For seismic contractors, the water between the surface and the seafloor is primarily an obstacle that must be overcome.
For some scientists, however, that same water is a target of opportunity -- a chance to better understand the very nature of oceans themselves -- and seismic reflection data may help unlock the secrets of ocean processes.
A new application for seismic reflection profiling is allowing scientists to "see" layers in the oceans, which provides new insight on the structure of the ocean currents, eddies and mixing processes.
This recent collaboration by geophysicists from the University of Wyoming and oceanographers from Woods Hole Oceanographic Institute to apply seismic data to oceans could be a major step forward in the ability to remotely survey the interior of oceans.
Raymond Schmitt, senior scientist in the department of physical oceanography at Woods Hole, said that early research suggests seismic data can be used to:
- Create detailed pictures of eddies, internal waves and other ocean features that affect climate, fisheries and the spread of pollution.
- Help scientists locate yet-undiscovered mixing sites, which may improve understanding of how the ocean absorbs heat and moves it from the equator to the poles.
- Help in improving climate models.
Serendipity
Serendipity intervened in the summer of 2000 when Steven Holbrook, a rofessor of geophysics with the University of Wyoming, was on a National Science Foundation-funded cruise acquiring data to study the deep crustal structure of the continent-ocean transition in the Newfoundland Basin near Grand Banks off Newfoundland.
The program included three long transects that directly crossed the climatological path of the North Atlantic current -- and during acquisition the scientists noticed some unusual echoes coming from the water.
"We noticed clear reflections coming from within the ocean," Holbrook said. "After writing a two-page blurb in our cruise report, where we noted the intriguing possibility of 'doing physical oceanography' with seismic reflection profiling, we promptly dropped the topic for two years."
Then, in the fall of 2002, the Wyoming group noticed similar reflections from the water column in the Gulf of California -- so they decided to process the data with special attention paid to the water column.
"Pedro Paramo produced the first water-column stack from the Gulf of California data, which motivated Scott Pearse and me to go back to the Newfoundland Basin data and create images," Holbrook said.
(Holbrook, Paramo, Pearse and Schmitt reported on their work in a recent issue of Science. The paper was titled "Thermohaline Fine Structure in an Oceanographic Front from Seismic Reflection Profiling.")
Analysis of shot and common-midpoint gathers showed that the energy represents primary reflections rather than multiples, refractions or diffractions. Reflections are visible at zero offset, show hyperbolic move-out, asymptotically approach the direct water wave and are consistent from shot to shot.
The team then created conventional stacks of these reflections, including velocity analysis, filtering and median stacking.
All show striking images of reflectance in the water column.
Holbrook said the group is confident that, at least in the images from the upper 1,000 meters of the Newfoundland Basin, the reflections come from the boundaries between layers with contrasting temperature/salinity properties.
"In this area, the layers are caused by 'thermohaline intrusions,' which are double diffusive phenomena caused by the juxtaposition of the warm, salty North Atlantic current water against the cold, fresh Labrador current water," he said. "This is a well-known location for strong thermohaline intrusions, which Ray Schmitt has studied for years."
Reflection seismic technology was never considered as a technique for studying the oceans because ocean acousticians typically use much higher frequencies of many tens to hundreds of kilohertz to look at scattering from upper ocean microstructure or zooplankton, which are not the same phenomenon the University of Wyoming scientists saw, according to Holbrook.
"Ocean thermohaline fine-structure, which is characterized by layers of several meters to tens of meters thick, appears to be well tuned to seismic energy in the 10-100 hertz band -- exactly the sound produced by marine seismic vessels probing the solid earth," he said. "So, in hindsight, it's not surprising that we're able to image fine-structure with seismic reflection profiling.
"The only surprise," he added, "is that it's taken us seismologists so long to discover these reflections in our data."
Corroboration
Of course, like any good scientists, the group needed independent corroboration of these results.
Previous oceanographic work in the region shows strong intrusions between the North Atlantic current and the Labrador current very near the site of the strong seismic reflections.
They had one point of direct comparison between ocean temperature and seismic character -- an expendable bathythermograph (XBT) that was collected along one of the profiles shows temperature inversions of up to 5 degrees centigrade.
Holbrook and his team took their seismic data to Schmitt, who has worked in the Grand Banks region for years.
The images appeared to be of intrusions -- or masses of water of similar density but of contrasting temperature and salinity -- that Schmitt had measured in the region more than 20 years before.
Previous oceanographic work in the region shows strong intrusions between the North Atlantic current and the Labrador current very near the site of the strong seismic reflections.
Schmitt said the team of scientists assumed a reasonable temperature-salinity relationship that is typical with such intrusions and then generated a salinity profile for the XBT and used the temperature and salinity profiles to calculate sound speed and density, with temperature having the dominant influence.
A comparison of the predicted sound speed profile and the stacked seismic reflection showed a close match, which confirmed that the low frequency sound energy was reflecting off the gradients in physical properties in the water column.
"The temperature versus depth profile matches the coincident seismic images very well, in the same way that a well log matches seismic reflections from within a sedimentary column," Holbrook said.
"That one XBT was the bit of convincing evidence we needed to show that the seismic reflections did represent physical changes in the water," Schmitt said. "We had the proof we needed."
Potential
Two aspects of seismic reflection profiling make it attractive as a survey tool for oceanic water column structure:
&9679; The acoustic echoes are obtained throughout the whole water column and the lateral spacing of the returns is only about 20 feet, making it easy to track the horizontal extent of reflecting features.
(Most traditional water column data are collected by repeated lowerings of towed instrument packages, which limits the depth range to a few hundred meters, with 500 to 1,000 meter or 1,500 to 3,300 feet spacing between profiles.)
&9679; The technique allows rapid surveys of ocean structures that change more quickly than traditional methods can observe.
The result?
"(This) gives us a very promising new tool for studying ocean structure and dynamics, which is important for understanding climate on our planet," Holbrook said. "We may be able to visually spot and track sites of mixing between water masses, which helps regulate the transfer of heat and salt from the equator to the poles."
Other processes, such as subduction of water, formation of deep water and mesoscale eddies, are also likely amenable to study with seismic visualization.
"Perhaps most significantly," he added, "our results imply that a large, untapped resource exists for studying ocean dynamics, in the form of the many marine seismic profiles that are sitting around on computer disks across the world, but which haven't yet been reprocessed for water column reflections."
Tomorrow?
Schmitt said he and Holbrook would like to do a more sophisticated study with perhaps two ships -- a seismic vessel and a physical oceanographic ship to lower instruments and make more precise measurements of temperature.
The tropical Atlantic east of Barbados is one region he would like to study.
"There's a region of layered structure in the ocean that should show up very strongly on seismic reflection data," Schmitt said.
Holbrook, too, believes "seismic oceanography" holds great promise for the study of ocean structure and dynamics.
"The existing archives of reflection data, especially in industry, represent a potentially huge resource that could add to our knowledge of ocean structure at relatively little cost," he said. "To fully exploit this resource, access to pre-stack data, with no seafloor mute, is required so that the data can be reprocessed with special attention to the water column. This may not always be possible, but in many cases it should be.
So, if reflection seismic can be so beneficial to oceanography, why did it take so long to realize this potential?
Holbrook blames the "invisible walls" that exist between scientific disciplines.
"Basically, reflection seismologists aren't trained to study the ocean directly, and have always viewed the water column as 'layer zero' in their models -- a featureless medium through which their sound waves have to propagate to get to the interesting stuff," he said. "In fact, reflection seismologists often 'mute' their images above the seafloor."
The fact that the reflections are extremely weak compared to the geological reflections -- about 100 to 1,000 times weaker -- also has helped to keep them a secret.
"The earth shouts back at us, but the ocean only whispers," he said. "We haven't been listening for the whispers. Similarly, physical oceanographers generally aren't familiar with the methodology of reflection seismology, so they haven't been asking us seismologists to look for things in the ocean."
Holbrook and his colleagues did discover that they weren't the first seismologists to image the water column. Two earlier papers published in 1988 and 1991 showed somewhat fuzzier pictures of ocean structure, he said.
"So, our discovery is more accurately a re-discovery," he said.
Who did what first, however, isn't the real issue. What happens next matters most.