Stanford scientists use ocean waves to monitor offshore oil and gas fields

A new technique for passively probing the sea floor using weak seismic waves generated by the ocean was tested at the Ekofisk oil field in the North Sea. 

A technology developed by Stanford scientists for passively probing the seafloor using weak seismic waves generated by the ocean could revolutionize offshore oil and natural gas extraction by providing real-time monitoring of the subsurface while lessening the impact on marine life.

"We've shown that we can generate images of the subsurface nearly every day instead of taking snapshots just two or three times a year," said Biondo Biondi, professor of geophysics at Stanford's School of Earth Sciences.

Currently, many energy companies use a technique called time-lapse reflection seismology to monitor offshore oil and gas deposits to optimize production and look for hazards such as hidden gas pockets. Reflection seismology involves ships towing arrays of "air guns" that explode every 10 to 15 seconds to produce loud sound pulses. The pulses bounce off the seafloor and geological formations beneath, then journey back to the surface, where they are recorded by hydrophones. The data are then deciphered to reveal details about subsurface structures.

Each survey can cost tens of millions of dollars, and as a result they are only conducted two to three times a year. Environmental groups and marine biologists have expressed concerns about the use of air guns for contributing to noise pollution in the ocean that can disturb or even injure marine animals, including humpback whales and giant squid.

The new technique developed by Biondi and Sjoerd de Ridder, a student of Biondi's who is now a postdoctoral scientist at the University of Edinburgh, is different. It exploits naturally occurring seismic waves generated by Earth's oceans that are several orders of magnitude weaker than those produced by earthquakes.

Ambient seismicity

As ocean waves collide with one another, they create pressures on the sea floor, where they generate seismic waves that then propagate in every direction. Scientists have known about this "ambient seismic field" for nearly a century, but it was only recently that they understood ways to harness it.

"We knew the ambient seismic energy was there, but we didn't know what we could do with it," De Ridder said. "That understanding has only been developed in recent years. Our technique provides the first large-scale application to harness it for oil and gas production."

The technique that Biondi and De Ridder developed, called ambient seismic field noise-correlation tomography, or ASNT, uses sensors embedded in the seafloor. The sensors, which are typically installed by robotic submersibles, are connected to one another by cables and arranged into parallel rows that can span several kilometers of the seafloor. Another cable connects the sensor array to a platform in order to collect data in real time.

The sensors record ambient seismic waves traveling through Earth's crust. The waves are ubiquitous, continuously generated and traveling in every direction, but using careful signal-processing schemes they developed, Biondi and De Ridder can digitally isolate only those waves that are passing through one sensor and then another one downstream. When this is done repeatedly, and for multiple sensors in the network, what emerges is a "virtual" seismic wave pattern that is remarkably similar to the kind generated by air guns.

Less disruptive

Because the ASNT technique is entirely passive, meaning it does not require a controlled explosion or a loud air gun blast to create a seismic wave signature, it can be performed for a fraction of the cost of an active-reflection-seismology survey and should be far less disruptive to marine life, the scientists say.

Since 2007, Biondi and De Ridder have been testing and refining their technique in a real-world laboratory in Europe. The scientists worked with the energy companies BP and ConocoPhillips to study recordings from existing sensor arrays in the Valhall and Ekofisk oil fields in the North Sea that are capable of recording ambient seismic waves.

The proof-of-concept experiment has been successful, and the scientists have demonstrated that they can image the subsurface at Valhall down to a depth of nearly 1,000 feet. "We've now shown that our technique can very reliably and repeatedly retrieve an image of the near-surface," De Ridder said. "Our hope is that they can also reveal changes in the rocks that could signal an impending problem."

Source: Stanford

Black holes follow the rules

Artist's impression of a black hole at the centre of a galaxy. Credit: Gabriel Pérez Díaz.

Rather than having random sizes, massive black holes seem to follow a predictable rule in relation to the physical properties of the galaxy in which they are located.

Research at Swinburne University of Technology has shown that it is possible to predict the masses of black holes in galaxies for which it was previously thought not possible.

In large galaxies, the central black hole is related to the mass of the spheroid-shaped distribution of stars at the centre of the galaxy, known as the galaxy’s 'bulge'.

Some astronomers have claimed that the size of black holes at the centres of galaxies with small bulges was unrelated to the bulge.

Even the four million solar mass black hole in the bulge of our Milky Way galaxy was thought to be arbitrarily low relative to trends defined by their more massive, and therefore easier to detect, counterparts.

However, in previous work Swinburne Professor Alister Graham, lead-author of the current research, identified a new relationship involving black holes in galaxies with small bulges.  He demonstrated that the black hole in the bulge of the Milky Way was not set by chance but instead followed an astronomical rule.

“The formula is quadratic, in that the black hole mass quadruples every time the bulge mass doubles,” Professor Graham said. “Therefore, if the bulge mass increases 10 times, the black hole mass increases 100 times.”

Now, after studying more than 100 galaxies with black holes 4 to 40 times less massive than our Milky Way's black hole, they too have been found to follow this same rule.

"It turns out that there is yet more order in our Universe than previously appreciated,” Professor Graham said.

"This is exciting not just because it provides further insight into the mechanics of black hole formation, but because of the predictions it allows us to make."

The gravitational collapse of massive stars can produce black holes up to a few tens of times the mass of our Sun. And black holes that are one-tenth of a million to ten billion times the mass of our Sun have been identified at the centres of giant galaxies. However, there is a missing population of intermediate-mass black holes.

Astronomers don't know if this is because of observational difficulties in finding them, or if the massive black holes at the centres of galaxies start life as 100,000 solar mass seed black holes that formed in the early Universe.

This latest result, which extends the new rule to 40-times lower masses, gives astronomers some confidence that it may extend even further, so the smallest bulges might host these missing intermediate-mass black holes. 

"If confirmed, it would imply tremendous black hole appetites", co-author of the study, Dr Nicholas Scott, said. "There would need to be a dramatic growth of these small black holes relative to their host bulge, with the bulges growing via the creation of stars out of gas clouds while the black holes devour both gas and stars."

The researchers have identified a few dozen candidate galaxies in which they think intermediate-mass black holes may be hiding.  Future observations, with facilities such as the Square Kilometre Array and space-based X-ray telescopes, are expected to help resolve this black hole mystery.

Source: Swinburne

Protect the world's deltas, experts urge

A catastrophic landslide, one of the largest known on the surface of the Earth, took place within minutes in southwestern Utah more than 21 million years ago. Credit: Image courtesy of Kent State University

A catastrophic landslide, one of the largest known on the surface of the Earth, took place within minutes in southwestern Utah more than 21 million years ago, reports a Kent State University geologist in a paper published in the November issue of the journal Geology.

The Markagunt gravity slide, the size of three Ohio counties, is one of the two largest known continental landslides (larger slides exist on the ocean floors). David Hacker, Ph.D., associate professor of geology at Kent State University at Trumbull, and two colleagues discovered and mapped the scope of the Markagunt slide over the past two summers.

His colleagues and co-authors are Robert F. Biek of the Utah Geological Survey and Peter D. Rowley of Geologic Mapping Inc. of New Harmony, Utah.

Geologists had known about smaller portions of the Markagunt slide before the recent mapping showed its enormous extent. Hiking through the wilderness areas of the Dixie National Forest and Bureau of Land Management land, Hacker identified features showing that the Markagunt landslide was much bigger than previously known.

The landslide took place in an area between what is now Bryce Canyon National Park and the town of Beaver, Utah. It covered about 1,300 square miles, an area as big as Ohio's Cuyahoga, Portage and Summit counties combined.

Its rival in size, the "Heart Mountain slide," which took place around 50 million years ago in northwest Wyoming, was discovered in the 1940s and is a classic feature in geology textbooks.

The Markagunt could prove to be much larger than the Heart Mountain slide, once it is mapped in greater detail.

"Large-scale catastrophic collapses of volcanic fields such as these are rare but represent the largest known landslides on the surface of the Earth," the authors wrote. The length of the landslide -- over 55 miles -- also shows that it was as fast moving as it was massive, Hacker said.

Evidence showing that the slide was catastrophic -- occurring within minutes -- included the presence of pseudotachylytes, rocks that were melted into glass by the immense friction. Any animals living in its path would have been quickly overrun. Evidence of the slide is not readily apparent to visitors today. "Looking at it, you wouldn't even recognize it as a landslide," Hacker said.

But internal features of the slide, exposed in outcrops, yielded evidence such as jigsaw puzzle rock fractures and shear zones, along with the pseudotachylytes.

Hacker, who studies catastrophic geological events, said the slide originated when a volcanic field consisting of many strato-volcanoes, a type similar to Mount St. Helens in the Cascade Mountains, which erupted in 1980, collapsed and produced the massive landslide.

The collapse may have been caused by the vertical inflation of deeper magma chambers that fed the volcanoes. Hacker has spent many summers in Utah mapping geologic features of the Pine Valley Mountains south of the Markagunt where he has found evidence of similar, but smaller slides from magma intrusions called laccoliths.

What is learned about the mega-landslide could help geologists better understand these extreme types of events. The Markagunt and the Heart Mountain slides document for the first time how large portions of ancient volcanic fields have collapsed, Hacker said, representing "a new class of hazards in volcanic fields."

While the Markagunt landslide was a rare event, it shows the magnitude of what could happen in modern volcanic fields like the Cascades.

"We study events from the geologic past to better understand what could happen in the future," he said. The next steps in the research, conducted with his co-authors on the Geology paper, will be to continue mapping the slide, collect samples from the base for structural analysis and date the pseudotachylytes.

Hacker, who earned his Ph.D. in geology at Kent State, joined the faculty in 2000 after working for an environmental consulting company. He is co-author of the book Earth's Natural Hazards: Understanding Natural Disasters and Catastrophes, published in 2010.

Source:  Kent State University

Subtle shifts in the Earth could forecast earthquakes, tsunamis

University of South Florida graduate student Denis Voytenko prepares a GPS unit for a high-precision geodetic measurement.
Credit: Jacob Richardson

Earthquakes and tsunamis can be giant disasters no one sees coming, but now an international team of scientists led by a University of South Florida professor has found that subtle shifts in Earth's offshore plates can be a harbinger of the size of the disaster.

In a new paper published in the Proceedings of the National Academy of Sciences, USF geologist Tim Dixon and the team report that a geological phenomenon called "slow slip events" identified just 15 years ago is a useful tool in identifying the precursors to major earthquakes and the resulting tsunamis. The scientists used high precision GPS to measure the slight shifts on a fault line in Costa Rica, and say better monitoring of these small events can lead to better understanding of maximum earthquake size and tsunami risk.

"Giant earthquakes and tsunamis in the last decade -- Sumatra in 2004 and Japan in 2011 -- are a reminder that our ability to forecast these destructive events is painfully weak," Dixon said.

Dixon was involved in the development of high precision GPS for geophysical applications, and has been making GPS measurements in Costa Rica since 1988, in collaboration with scientists at Observatorio Vulcanológico y Sismológico de Costa Rica, the University of California-Santa Cruz, and Georgia Tech. The project is funded by the National Science Foundation.

Slow slip events have some similarities to earthquakes (caused by motion on faults) but release their energy slowly, over weeks or months, and cannot be felt or even recorded by conventional seismographs, Dixon said. Their discovery in 2001 by Canadian scientist Herb Dragert at the Pacific Geoscience Center had to await the development of high precision GPS, which is capable of measuring subtle movements of the Earth.

The scientists studied the Sept. 5, 2012 earthquake on the Costa Rica subduction plate boundary, as well as motions of the Earth in the previous decade. High precision GPS recorded numerous slow slip events in the decade leading up to the 2012 earthquake. The scientists made their measurements from a peninsula overlying the shallow portion of a megathrust fault in northwest Costa Rica.

The 7.6-magnitude quake was one of the strongest earthquakes ever to hit the Central American nation and unleased more than 1,600 aftershocks. Marino Protti, one of the authors of the paper and a resident of Costa Rica, has spent more than two decades warning local populations of the likelihood of a major earthquake in their area and recommending enhanced building codes.

A tsunami warning was issued after the quake, but only a small tsunami occurred. The group's finding shed some light on why: slow slip events in the offshore region in the decade leading up to the earthquake may have released much of the stress and strain that would normally occur on the offshore fault.

While the group's findings suggest that slow slip events have limited value in knowing exactly when an earthquake and tsunami will strike, they suggest that these events provide critical hazard assessment information by delineating rupture area and the magnitude and tsunami potential of future earthquakes.

The scientists recommend monitoring slow slip events in order to provide accurate forecasts of earthquake magnitude and tsunami potential.

Source: University of South Florida (USF Health)