If the line is broken then the system automatically finds an alternate route to transmit the data back to the recording truck (assuming one exists of course). A new non-commercial system, first introduced in 2018, is the ‘nimble node’ system designed to be used as efficiently as possible to enable crews employing hundreds of thousands of channels (Brooks et al. 2018, Manning et al. 2018). The node itself is 13 x 4 cm and weights just 150 g. To avoid external connectors, it employs optical downloading and inductive charging. The node does not employ a geophone but its size and expected cost, does allow for denser receiver spacing thereby providing equivalent or better data quality. The Nimble node, like the NuSeis node, has a form factor focused on optimising sensor ground coupling.

Figure 10. Nimble node (left), charging/download rack (right). Adapted from Manning et al. (2018). Another acquisition system currently under development is METIS (Multiphysics Exploration Technologies Integrated System). METIS is specifically designed for overcoming the logistical issues of acquiring surveys in the jungle of Papua New Guinea and is an acquisition philosophy that covers geometry, sources, and receivers. On the receiver side the



system employs nodes that are dropped into position by drones (Figure 11). The darts are based on the Wireless Seismic RT system 2 that transmits data in real-time to

Figure 11. Photo of a Metis node or DART (Downfall Air Receiver Technology).

DISCUSSION AND CONCLUSIONS

Nodal seismic acquisition systems have developed rapidly over the last 10 years. Hardware wise, node weights have decreased dramatically whilst recording durations have increased. Although systems with separate batteries, digitisers, and geophones were common ten years ago, most recently introduced systems incorporate these elements into a single unit. This removes any remaining issues with cables and connectors (the cause of many issues on cabled crews) and makes the systems simpler to use. As developments progress we expect that these nodes will continue to get smaller and lighter, consistent with the new Nimble Node (Figure 10). The continued move to integrated nodes is coupled with an increasing trend to replace analogue strings with individually recorded sensors. Of course, having smaller nodes is only part of the solution in some areas, the development of METIS shows that survey efficiency also comes from how you deploy, collect, and harvest the nodes. Particularly with the interest in higher channel count crews we expect all these issues to be addressed, to some extent, through automation. The question of which nodal system to use depends to a large extent on the survey location and requirements. If weight is the biggest concern (e.g. for heliportable crews) then clearly one of the lighter nodes is advantageous. If different sensors need to be deployed then a node that supports multiple types of external sensors (Figure 7), or that can be adapted to employ external sensors (NuSeis, RT3) would be required. Overall, we see a trend towards using larger numbers of lighter nodes. Coupled with these advances in hardware technology will come changes in the way that nodes are handled in the field, with a greater emphasis on automation. Data comparisons that we have acquired to date suggest that the use of nodes has data quality as well as logistical

advantages (Dean and Sweeney 2019b, a). The acquisition geometries that these nodes might enable is also an area of future study.

ACKNOWLEDGEMENTS

Thanks to the various equipment companies. In particular to Geophysical Technologies Incorporated (GTI) who provided a test system to Curtin University.

REFERENCES

Brooks, C., A. Ourabah, A. Crosby, T. Manning, J. Naranjo, D. Ablyazina, V. Zhuzhel, E. Holst, and V. Husom. 2018, 3D field trial using a new nimble node - West Siberia, Russia. Paper read at SEG Technical Program Expanded Abstracts. Burg, K. E. 1941, Prospecting method and apparatus. US. Dean, T., and D. Sweeney. 2019a, The effect of land seismic recording system noise levels on survey productivity. Paper read at 81st EAGE Conference & Exhibition, at London. Dean, T., and D. Sweeney. 2019b, The use of nodal seismic acquisition systems to acquire limited-scale surveys. First Break, 37, no. 1. Dean, T., J. Tulett, and R. Barnwell. 2018, Nodal land seismic acquisition: The next generation. First Break, 36,47-52. Lansley, M., M. Laurin, and S. Ronen. 2008, Modern land recording systems: How do they weigh up? : The Leading Edge,888-894. Manning, T., C. Brooks, A. Ourabah, A. Crosby, M. Popham, D. Ablyazina, V. Zhuzhel, E. Holst, and N. Goujon. 2018, The case for a nimble node, towards a new land seismic receiver system with unlimited channels. Paper read at SEG Technical Program Expanded Abstracts. Olofin, D. K., and R. A. Will. 1989, Acquisition and Processing of Shallow Water 3-D Seismic Surveys Over Producing Fields in the Northwest Niger Delta. Paper read at Offshore Technology Conference, at houston. Shave, D. G. 1982, Seismic group recorder system. Paper read at SEG Technical Program Expanded Abstracts

recent advances in nodal land seismic acquisition …recent advances in nodal land seismic...

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