Offshore towed hydrophone linear array: principle, application, and data acquisition results
© Chen et al.; licensee Springer. 2013
Received: 11 December 2012
Accepted: 21 January 2013
Published: 19 February 2013
An underwater acoustic sensing array was presented in this article. With the high-precision sampling clock generation and transmission system, the array can acquire signals synchronously in sub-microsecond level, which is important in offshore environment. Meanwhile, real-time data transmission and storage system was established. All of the data received in host computer can be saved and displayed immediately. Data acquisition experiment was implemented in freshwater reservoir near Tianjin city,China, and the results of the signal wave show that the acquisition and transmission system of hydrophone array can be used to get the underwater information by acoustic exploration.
Acoustic exploration is one of the most important methods to acquire information from the ocean. Conventional wisdom holds that cabled ocean observatories are more widely used compared to wireless systems. While application fields of the wireless underwater sensor networks (WNSN) are relatively narrow. For example, VENUS (Victoria Experimental Network Under the Sea) and NEPTUNE(North-East Pacific Undersea Networked Experiments) ocean observatories are established in Canada, which are the two oceanic projects of University of Victoria in Canada . VENUS is used in the coastal ocean, while NEPTUNE in the deep ocean . But things have been changed in recent years. One application of WNSN was depicted in . Deployment of networks can be cabled, fixed, and wireless. With the development of sensor technology and wireless communication technology, WNSN are no longer just in the secondary status nowadays. As another type of wireless sensing tool, autonomous wave glider was reported in , which is used for long-term working to conduct acoustic exploration. The power of glider is generated autonomously by waves. Glider and rudder are connected to a float by an umbilical. Compared to other vehicles, the self-noise of wave glider is very low, so the acoustic detection sensitivity of it can be improved considerably. An underwater network lab testbed was described in , which contains a real physical environment, such as a set of communication hardware, a programming library, and an emulator.
In the past several decades, the trend of using hydrophone array, rather than single hydrophone, as underwater acoustic information detection method is becoming evident [5–9]. Depending on the detection methods, the ocean acoustic detection can be divided into two kinds: active detection and passive detection. As one typical application of passive acoustic exploration in ocean, the feasibility of short-term seabed earthquake forecasting on East Pacific Rise transform faults was discussed in , which is based on the acoustic data acquired passively from six hydrophones emplaced in the eastern Pacific Ocean, Chile. Through research, it is found that it will have a high probability of foreshocks before the main earthquake in some special seabed strata (e.g., in the Eastern Pacific Rise transform faults), compared with very low probability of the aftershocks earthquake. This feature provides specific short-term earthquake prediction possibility of underwater acoustic array data records, which can predict earthquakes above 5.49° by foreshocks information .
As a type of active acoustic detection, underwater seismic exploration is one of the main technical means to implement the marine acoustic exploration of potential seabed oil, gas resources reservoir discovery, and refinement of in-fill drilling monitor. For instance, four time-lapse seismic measurements in Gullfaks oil field were fulfilled in 1985 (baseline data), 1995, 1996, and 1999. Through time-lapse seismic data, the movement of water injected was analyzed for forecasting recovery factor of the oil field. As a result, the resources recovery can be increased . In the monitoring of oil fields which have been mined, seismic exploration also can be used. By comparing the seismic data of drilling platforms in before mining and mining with the ongoing oil exploration, the difference of sound response between water and the hydrocarbon compound can be used for resources monitoring . Active acoustic detection methods can also be used for real-time monitoring of marine fish density and behavior [8–10]. Compared with the traditional way, the method of ocean acoustic waveguide and hydrophone linear array can implement thousands of square kilometers of real-time imaging, and continuous monitoring in specified sea water. Acoustic data also can be used for monitoring carbon fixation in the deep ocean. By analyzing the data of 1994, 1999, and 2001 in the same seismic reflection exploration region, it can clearly draw the conclusion that data reflected CO2 changes .
A distributed data acquisition system for large-scale land-based seismic data acquisition, which called rDAQ, was reported in . Its data transmission medium is Gigabit Ethernet, file storage format is SEG-D. Clock synchronization of multiple data acquisition node in chain system is useful to improve the performance of hydrophone array. In , a new type of synchronous correction method was discussed, which pass through the master–slave clock recovery system to achieve multiple ADC synchronization of data acquisition in distributed data acquisition nodes. The system described in  is the basis of this article.
This article presented a type of hydrophone linear array which can be used in the acoustic explorations in shallow water. Its high-precision sampling clock synchronization mechanism was illustrated detailed, as well as real-time data storage method. In the end of the article, we implement a field data collection experiment in Qilihai reservoir in the suburban of Tianjin. The data results show that the system is stable, and can be used for acoustic detection of shallow waters.
2. Composition of the hydrophone linear array
Depending on the different location, hydrophone linear array can be divided into on-board equipment and underwater equipment. On-board equipment mainly includes host computer, on-board interface node and power supply, and so on. Underwater equipment is the main body of the system, includes hydrophone groups, which are uniform distribution of linear type; acquisition nodes, which are used to digitize and transmit underwater acoustic signal; head node, which dedicate to communicate with the on-board equipment; and so on. Every data acquisition node includes Data Acquisition Unit (DAU), Data Transmission Unit (DTU), Synchronous-clock Transmission Unit, and Commands Transmission Unit. Every DAU processes 16 channels acoustic signals by 24-bits analog-to-digital converter. Meanwhile, DTU is used to transmit all the data acquired by cascade-type communication channel.
2.1. Sampling clock synchronization
Each node interval cascade arranged about 18 m away. The output frequency of the TCXO is 16.384 MHz which is called f h . Transmission clock fl ~ fml and each node sampling data output pulse frequency of f1d ~ fmd was 4 kHz. As previously mentioned, the synchronization error of data output ticks in all acquisition nodes (i.e., f1d ~ fmd phase error) should as small as possible. Because the array transmission channel did not support the whole Ethernet protocol, thus high-speed clock f h or sync message cannot be transmitted directly in the channel. Instead, the frequency should be divided on fl before transmission. Meanwhile, the high edge steepness and time precision of the waveform still need to be keeping. Each slave clock in acquisition nodes is generated by frequency-doubling from the transmitted clock through a phase-locked loop (PLL). Two inputs of the comparator for PLL error are, respectively, connected with fl ~ fml and f1d ~ fmd. The purpose of phase locking is eliminating the phase difference of two clock signal inputting the comparator. The PLL output is restored from the high-speed sampling clock f1h ~ fmh.
When the electrical signal transmitted in a unit length of the conductors or printed circuit traces, it is proved that there will be a certain time delay between terminal and signal source (called propagation delay unit, tp), which is proportional to the square root of the dielectric permittivity of the signal channel insulate material. For example, the dielectric constant of air is 1.0, thus tp of radio wave is about 3.35 ns/m. The interval of dielectric constant for outer polyethylene sheath in unshielded twisted wire is between 1.8 and 2.8. Its tp is between 4.45 and 5.60 ns/m.
where td is the time delay of frequency divide in the clock source; tc is the time delay of differential interface; l n tp is the time delay of transmission delay for n th acquisition node; tnm is the time delay of the PLL frequency multiplication in the n th acquisition node.
The distance between clock source and acquisition nodes named ln increases linearly with the increase of number n th in formula (2). All the values of ln are known exactly, other errors in synchronous system are determined as well. Therefore, it is possible by means of software to compensate for the total delay in the n th node clock tns, to further improve the accuracy of the clock.
2.2. Real-time data storage
By using the memory-mapped data files in Windows system, we can map the files on the disk to the address space of the software processes. Before the process can access memory-mapped file data from its own address space, Windows requires a process to obtain a predetermined address space in the mapped view area, and to ensure that the view area is accessible for this process. The view mapped only a small part of the data to a disk file every time. It will re-establish a new view of mapping each time when the view storage is done. The starting address should be increased properly.
3. Workbench testing and field data acquisition
Underwater acoustic measurement has practical significance in the ocean resources exploration, the ecological environment monitoring, and other occasions. Hydrophone array with linear formation is widely used in such fields. With high-precision sampling clock synchronization system, we can obtain more accurate data which contain more information, through the experiment carried on QILIHAI reservoir in Tianjin, China.
Moreover, it is shown that towed linear array can be used in shallow environment with high-speed real-time data storage. The synchronous acquisition method is less influenced by environment, and the data storage system can accurately implement the mass data storage.
This study was supported by the grants Program for New Century Excellent Talents in University (NCET), TOA (KX2010-0006), and TSTC (11ZCKFGX03600) in China.
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