It is now possible to monitor parameters such as temperature or strain quasi-continuously over many tens of kilometers using optical fiber technology. These fiber based systems have been developed by the oil industry to monitor the flow and the structural integrity of undersea pipelines. The approach is to transmit high-power laser light down an optical fiber. A small fraction of the transmitted signal is reflected back by the Brilloiun scattering process. The optical frequency of this scattered light differs from the transmitted light by an amount proportional to temperature. The PI's have requested an EAGER award to quantify the Brillouin system?s performance and to evaluate the potential role of this technology to measure precise temperature in ocean monitoring by using it in several demonstrational experiments.
Broader Impact:
The Brillouin scatter technology has a huge range of potential applications across all fields of oceanography. A fiber laid down the axis of an active spreading center can monitor the space-time injection of warm fluid along the rift. The elevation offset between isotherms on one side of a channel relative to the other can be used to infer the geostrophic flow through the channel, in real time. A fiber floating on the tropical sea surface can monitor the patchy fall of rain (cold) and the daily cycle of heating and cooling.
Robert Pinkel Scripps Institution of Oceanography Away from the sea surface, the temperature of a parcel of sea-water changes but slowly in time. Over short times, variations in temperature can thus be used to track the motion of water parcels of different temperature. To detect patterns in the motion, large numbers of temperature sensors sampling at known positions in space and time are required. Unfortunately, such instruments are relatively expensive and determining positions underwater is difficult. It would be a great advance if one could simply unroll a ~2-mile length of "string" in the ocean, plug one end of the string into some sort of magic box, and measure the temperature all along the string. This can now be done- if the string is an optical fiber. The technology is based on the Raman back-scattering of short pulses of light that are transmitted down the fiber. Termed "Distributed Temperature Sensing" (DTS), the technology was developed initially for the petroleum industry. Over the past decade the precision of the approach has improved from several degrees C to several hundredths of a degree, after averaging in space (~10 m) and in time (~1 minute). In this program, we attempted first applications of DTYS technology to oceanography, focusing on applications appropriate to the present state of the art. The DTS sensing proved to be a game-changing technology when applied to near-shore measurements of internal wave shoaling and coastal mixing (Figure 1). A ~ 1.5 mile fiber was laid on the sea floor extending off shore from Scripps pier in La Jolla CA. In experiments conducted in 2011 and 2013, intriguing patterns of temperature fluctuation were seen from the shoaling internal tide, Figure 2. In Fall 2012, a 1.5 mile floating optical fiber was towed behind the RV Roger Revelle as the ship steamed eastward through the equatorial Pacific (00N, 1390-1410 W). The fiber detected ~0.1 C sea-surface temperature fluctuations associated with internal waves propagating in the upper thermocline, Figure 3. These fluctuations are thought to be caused by internal wave modulation of the warm surface layer that forms on the sea every afternoon as the sun heats the water. The optical approach provides an ideal "ground-truth" for airborne and space-borne measurements of temperature inferred from infra-red imagery. Further applications include the observation of deep current interactions with topography, the monitoring of low frequency flows through channels. Based on these successful initial applications of DTS technology to oceanography, other research groups are beginning to use the approach for their own oceanographic applications.
