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Measuring Currents in Submarine Canyons: Technological and Scientifi C Progress in the Past 30 Years

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Measuring Currents in Submarine Canyons: Technological and Scientifi C Progress in the Past 30 Years Exploring the Deep Sea and Beyond themed issue Measuring currents in submarine canyons: Technological and scientifi c progress in the past 30 years J.P. Xu U.S. Geological Survey, 345 Middlefi eld Road, MS-999, Menlo Park, California 94025, USA ABSTRACT 1. INTRODUCTION processes, and summarize and discuss several future research challenges constructed primar- The development and application of The publication of the American Association ily for submarine canyons in temperate climate, acoustic and optical technologies and of of Petroleum Geologists Studies in Geology 8: such as the California coast. accurate positioning systems in the past Currents in Submarine Canyons and Other Sea 30 years have opened new frontiers in the Valleys (Shepard et al., 1979) marked a signifi - 2. TECHNOLOGICAL ADVANCES submarine canyon research communities. cant milestone in submarine canyon research. IN CURRENT OBSERVATION IN This paper reviews several key advance- Although there had been studies on the topics of SUBMARINE CANYONS ments in both technology and science in the submarine canyon hydrodynamics and sediment fi eld of currents in submarine canyons since processes in various journals since the 1930s 2.1. Instrumentation the1979 publication of Currents in Subma- (Shepard et al., 1939; Emory and Hulsemann, rine Canyons and Other Sea Valleys by Fran- 1963; Ryan and Heezen 1965; Inman, 1970; Instrument development has come a long way cis Shepard and colleagues. Precise place- Drake and Gorsline, 1973; Shepard, 1975), this in the past 30 yr. The greatest leap in the tech- ments of high-resolution, high-frequency book was the fi rst of its kind to provide descrip- nology of fl ow measurements was the transition instruments have not only allowed research- tion and discussion on the various phenomena from mechanical to acoustic current meters. ers to collect new data that are essential discovered in submarine canyons and sea val- Advances in sensor development and semicon- for advancing and generalizing theories leys, presenting the most detailed fi eld data ductor engineering drastically improved the pre- governing the canyon currents, but have collected with state of the art instrumentation cision and accuracy, from centimeter per second also revealed new natural phenomena that deployed at locations in almost all large water to millimeter per second, of current meters, challenge the understandings of the theo- bodies (oceans, seas, lakes) on Earth. In the three reduced the physical size of the instruments, rists and experimenters in their predictions decades since the Shepard et al. (1979) book, and increased their data storage capacities. The of submarine canyon fl ow fi elds. Baroclinic the submarine canyon research community has concurrent reduction of the power draws of sen- motions at tidal frequencies, found to be seen large strides in both science and technol- sors and the improved battery technology made intensifi ed both up canyon and toward the ogy. New instruments with high precision and long fi eld data collection possible. Coupled canyon fl oor, dominate the fl ow fi eld and sampling frequencies that were not imaginable with improved mooring designs, it is now quite control the sediment transport processes 30 yr ago are being developed and utilized. routine to have continuous year-long observa- in submarine canyons. Turbidity currents New discoveries are being made in the fi eld and tions of fl ow fi elds in canyons (Khripounoff et are found to frequently occur in active sub- laboratory and new theories are being formu- al., 2003; Xu et al., 2004), a big improvement marine canyons such as Monterey Canyon. lated based on those discoveries (see review in compared to the days- and month-long time These turbidity currents have maximum Allen and Durrieu de Madron, 2009). I do not series collected 30 yr ago (Shepard et al., 1979). speeds of nearly 200 cm/s, much smaller attempt here to review all aspects of canyon Improvement in material, design, and machin- than the speeds of turbidity currents in geo- hydrodynamics (e.g., numerical and physical ing afforded sensors and pressure cases that can logical time, but still very destructive. In modeling on turbulence mixing or exchanges now withstand pressure at full ocean depth, and addition to traditional Eulerian measure- between canyons and continental shelf and/or thus currents can now be measured thousands of ments, Lagrangian fl ow data are essential in slope); instead, I review and summarize sev- meters below the sea surface (Khripounoff et al., quantifying water and sediment transport eral key advances made in the past 30 yr in the 2003, 2009; Xu et al., 2002, 2004). in submarine canyons. A concerted experi- research of submarine canyon hydrodynamics The most signifi cant leap forward in fl ow ment with multiple monitoring stations that are directly related to sediment transport. measuring technology is probably the devel- along the canyon axis and on nearby shelves Herein I review technological advances includ- opment and wide use of acoustic Doppler cur- is required to characterize the storm-trigger ing fi eld and laboratory instrumentation as well rent profi lers (ADCP). Before the ADCP was mechanism for turbidity currents. as data analysis, then review a number of key invented, velocity profi les were measured at advances in submarine canyon hydrodynamic only a few locations in the canyon by current Geosphere; August 2011; v. 7; no. 4; p. 868–876; doi: 10.1130/GES00640.1; 5 fi gures. 868 For permission to copy, contact [email protected] © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/4/868/3339895/868.pdf by guest on 30 September 2021 Currents in submarine canyons meters deployed at different depths on a sin- is its nonintrusive nature, which allows in situ pended particles in the same water column gle vertical mooring line (Fig. 1). Limited by measurements in highly energetic and hazard- where velocity profi les are measured. There resources and complexities in mooring design ous fl ows such as turbidity currents. Although have been numerous attempts (Thorne et al., and deployment, a typical mooring consisted of ADCP started to appear on the mass market in 1991; Hay and Sheng, 1992; Holdaway et al., 3–5 point current meters that spanned a few hun- the early 1980s, most of the installations were 1999; Gartner, 2004) to convert the acoustic dred meters vertically (Ferentinos et al., 1985; either downward-looking from ships or upward- backscatter signal to sediment concentration, Hunkins, 1988; Xu et al., 2002; Palanques et al., looking on bottom platforms deployed on con- with limited success. One major diffi culty that 2005). These limited numbers of points were tinental shelves, estuaries, and lakes. It was not remains to be overcome is that the grain size dis- clearly insuffi cient to fully resolve the current until early 2000 that ADCPs were mounted on tribution in the water column, and particularly profi le. An ADCP, however, can provide current subsurface moorings to measure current profi les in energetic fl ows like turbidity currents, is still profi les consisting of 20 or more data points in submarine canyons (Xu et al., 2004). ADCPs unknown. Without the grain size information, within the same vertical span depending on the can also be mounted on a vessel looking down- the attenuation of the acoustic backscatter due frequency of the sound source, water depth, ward to collect velocity profi les when the ves- to the presence of sediment particles cannot be mooring confi guration, and sampling rate. For sel is at anchor at multiple locations (Petruncio accurately estimated. example, a low-frequency (75 kHz) ADCP sys- et al., 1998; Flexas et al., 2008), or in transit to tem has a bin size (distance over which data provide information on the spatial variability of 2.2. Seafl oor Mapping are averaged) of tens of meters, while a high- currents (Wang et al., 2008). frequency (1000 kHz) can have a bin size as ADCPs are also useful in obtaining, at least Detailed maps of seafl oor bathymetry are small as 5 cm. Another advantage of the ADCP semiquantitatively, the concentration of sus- very important for correctly and accurately col- lecting and interpreting current observations in the areas of complex topography such as sub- marine canyons. Multibeam maps are a vast float improvement over the old bathymetric charts, as can be seen by overlaying a shaded relief map of bathymetry based on a multibeam sur- vey on top of an old bathymetric contour of a canyon. Traditionally the majority of fl ow mea- surements in submarine canyons are obtained from moorings. In areas where the topography often varies on scales of a few hundred meters, good bathymetry maps are essential to place the moorings at a desired location. In addition, good upward looking bathymetry greatly aids the interpretation of ADCP observations that may be strongly infl uenced by the surrounding topography. An accurate bathy- downward looking metric map and three-dimensional multibeam ADCP images along with precise global positioning system (GPS) data and accurate acoustic release floats mechanisms can enable scientists to deploy the instrumented moorings to the planned locations current (Xu et al., 2010; Fig. 2). meter The use of remotely operated vehicles (ROVs) and automated underwater vehicles acoustic acoustic (AUVs) in submarine canyon research greatly releases releases improved the accuracy of both seafl oor map- ping and instrument placement on the seafl oor. Guided by a multibeam bathymetry obtained anchor anchor from a surface vessel that has a resolution of meters, an AUV equipped with a specially fi tted Figure 1. Schematic diagram of typical subsurface moorings used to multibeam system can collect bathymetric data measure currents in submarine canyons. The mooring on the left is made with centimeter-scale resolution (Paull et al., out of three point current meters that measure time series horizontal 2010).
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