FROM analyzing speech to recording earthquakes, tracking submarines, or imaging a fetus, measuring and analyzing acoustic signals are increasingly important in modern society. Acoustic waves are simply disturbances involving mechanical vibrations in solids, liquids, or gases. Lawrence Livermore researchers are developing advanced techniques for extracting and interpreting the information in these waves. In the course of extracting data from acoustic signals, the researchers have developed complex and creative algorithms (mathematical relationships implemented in computers) that at times mimic the reasoning processes of the human brain.|
One leader of Livermore's acoustic signal-processing research is electronics engineer Greg Clark, who is involved in three disparate acoustics projects: heart valve classification, where acoustic signal processing is determining whether an artificial heart valve is intact or needs replacing; oil exploration, where Livermore experts are automating a key procedure used for locating undersea oil deposits; and large-structure analysis, where Livermore is preparing to use acoustic wave vibrations to assess the integrity of several large mechanical structures in northern California.
Making sense of acoustic signals requires researchers to develop realistic computer models and develop algorithms for separating signals from contaminating noise.
Computer models may be based on prior knowledge about the source and underlying physics of the signal, as they are in the large-structure project that studies the San Francisco-Oakland Bay Bridge. Knowledge about the bridge and a detailed numerical model guide the development of signal-processing algorithms for the project.
But in the heart valve and oil exploration projects, knowledge of the signals is lacking or cannot be linked to a strong physical model, at least at the outset. For these cases, a "black box" model, derived only from the data (that is, input and output signals), without details of the underlying physics, is used to guide the development of algorithms.
For the three projects, Clark uses advanced signal-processing techniques, including statistical neural networks, which are systems of computer programs that approximate the operation of the human brain. Current uses for neural networks include predicting weather patterns, interpreting nucleotide sequences, and recognizing features in images. In a supervised learning mode, a neural network is "trained" with large numbers of examples and rules about data relationships. This training endows the network with the ability to make reasonable yes-no decisions on whether, for example, a geologic data plot indicates a geologic layer that could mark the presence of an oil deposit, or whether energy in a frequency spectrum of a recording signifies a damaged artificial heart valve.
In every acoustic signal project, vital data must be separated from noise that contaminates and inevitably degrades signal quality. The noise is caused both by the surrounding environment and the very system recording the signals. For example, the remote system designed by Livermore engineers for monitoring large structures can introduce noise into the signal in the course of relaying it over cellular phone lines to the Laboratory for analysis. In another example, the delicate sounds of a heart valve flipping up and down can be buried by acoustic scattering inside the body. Similarly, multiple ricocheting reflections from underwater explosions can contaminate the precise data needed to isolate geologic strata.
Often, Livermore engineers can reduce noise by using filters for certain frequency spectra. For example, if they know that the structural failure of a bridge will cause a vibrational response at 5 hertz (cycles per second), they can design a filter to monitor the energy content in that part of the frequency spectrum.
Hearing the Heart
The team's efforts are focused on the sounds made when the valve opens-sounds caused by the disk hitting the outlet strut-because those sounds yield direct information on the condition of the strut. A closing valve, in contrast, causes the entire ring to vibrate, and
that masks the strut's vibrations. Clark compares the sound of a faulty valve
to the thud of a cracked bell.|
Unfortunately, the opening sounds have much lower intrinsic signal levels than the valve's closing sounds. Candy notes that measuring heart sounds noninvasively in this noisy environment puts significant demands on the signal-processing techniques to extract the desired signals, especially when the data of interest last only 10 to 20 milliseconds. "Finding the opening sound caused by a strut separating from the heart valve is more difficult than searching for a single violin string that is out of tune in an entire orchestra," Candy says.
Clark points out that every valve recording is distorted by the body cavity and the recording process itself. That is why Livermore researchers are conducting studies at a U.S. Navy laboratory in San Diego in which acoustic sensors are submerged in water while collecting the sounds of valves that have been surgically removed from patients. This submersion isolates the pure sounds of both intact and damaged valves. "The test will allow us to measure the heart valve sounds without the acoustic scattering effects caused by the body," Clark says. "The pure data should allow us to mitigate the distortion in patient recordings."
Listening for Oil
One such mapping project is sponsored by the Department of Energy National Gas and Oil Technology Partnership. It is a cooperative effort involving Shell Oil of Houston, Texas, as Lawrence Livermore's industrial partner and a Ph.D. student from the University of California at Davis who is being supported by the Laboratory to work on the project. |
The purpose of the Livermore- Shell-UC Davis project is to automate a technique used to analyze acoustic oil exploration data. The current technique is a significant bottleneck because it is performed manually, at great cost in time and money. The project's goal is to reduce manual effort to only about 0.1 percent of the data processed. Current results show that this goal is achievable, says Clark.
Oil companies obtain acoustic signals by having a ship set off underwater explosions that generate acoustic waves that reflect from geologic layers as deep as 15 to 20 miles under the sea. A 5-mile-long linear array of some 100 hydrophones, which is towed by the ship, measures the signals (Figure 4).
The signals are organized into two-dimensional images, called common reflection point (CRP) panels, which represent vertical sections of the earth (Figure 5a). The panels show multiple reflections of the acoustic waves as they bounce off various strata of earth (Figure 5b). "You get a horrendous number of reflections," says Clark. "It gets very messy to sort out."|
Sorting out the reflections depends on correcting for the velocity of the sound waves as they travel through different layers of the earth. If the velocity computer model is correct, the imaged events appear as approximately straight horizontal (flat) lines in the CRP panel, because the true depth of the event horizon is approximately constant across the panel.
If, as typically occurs, the initial velocity computer model is incorrect, the event depths vary across the panel and do not appear flat. As part of an iterative velocity estimation process, an expert must visually inspect the panels to pick out event locations manually. The expert's picks are then used as input to refine the velocity model. This process is repeated several times, until the model produces events imaged as flat lines. The corrected panels are combined to obtain a two-dimensional image of the subsurface strata to help geologists determine where to site an offshore drilling platform.
When Clark inspected the CRP panels at Shell facilities in Houston, he suggested a new approach for analyzing reflection data. Instead of analyzing one signal or a few signal traces at a time, as is conventionally done, he proposed treating the set of 45 traces that forms a CRP panel as a single image. "I pointed out that if you treat the panel as an image, there's a whole set of literature and a lot of powerful tools available to you," he said (Figure 5c).
The Livermore team developed a technique that breaks the panels into small pixels (picture elements) to determine if the data represented within each pixel are part of an event or simply background noise. The technique uses advanced algorithms from the areas of automatic target recognition, computer vision, and signal-image processing. For example, using algorithms similar to those employed for computerized military target recognition, the technique scrutinizes the neighborhood of each pixel at various orientations and scales and looks for common features with neighboring pixels. Whenever possible, prior knowledge of known events is incorporated into the procedure. The results of the Livermore process compare favorably with those attained by experts (Figure 6).|
Clark has made many trips to Shell to collaborate with their scientists on the project. "It's been a wonderful marriage of disciplines," he says. "I introduced them to a lot of advanced signal-processing techniques, and they educated me about their world of exploration geophysics." Clark's Ph.D. research involved analysis of seismograms for Livermore's Comprehensive Test Ban Program, so he already had an elementary geophysical background.
Shell Oil estimates that the Livermore work will significantly affect the oil industry. Shell's current implementation of the Livermore algorithms already reduces data-picking time from about one work day to about 90 minutes. Once the software is completely converted from research code to production code, Shell estimates the cost of performing a single velocity analysis could be reduced from $75,000 and 12 weeks to $6,000 and one week. An oil company performing 100 velocity analyses per year could save nearly $7 million annually. Potential annual savings for the U.S. oil industry could amount to roughly $140 million.|
Clark points out that it costs about $1 billion to erect an oil platform in the Gulf of Mexico. Such costs make it crucial to respond quickly to business opportunities. The Livermore signal-processing techniques, providing time savings of a factor of 12, could significantly enhance the industry's responsiveness.
The numerical models provide information useful for designing sensors (accelerometers) that Livermore researchers will install on large structures for remote monitoring. The models indicate what frequencies are of interest and also help determine the best locations for the sensors.|
The monitoring system developed by Livermore engineers will continuously record, time stamp, and store sensor data. Researchers can contact the system at any time by cellular phone to download the data for analysis.
In developing signal-processing algorithms, Clark and colleagues integrate data from the sensors, models of sensor noise and the structure's unique environment, and a state-space numerical model. State-space models (common in electronics engineering) transform standard finite-element computer models (common in mechanical engineering) to render them more suitable for signal processing.
The resulting algorithms compare numerical model simulations with measurements of the real structure. A discrepancy between the two is a sign that the structure has suffered damage. Future algorithms will attempt to determine the source of discrepancies between the numerical model and the structure. Statistical classifiers, including artificial neural networks, could then come into play to classify what kind of damage the structure has sustained.
Data were collected recently to test the ability of signal-processing algorithms to detect differences between the numerical model and the actual structure. An experimental structure at the Nevada Test Site, a scale model of a five-story building some 14 feet tall, was the testbed for verifying these algorithms (Figure 8).
Engineers used the experimental structure both to evaluate the sensor systems and to acquire data for evaluating the signal-processing algorithms. For the latter purpose, they simulated an earthquake using a small amount of explosives contained in a rubber bladder. They also vibrated the structure continuously to excite every frequency at which it might vibrate.|
This summer, a dozen sensors will be placed on the Bixby Creek Bridge to record vibrational ground motion from small earthquakes. Placement and design of the sensors were guided by the existing numerical model of the bridge, made by Livermore engineers for Caltrans to evaluate the retrofitted bridge in a large earthquake.
Also this summer, a remotely monitored sensor and data acquisition system will be installed at various locations on the San Francisco- Oakland Bay Bridge. The system will monitor both ambient vibrations from traffic and wind and the structure's response to ground motion from small earthquakes. The guiding numerical model of the bridge is a product of a Livermore-University of California at Berkeley project (see S&TR, December 1998, "A Shaker Exercise for the Bay Bridge"). Data from the system will allow researchers to identify the bridge's "healthy fingerprint" and assess how well the signal-processing tools detect and identify discrepancies between model simulations and measured structures.
Prototype instruments are also being placed on NIF for long-term structural monitoring. The monitoring will help ensure that the giant laser facility's sensitive optical systems can perform under ambient vibration conditions, which include traffic, air conditioners, and other "cultural noise" effects, as well as microearthquakes. The NIF numerical model was made prior to beginning construction of the $1.2-billion laser facility.
The Livermore team hopes the work will provide a structural monitoring capability for Caltrans that can also be applied to critical DOE sites such as hazardous material facilities. In this way, says McCallen, authorities could have a much better handle on assessing damage to important structures and determining response and upgrade priorities.
Listening to the World
Key Words: accelerometers, acoustic signals, algorithms, artificial neural networks, Bixby Creek Bridge, Caltrans, Department of Energy National Gas and Oil Technology Partnership, heart valves, National Ignition Facility, Nevada Test Site, numerical models, oil exploration, San Francisco-Oakland Bay Bridge, signal processing, structure analysis.
For further information contact Greg Clark (925) 423-9759 (email@example.com).
ABOUT THE SCIENTIST
GREGORY A. CLARK received his B.S. and M.S. in electrical engineering from Purdue University in 1972 and 1974, respectively, and his Ph.D. in electrical and computer engineering from the University of California at Santa Barbara in 1981. His research activities are in the theory and application of automatic target recognition, computer vision, sensor fusion, pattern recognition-neural computing, estimation-detection, signal and image processing, and automatic control. He joined Lawrence Livermore in 1974 and is currently a principal investigator for several projects in the Defense Sciences Engineering Division. He has contributed to over a hundred technical publications and serves as a reviewer for several professional journals.