Recent Land and Marine Equipment Mitigate Environmental Footprint of Seismic Acquisition
Nicolas Tellier
Chief geophysicist
Sercel

Laurent Guérineau
Manager - R & D Special Projects
Sercel

Environmental rules & regulations, coupled with the recent COP 25 Climate Commitment, have mandated the Seismic Industry to reduce its impact to the environment. The article shares insights on two recently released products for land acquisition and marine acquisition to reduce the Environmental Impact and Carbon Footprint.

For many years, the seismic industry has strived to reduce its impact to the environment. This issue is increasingly topical as the awareness of both public and governments increases. The COP25 summit, ongoing when writing this paper, reminds us that regulations will go ever stronger with time, and that all companies, whatever their business, now have to take all necessary measures to mitigate their environmental footprint.

In land acquisition, the main environmental concern is generally related to vehicle traffic, be it the source vehicles roaming thousands of kilometers in often fragile soil and ecosystems, or the vehicles required for the logistics of the field operation.

In marine seismic, the potential impact of anthropogenic sound generated during geophysical surveys on marine mammals is receiving increased attention from government regulators of several countries, leading to the introduction of mitigation and monitoring guidelines aimed to reduce the potential impact of marine seismic sources on marine mammals.

To adapt to this new paradigm, manufacturers focus their efforts to reduce the environmental impact of their equipment, and help contractors mitigating their environmental impact on prospected areas. This article presents two recently released products: for land acquisition - a compact vibrator, designed to reduce the impact of seismic sources; and for marine acquisition , a system that detect and localize marine mammals in the vicinity of seismic operation.

REDUCING THE FOOTPRINT OF SEISMIC SOURCES
A compact vibrator (Figure 1) was designed to reduce the vibrator's environmental impact, improve accessibility and enable broadband performance. Called Nomad 15 and having a 17,000 lbf peak force, it completes the range of heavier vibrators of the Nomad family (Nomad 65 with 62,000 lbf, and Nomad 90 with 90,000 lbf ).


Figure 1: Nomad 15

Emission Standards Compliance
United States regulations that define the acceptable limits of exhaust emissions (mainly nitrogen oxides (NOx) and par ticulate matter (PM)), applicable to non-road equipment powered by diesel operating within their territories. For the Nomad 15 class of engine, NOx have been reduced by 50% and PM by 90% compared to the previous Stage 3a/Tier3 standard. Compared with the Tier1/Stage1 in force until 1999, the reduction reaches 90% for NOx and 95% for PM. The large majority of vibrators currently available on the seismic market are equipped with engines compliant with lower steps of Stage /Tier norms, than Nomad 15.

Fuel Consumption Reduction
In Vibroseis operation, vibrator groups usually pass through production/ stand-by cycles. The stand-by periods are due to various factors: for example, short weather stand-by, line testing and repair, or third-party interferences. Vibrator stand-by is also dependent on the topography of the survey area (flat desert requiring less stand-by than detour intensive small fields or uneven terrain), and on the methodology used (single source blended acquisition yielding less stand by than single fleet or flipflop).

So while some Middle East or Nor th Africa crews may dramatically reduce stand-by time, it remains significant in many other areas. Unlike road vehicle engine operation, the engines used on vibrators are driven at a constant speed measured by the Rotations Per Minute (RPM).


Figure 2: The Intelligent Power Management concept

Even when vibrator is in stand-by, the engine runs at its full RPM, thus greatly exceeding the power requirements of the vibrator at that given moment. Idling the engine is possible, but requires a driver inter vention that is rarely performed during shot stand-by periods.


Figure 3: Directional steering modes: in addition to the standard mode, where only the front wheels provide steering, the Nomad 15 has two special directional modes to ease access to difficult areas: coordinated mode (left ), the wheels turn in opposition offering an unequalled turning radius, and synchronized mode (right), which allows the vibrator to move in a sideways direction, often referred to as "crabbing".

The Intelligent Power Management (IPM, Figure 2) is a unique feature developed for the Nomad 15, and made available for the entire Nomad family. The IPM automatically adjusts the engine RPM to equal the power required for the vibrator's current mode of operation, without any action from the driver. This is accomplished through measurement of the engine load and the signal denoting the accelerator pedal position. This innovative feature can significantly reduce fuel consumption, as well as noise and exhaust emissions.

A field test carried out by a seismic crew using five vibrators, each operated for more than 2,000 hours, showed fuel savings of up to 15% on the two vibrators equipped with IPM. A second large-scale test enabled to confirm the 15% savings on fuel consumption enabled by the solution.

Noise Mitigation
Another important aspect in Vibroseis operation is noise emission, as the power required to shake the ground necessitates the use of large engines. The vibrators available on the market, in their standard configuration, do not come equipped with noise mitigation devices. Optional soundproof covers are available, but are rarely purchased by contractors. Note that soundproof covers are now mandatory for operation in some areas, such as the European Economic Area, and that is requirement is likely to spread. Noise assessment in the field is not easy, as it is highly dependent on measurement distance and location, surrounding noise, engine load, engine cooler and air conditioning operation. Manufacturers do not communicate much on vibrators noise level. Nonetheless, without an engine soundproof cover, it is common to have noise levels above 95 dB one meter from the engine, and 85 dB seven meters away. Such levels of noise become an important issue when operating in populated or fragile wildlife areas.

On the Nomad 15 vibrators, noise reduction is ensured by the means of an included soundproof engine housing, as well as the IPM system that reduces engine noise during stand-by periods. The maximum noise level is then reduced to 77 dB seven meters from the side of the vibrator, and less than 70 dB seven meters in front of the vibrator.

Accessibility and Broadband Capacity
The compact vibrator proposed offers unequalled accessibility to numerous areas usually closed to larger or heavier vibrators. Compact dimensions, high maneuverability and an excellent turning radius (Table 1) enabled by four directional wheels and several direction modes (Figure 3) ease access to the tough areas that constitute a large part of seismic operation playgrounds in India, such as settlements, narrow access agricultural lanes, roads where oversized vehicles are not permitted or mountainous areas. The different directional modes offer the best chance to get out of difficult terrains or mud stuck without external winching or assistance. A powerful hydraulic transmission makes it also a capable climber of even the steepest slopes (over 55%).

To foster the development of broadband acquisition, Nomad 15 offers high performance at both low and high frequencies. The Nomad 15 can initiate a sweep at 1 Hz with reduced force, reaching full force at 7 Hz. High frequencies of up to 400 Hz are achievable, depending on ground characteristics, and are made possible by an extra stiff circular baseplate and an hydraulic peak force of 17,364 lbf that exceeds the hold-down weight (16,135 lbf ). Then latter feature helps to compensate for the massto- baseplate phase shift above the ground cutoff frequency, thus enabling to gain an extra bandwidth of high-fidelity signal in the high frequencies.

QUIETSEA MARINE MAMMAL MONITORING SYSTEM
Over the years, we have seen an ever-growing number of regulatory agencies requiring or encouraging the use of Passive Acoustic Monitoring(PAM) for real-time detec tion and localization of marine mammals(Figure 4) within an Exclusion Zone (EZ) around source vessels, in order to minimize the environmental impact of marine seismic sources to the wildlife present in the survey area.

The Exclusion Zone, usually defined as the radius around the seismic sources within which mitigation measures (such as seismic source shutdown) applies is generally set at 500 m.


Figure 4: Sperm Whale

Previously Available PAM Systems
Previously available PAM systems were typically comprised of a dedicated towed array containing several hydrophones, an onboard signal conditioning and data acquisition device, all of them connected to a dedicated computing system.

Although the potential value of Passive Acoustic Monitoring as a realtime mitigation tool has been recognized by most regulatory agencies, the previously available PAM systems, while well suited for research and scientific use, were quite limited in terms of effec tiveness for commercial marine seismic surveys. Indeed, on the acquisition vessel, the management of a dedicated PAM towed array poses safety concerns for operators during the deployment and retrieval phases. These PAM systems also greatly increased the risk of entanglement with lead-ins and streamers, which increases the likelihood of unnecessary downtime and equipment replacement costs for the seismic contractor (Figure 5).


Figure 5: PAM towed array entanglement with lead-in

PAM towed arrays are usually deployed a few hundred meters from the back deck of the seismic vessel. Thus, the boat-induced noise masks the vocalization of marine mammals and the vessel wash acts as an acoustic barrier, both of which hinder the system's ability to detect cetaceans.

Besides, these PAM systems typically rely on a single linear antenna containing a limited number of hydrophones, which leads to several restrictions in terms of system performance:
  • Limited detection and localization performance in the vessel forward direction (while it is a direction of particular interest), and inability to solve the port/starboard localization ambiguity.
  • The limited number of hydrophones may not provide enough information for proper localization in some cases, and does not offer any redundancy in case of hydrophone malfunction.
  • The use of a single antenna results in operational downtime at night in case of entanglement. Indeed, PAM being the only mammal-monitoring tool available at night (as marine mammal observers cannot visually check the presence of mammals), its unavailability leads contractors to wait until dawn to resume operations.
  • Towed arrays do not provide any QC status concerning their state of health, thus increasing the risk of operating a malfunctioning system.
  • Poor low frequency response that may exclude some whale species from being identified through acoustic monitoring.
Previous PAM system software are also quite not intuitive, making it unfriendly to configure and operate:
  • Expert PAM operators are required for configuration and operation as there are no standard software settings for optimal results. System performance is inconsistent and highly dependent on the skills, ability and experience of the operator.
  • Expert skills are required to analyze the data, confirm acoustic detections, reject false alarms, provide range estimates, etc. This subjective interpretation is operatordependent and results in inconsistent, unreliable performance.
The QuietSea Solution

Figure 6: QuietSea in-sea architecture

A recent fully integrated passive acoustic monitoring system, called QuietSea, overcomes most limitations of the current PAM systems. Designed to integrate with the Seal 428 seismic acquisition system and SeaProNav navigation system, and incorporated in the Figure 6: QuietSea in-sea architecture Sentinel streamer (Figure 6), this system offers various benefits to seismic contractors.

By eliminating the need to deploy additional PAM antennas at sea, the QuietSea system mitigates the probability of accidents during deployment, retrieval and operation, thus reducing the subsequent probability of downtime and equipment replacement cost.

The bidirectional communication with the navigation software, coupled with the network of broadband in-sea modules seamlessly integrated within the Sentinel streamers, provides improved cetacean localization accuracy and real time repor ting of detected events for faster decision making (Figure 7 ). QuietSea offers an enhanced monitoring of the Exclusion Zone (EZ) and beyond. Thanks to the very low noise Sentinel hydrophones, the system utilizes up to 512 sensors to monitor baleen whales, such as blue whales, down to 10 Hz.

Additionally, numerous broadband hydrophones seamlessly integrated within the streamers and placed in strategic positions (on streamer heads, outside the vessel wash and close to the center of the EZ) constitute large, redundant array that provides good detection and localization of baleen and toothed whales, regardless of the listening direction.

QuietSea relies on advanced automated detection and localization algorithms, which drastically decrease the false alarm rate, delivering truly objective, consistent and reliable information for decision making, regardless of the skills, ability or experience of the operator.


Figure 7: SeaProNav display showing the Exclusion Zones and a cetacean localized by the system, at the front starboard side of the vessel. This cetacean, located in front of the vessel, would not have been detected by traditional PAM systems.

The QuietSea GUI is also designed to be intuitive and user-friendly, requiring minimal settings and relying on self-adjusted software parameters to deliver stable performance across various environments. The rugged and reliable in-sea modules are based on a field proven design, with a built-in Quality Control capability that allows the system to assess the health of the hydrophones as well as the detection performance of the modules.

QuietSea provides seismic contractors with a reliable PAM system that optimizes the control of their environmental footprint. It already equips numerous vessels in the seismic industry, and its performance and benefits have been recognized and underlined by all users.

Conclusion
New equipment accompanies the efforts of seismic exploration industry to reduce its environmental impact.

In land acquisition, noise, particle emissions as well as fuel consumption are significantly reduced thanks to compact vibrators that include features such as IPM, engines complying with emission reduction standards and soundproof housings. Combined with high accessibility and broadband capacity, such vibrators are ideal seismic sources for various applications, such as shooting in the limited access areas widely encountered in India, mixed source shooting (where large and small vibrators operate according to terrain type for optimal productivity) or imaging of shallow targets with high frequencies (e. g., for mining or Smart Cities). It proves a good solution also to replace partially or completely explosive, by using safer and more productive sources. As environmental regulations a rebecoming more and more stringent, manufacturers have to adapt their equipment in order to fulfill the latest requirements.

In marine acquisition, the traditional Passive Acoustic Monitoring is recognized as a promising tool to complement the mitigation measures adopted during marine seismic acquisition. QuietSea integrated Passive Acoustic Monitoring system addresses most of the limitations encountered in previous PAM solutions, making it the most intuitive PAM system available for marine seismic surveys. By carefully balancing both the expectations of the regulatory agencies and the operational constraints of the seismic contractor, systems such as QuietSea will help Passive Acoustic Monitoring gain the wide acceptance it deserves among the marine seismic industry while actively contributing to the reduction of the environmental footprint of marine seismic surveys.