What's new in exploration

	Vol. 226 No. 9
PERRY A. FISCHER, EDITOR

Subsea meets space. Researchers studying how to measure and detect extremely faint gravitational wave signals from outer space hit upon a novel idea that led to a breakthrough in fiber optic acoustic sensing. The research team, from the Centre for Gravitational Physics at Australian National University, worked with Dr. Ian Littler from the University of Sydney to develop the ground-breaking technique.

The technology is 100 times more sensitive than current acoustic techniques and the fiber optic sensors are deployable up to 100 km away from a central recording station without significant degradation of signal quality. Our industry could benefit tremendously.

The ANU-led team bettered the world record in fiber strain sensitivity by more than a factor of 100, and achieved it at the end of a 5-km long optical fiber. Patents have been applied for. The sensor is reported to detect acoustic waves “with almost unimaginable sensitivity.” It is described as being able to detect changes on the order of a human hair (100 micrometers) at an Earth-Moon distance, a sensitivity level known as sub-picostrain.”

The acoustic sensor is a variation on the well-known Fabry-Perot interferometer, where laser light is sent through the fiber and reflected at the end. If the end sensor has moved even minutely, it will show as a change in the interference pattern created by the sent light and the reflected light. This is already commercially available, but the difference here is that there are reflectors at both ends of the fiber. Light bounces back and forth many times before exiting the system, effectively magnifying the sensitivity threshold. This is a technique that is used in gravitational wave detection, to measure an extremely tiny warp in space-time due to massive astrophysical objects such as black holes and supernova.

The breakthrough is not just in the signal mechanics, but also in the ability to extract the laser interference signal due to minute changes from a sensor located at a great distance away, and with no loss in fidelity.

A Sydney-based Australian marine geotechnical services company, called Benthic Geotech, was the first to approach the team about jointly working toward full commercialization of the fiber sensors.

The obvious application would be a radical sensitivity increase in passive seismic monitoring, perhaps even hearing fluid flow itself rather than just the cracks and creaks from rock movement. On the potential problem side, if crowded areas and permanent seismic systems already suffer from noise pollution, this could prove to be too sensitive a receiver for those areas.

Paulsson Geophysical Services of Brea, California, is recording hundreds of small earthquakes in the SAFOD borehole, using the world’s longest borehole seismic receiver array. Borehole trajectory (blue); 80-level borehole array (red); San Andreas Fault (SAF) plane is the thin red line.

Unusual geo-dating technique. New isotopic measurements could show that some estimates of oil generation timing may be off by tens of millions of years. The technique of using rhenium-osmium as a radiogenic clock has only been possible within the last decade, But nowadays we can detect isotopes in the parts-per-trillion range. The technique has the potential to date many oil reservoirs worldwide, and could help identify the ages of source rocks from which known oil deposits originated.

Rhenium occurs at about 200 parts per trillion outside the Earth’s core. It comes in isotopic weights of 185 and 187. Rhenium-187 is very slightly radioactive, turning into osmium-187, with a half-life of 42.3 billion years. Thus, osmium very slowly collects more of the 187 isotope. It is a dreadfully slow and extremely rare radioactive clock. But we now have the technology to reliably measure this isotopic ratio.

The ratio can be affected, that is, reset, by various high-temperature processes acting on its parent rock. There is some noise introduced in ratios that are this small, and geo-dating may be impossible if the source rock is unknown.

Geologists David Selby and Robert Creaser from the University of Alberta, recently used the ratio to date the oil in Alberta’s oil sand deposits. They tested the black shale source rock, and then the oil, and compared the isotopic ratios in the two samples. The result was 112 ± 5.3 mya oil, which is 60 million years earlier than previously thought. What was important was the accuracy of the finding. It tries to pinpoint the time of oil genesis.

For petroleum exploration, knowing the age of hydrocarbons places constraints on where they might accumulate. In my view, the utility and accuracy of the new technique still needs some further validation before its utility is fully understood. But, it offers a new potential tool in certain circumstances.

SAFOD installed. The San Andreas Fault Observatory at Depth has successfully completed an 8.5-in., 13,082-ft borehole into the fault. “As we were drilling, all of a sudden the drill rate jumped and all this gas started to come up. This gave us an indication that we were going through a fault zone,” Gregory van der Vink said. He is project director for EarthScope, which is financed by the National Science Foundation to explore the structure and evolution of the North American continent in collaboration with the US Geological Survey.

The San Andreas Fault is one of the most active in the world, averaging a magnitude 2.0 earthquake every two years. The hope is to collect enough continuous pressure, temperature and seismic data during the 10- to 15-year life of the project to identify any patterns for earthquake prediction.