A field test was conducted with autonomous marine vehicles (AMVs) and 3D sensor arrays (3DSAs) to record and compare seismic data generated during an ocean-bottom-cable (OBC) survey. The test was a field verification to check that the AMV platform and the sensor array can deliver high-quality seismic data in a form that can be successfully processed and compared with ocean-bottom fixed-receiver data. The feasibility test conducted offshore Abu Dhabi demonstrated the successful and safe deployment, seismic-data acquisition, and retrieval capabilities of the AMV and 3D sensor array.
AMVs are an alternative to conventional methods of acquiring marine seismic data; they are designed with the aim of increasing offshore safety and reducing risk while delivering a quality service within lower-cost pricing models. These unmanned vehicles have expanded the envelope of offshore operations and have been instrumental in increasing productivity and safety in marine environments. Such vehicles, because of their low profile and flexibility in maneuvering around obstructions, can be placed close to obstructions, reducing the risk typically involved in operations of this nature. The vehicles have proved capable of carrying out a wide range of vital ocean-monitoring functions formerly assigned to manned vessels over a longer time period and provide a viable alternative or supplement to acquiring seismic data in cases where adverse existing factors may impede standard acquisition methods.
The AMV used for the test offshore Abu Dhabi is a hybrid sea-surface and underwater vehicle that has been proved to enhance exploration and production in marine environments by collecting and delivering real-time measurements in areas formerly too costly or too challenging for operation. The wave-powered sensor platform enables collection and delivery of data gathered at sea on missions lasting up to 1 year. The AMV is a two-part system that consists of a surface float and a submerged glider, connected by a high-speed communication umbilical tether (Fig. 1 above). The vehicle’s propulsion system is passive and mechanical; it exploits the natural difference in wave motion between the surface float and the submerged glider to convert energy from wave motion into thrust. Articulating fins attached to the submerged glider convert wave energy and generate thrust as they pivot vertically. The vehicle produces forward propulsion independently of wave direction as its float moves up and down with each wave and the submerged glider tows the float forward. The AMV is capable of holding station, or it can be programmed to travel directly from one location to another by following a specific route defined by multiple sets of geographic coordinates, or waypoints. The solar energy system on the float recharges the batteries onboard the vehicle that power the navigation system, payload electronics, and auxiliary thruster. The vehicle can carry a wide range of sensor payloads for various applications, including seismic-data recording.
The AMV is well-tested, with more than 300 vehicles having performed more than 4 years of client missions from the Arctic to Australia. Since 2010, these AMVs have traveled more than 1,000,000 nautical miles globally and completed the trans-Pacific mission from San Francisco, California, to Australia that set the world distance record for autonomous ocean vehicles (9,380 nautical miles). These AMVs have completed more than 150 missions, collected millions of discrete data points, and navigated through 17 hurricanes and three cyclones, continuously transmitting data while clocking more than 28,400 days at sea. To date, 33 missions have been conducted in the oil-and-gas environment.
Remote navigation by means of a secure web and satellite system allows real-time control of, and communication with, the vehicle, and enables operation of the vehicle with reduced risk to personnel and at a lower cost. A global-positioning-system receiver on the float determines the vehicle position and provides a precise time stamp for the data recorded on the mission. The vehicle’s flexible design makes it easy to integrate a broad range of custom sensors or plug-and-play payloads. Examples of sensor payloads that can be configured according to client specifications include meteorological sensors; wave sensors; acoustic modems to harvest data from sensors mounted on subsea structures or the seafloor; bathymetry sensors; current sensors; fluorometry systems to detect the presence of oil, turbidity, and chlorophyll in the water; magnetometers; and cameras to provide real-time imaging.
For seismic applications, the vehicle is equipped with a 3D sensor array (3DSA) attached by a motion-isolating tow cable. The 3DSA consists of 15 hydrophones (three in each of the five arms) mounted on a frame approximately 1 m in size. A buoyancy engine immediately below the top arm ensures that this arm remains on the high side and that the entire array is oriented and does not rotate on its axis while moving through the water. The 3DSA continuously records seismic data as well as array-orientation and -depth data. Seismic data are naturally organized in common receiver (3DSA) gathers, and, typically, the sum of the 15 individual hydrophone measurements per shot point is processed.
Seismic-Acquisition-Feasibility Test: Case Study
Three AMVs, each towing a 3DSA, were deployed during the acquisition of an OBC 3D survey in shallow water offshore Abu Dhabi. The test was conducted to assess the feasibility of seismic acquisition with AMVs and a 3DSA. The test included safe deployment and retrieval of the sensor array and evaluation of its performance on the basis of the vehicle’s ability to hold station. The test also measured the system’s ability to maintain desired depth and determine the accuracy of measurements of pitch and orientation and compared the quality of the acquired seismic data with the pressure data recorded in the OBC survey. The acquisition design of the OBC survey incorporated Managed Source and Spread (MSS), a technology designed to maximize acquisition production rates while managing interference noise from adjacent shots. MSS involves two sources firing nearly simultaneously but managed with a time- and distance-separated window so that the data have minimal seismic interference noise contamination within the objective zone of interest. Additionally, the survey was acquired with a small (2,740-in.2) source array volume towed at a depth of 5 m in order to minimize reverberation caused by shallow-water bathymetry and the hard seafloor.
The water depths across the acquired survey area and in the test vicinity averaged approximately 20 m. Because of the shallow water depths, towed-streamer acquisition is seldom used offshore Abu Dhabi, limiting the marine seismic-acquisition methods to either OBCs or ocean-bottom nodes (OBNs). The AMV and 3DSA are, therefore, ideally suited to operate and record seismic data in this environment and to potentially provide a viable alternative for off-bottom recording or as a method to supplement OBC or OBN data acquisition.
Seismic-Data Processing and Results
Three vehicles were deployed and collocated near an OBC receiver cable and recorded the data from two near-pass source lines. The OBC receiver nearest the offset to the source line was 0.9 km, while the vehicle nearest the offset to the source line was 1.35 km. A basic processing sequence, comprising a debias low-cut filter, swell noise attenuation by means of singular-value decomposition to suppress low-frequency swell noise from 2 to 4 Hz, and nonuniform coherent noise suppression to target Scholte wave energy, was applied to the 3DSA recorded data. The compressional-P component OBC recorded data were treated similarly; however, the Scholte wave energy was simply muted by use of an outer trace mute application. The receiver gather and stack results were then compared. Data analysis and frequency-band splits indicated that the spectral content of the data recorded by the 3DSA was comparable with the data recorded in the OBC hydrophone. Similar noise content was observed on the 3DSA data, in which the guided wave energy from the previous simultaneous source was recorded and visible at higher offsets, while the late arrivals originating from the second distance-separated source vessel are observable below 5.0 seconds.
In addition, visible Scholte wave energy was recorded in the 3DSA data, possibly attributable to the very thin water column of 20 m in the 3DSA vicinity combined with the sensor-array tow depth of 10 m. The recorded Scholte wave energy, predominantly in the 4- to 12-Hz pass band, is observed to be diminished at higher offsets for the 3DSA data, which was attributed to the OBC hydrophone being situated on the seafloor while the 3DSA was located off the bottom. Data-processing tests exploit the spatial distribution of the hydrophones in the 3DSA that enables estimating the spatial gradients (by use of finite difference) of the recorded pressure field. Both pressure and gradient data can be used in processing (e.g., receiver-side deghosting and determination of the propagation direction of the wavefield).