Feature Articles—October 2009 Issue
MVP CAST Gauge Optimizes Seismic and Hydrographic OperationsComputer-Assisted Sound Speed Technology Enhances The ODIM MVP Real-Time Water Column Profiling System
By Derrick R. Peyton
Director
Business Development
and
Mark Smith
Manager
After Sales & Service
ODIM Brooke Ocean
Nova Scotia, Canada
Sound speed information is an important component of seismic and seafloor mapping. Knowledge of oceanographic variations in the water column for these types of operations has been largely based on historical information integrated with sparse on-site observations.
Variations in sound speed can be caused by a variety of factors, such as tides, currents, precipitation, solar heating and freshwater estuaries. Relying on historical sound speed data will not give results accurate enough to meet today’s survey standards.
The introduction of the ODIM Moving Vessel Profiler™ (MVP) has shown that sound speed profiles can be collected at a high spatial and temporal resolution while the survey vessel is under way. The MVP allows for conductivity, temperature, depth (CTD) data to be logged in preparation for submission to onboard production systems.
Recent advances have facilitated the real-time monitoring and display of sound speed information, as well as other sensor parameters, for immediate review and analysis in the decision-making process. These advances allow for an optimization of the system such that sufficient sound speed profiles can be acquired to produce accurate information for seismic and hydrographic operations.
Seismic Exploration Operations
It can be argued that most oil used in the future will come from existing reserves or those that have been identified and scheduled for production. With dwindling oil reserves and the enormous cost of deepsea oil exploration, the seismic industry has been moving more and more to 4D seismic activities. The primary purpose is to determine accurate oil field reservoirs so that existing reserves are adequately mapped and all remaining amounts of oil and gas are efficiently recovered. Estimating the size of remaining reserves and the probability of recovery provides valuable data for monitoring and managing supply and demand.
Coupled with this are the technological advances that have been made in the seismic industry, whereby streamer counts have increased from four to 16 and streamer lengths have increased to eight kilometers. This, in turn, has resulted in a massive volume of data that presents an enormous challenge to geoscientists in the form of data management, processing and interpretation. For example, today’s turnaround times for seismic surveys can be a matter of weeks. Data can be streamed in real time to onshore geoscientists who obtain initial results and make survey vessel line shooting decisions while a project is under way.
One of the factors that limits the final resolution of seismic data is the speed of sound in the water column. Variations can be significant enough to cause problems with seismic processing like crossline discontinuities, mis-stacking, amplitude variations with offset distortions, and dip movement out and migration artifacts.
Seismic Effects on Marine Life
An important part of observing water column characteristics while conducting seismic operations is to model the potential effects of seismic air gun noise on marine mammals and fish. Seismic noise effects are believed to induce a shift in mammal hearing thresholds and even displace or mask calls between individuals.
The ocean is an acoustic environment in which sound travels greater distances than light. Consequently, marine organisms rely heavily on their hearing to find food, avoid danger and communicate.
Seismic operations are thus under stringent regulations to operate at a safe distance from marine mammals, but variations in the water column can result in the bending of sound rays. Consequently, the propagation of seismic noise is not trivial and requires a wave theory model to accurately calculate the distance vs. noise level from the seismic source. The results of sound propagation models are used by regulators to define the safety radius for marine mammals around seismic arrays.
Thus, as with the case of resolving seismic processing for timely and accurate geophysical results, the ability to collect real-time water column CTD information while under way can also be relevant for estimating the safe distance at which a seismic sound source interacts with marine mammals.
Multibeam Bathymetric Surveys
To obtain accurate acoustic propagation modeling, spatial and temporal sound speed profiles are important factors. In addition, accurate acoustic propagation modeling also requires knowledge of the bathymetry and sediment geoacoustic properties. Multi-beam hydrographic surveys have become mainstream and are now widely used for both hydrographic and seismic survey requirements.
Due to the wide swath width of multibeam systems and the large temporal and spatial sound speed oceanographic variations, knowledge of the speed of sound in the water column is very important to ensuring accurate survey results.
The International Hydrographic Organization’s Standards for Hydrographic Surveys requires that hydrographic surveys account for sound speed uncertainties in order to determine the total propagated uncertainty (TPU) of a sounding. The Canadian Hydrographic Service and NOAA’s Office of Coast Survey both stipulate in their field procedures and deliverables that the TPU of a sounding be determined and that frequent sound speed profiles be observed for best practices in acquiring this information.
Real-Time Water Column Profiling
The MVP greatly enhances the productivity of CTD, sound speed and other specialized profiling by allowing water column casts to be conducted from an underway vessel. The ODIM MVP consists of sensors housed in a small, streamlined free-fall fish; a conductor cable with strength member; a computer-controlled, high-speed hydraulic winch; and a complete cable metering, overboarding and docking system. The sensor information is transmitted in real time through the conductor cable to the survey vessel for immediate input into the multibeam collection system.
The MVP allows the CTD fish to free-fall near-vertically. Deployment is executed under computer control and can be restrained by three parameters, which are the desired depth of cast, the preset height above the bottom or the maximum cable out.
Using this concept, the system can achieve a much deeper depth for a given vessel speed than a comparable towed system. Once the programmed downcast depth has been reached, the fish is automatically winched back to a towed position near the surface, where it can be recovered or redeployed.
Analysis of Sound Speed Data
Refraction artifacts are typically dealt with in post-processing, but this is time consuming and could require significant processing expertise.
From a hydrographic perspective, the best approach is to monitor and assess refraction artifacts in real time by monitoring the sound speed variability that is the cause of the artifact. This involves isolating the ray tracing portion of the depth-reduction procedure and computing the bias in sounding depth and horizontal position that would be incurred had the most recent profile not been collected, i.e. if the previously collected cast had been used instead.
By comparing sequential pairs of sound speed casts, the hydrographer can ascertain if the current profiling rate is sufficient for maintaining accuracy. With a multibeam “uncertainty wedge,” the hydrographer is acutely aware of the impact of sound speed collection rates on sounding accuracies. Using this tool to monitor sound speed variability has the potential to greatly reduce refraction artifacts in the raw data—thus minimizing post-processing efforts.
The same approach could possibly be taken toward assessing marine biological range constraints on a seismic source by inputting sequential sound speed profile information into acoustic wave theory models.
The MVP CAST Gauge Advantage
The benefits of this approach are realized when this concept is integrated into the MVP technology. An MVP Computer-Assisted Sound Speed Technology (CAST) gauge has been developed as a means of visually displaying and quantifying the results of sequential real-time CTD casts.
The goal is to provide a mechanism that will enable the user to predict when to make a sound speed cast based on quantifiable information, such as the range uncertainty due to changing water column conditions as observed along a sequence of casts performed by the MVP in real time.
It should be clarified that the uncertainty value in this context is the difference between the epoch 2 sound speed profile and the sound speed profile of epoch 1. No consideration is given at this point to the measurement uncertainty of the sensor itself.
The MVP CAST gauge provides a color-intensity plot that gives an intuitive graphical presentation of sound speed variability over a survey line. When set to automatically update, the image of the developing variability is updated in real time.
The MVP CAST gauge will greatly enhance the user’s knowledge of the oceanographic environment in the survey area. This will reduce the overall cost of the survey by automating the acquisition of sound speed profiles while under way. In addition, a more automated system can be realized if the MVP CAST gauge results are submitted directly into the seismic or hydrographic operational systems in support of decision-making processes.
Conclusions
Due to potentially high spatial and temporal variability, the sound speed component of a water column is one of the most difficult parameters to monitor. To reduce this uncertainty, it is recommended that one increase sound speed profile acquisition rates, but this approach is quite costly if it involves stopping a survey vessel. The ODIM MVP provides the advantage of automatically acquiring sound speed profiles while the survey vessel is under way and transporting that data to the seismic or multibeam system in real-time.
The software behind the MVP CAST gauge has been developed to optimize operational cost and to monitor and maintain accuracy. The CAST gauge integrates the concept of computing and visualizing sound speed uncertainty by comparing ray path analysis from one epoch to the next and displaying this result in real time. In this way, the user can execute sound speed casts based on a near real-time quantitative analysis of sounding uncertainty.
Acknowledgments
The authors would like to acknowledge the consultation and guidance of Jonathan Beaudoin of the Ocean Mapping Group of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick. In addition, they would also like to thank Steven Smyth of ODIM Brooke Ocean and Mike Lamplugh of the Canadian Hydrographic Service for the recent development and testing of the CAST gauge.
Derrick R. Peyton holds a diploma in survey technology, a B.Sc. and an M.Sc. in geomatics engineering and an MBA. He has acquired certification as a P. Eng, a Canada lands surveyor and an International Hydrographic Organization Category A hydrographer. He has conducted various surveys in seismic exploration, dredging, U.N. Convention on the Law of the Sea, cable routes and nautical charting.
Mark Smith is manager of after sales and service at ODIM Brooke Ocean. He holds a diploma in mechanical engineering technology and is certified as an engineering technologist. He has worked on the ODIM Moving Vessel Profiler for more than 10 years, and he has extensive experience in geophysical exploration and seafloor mapping.
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