Deep Tow Instrument Package Research Articles

Synoptic fine-scale surveys of the seafloor with the new Deep Tow instrument package

Over the past 3 years, a multisensor digital Deep Tow vehicle has been developed at the Marine Physical Laboratory to carry out fine-scale measurements near the seafloor down to 6000 m depth. The suite of sensors include: a pair of 110-kHz side-looking sonars, a 4-kHz sub-bottom profiler, a 40-kHz obstacle avoidance sonar, a 12-kHz navigation sonar, a 23.5-kHz up-looking sonar, a CTD, a transmissometer, a magnetometer, a complement of still and TV cameras, and a 100-kHz multibeam echo-sounder. The latter provides fine-scale swath bathymetry and acoustic backscatter imagery of the bottom over a swath roughly 60 deg wide. All the sensor data are digitized in the vehicle and are telemetered to the ship over a standard 0.680-in. electromechanical tow cable at bit rates between 500 and 1000 kb/s depending on the length and condition of the cable. Examples of the data collected during sea trials and during deep-water search operations are presented.

A Deep-Towed ADCP-CTD Instrument Package Developed for Abyssal Overflow Measurements in the Northeastern Caribbean Sea

Abstract A deep-towed instrument package has been developed to study the velocity and tracer signature of abyssal overflows in the northeastern Caribbean. Primary package components include a conductivity-temperature-depth (CTD) instrument and an acoustic Doppler current profiler (ADCP), allowing for simultaneous measurement of density, watermass tracers, and absolute velocity. A description of package construction and operation is supplemented by examples from a set of 17 deployments during two oceanographic cruises in January 1991 and March 1992. A new method for determining the three-dimensional position of the instrument package is described, based on the ability of the ADCP to acquire reference velocities corresponding to its motion over the seafloor. Factors affecting ADCP data quality are discussed, particularly those stemming from the low-scatterer environment at abyssal depths and the impact of large vertical accelerations due to surface wave-induced ship heave.

Detailed study of the Brunhes/Matuyama reversal boundary on the east Pacific rise at 19°30′ S: Implications for crustal emplacement processes at an ultra fast spreading center

We have conducted the first detailed survey of the recording of a geomagnetic reversal at an ultra-fast spreading center. The survey straddles the Brunhes/Matuyama reversal boundary at 19°30′ S on the east flank of the East Pacific Rise (EPR), which spreads at the half rate of 82 mm yr-1. In the vicinity of the reversal boundary, we performed a three-dimensional inversion of the surface magnetic field and two-dimensional inversions of several near-bottom profiles including the effects of bathymetry. The surface inversion solution shows that the polarity transition is sharp and linear, and less than 3–4 km wide. These values constitute an upper bound because the interpretation of marine magnetic anomalies observed at the sea surface is limited to wavelengths greater than 3–4 km. The polarity transition width, which represents the distance over which 90% of the change in polarity occurs, is narrow (1.5–2.1 km) as measured on individual 2-D inversion profiles of near-bottom data. This suggests a crustal zone of accretion only 3.0–4.2 km wide. Our method offers little control on accretionary processes below layer 2B because the pillow and the dike layers in young oceanic crust are by far the most significant contributors to the generation of marine magnetic anomalies. The Deep-Tow instrument package was used to determine in situ the polarity of individual volcanoes and fault scarps in the same area. We were able to make 96 in situ polarity determinations which allowed us to locate the scafloor transition boundary which separates positively and negatively magnetized lava flows. The shift between the inversion transition boundary and the seafloor transition boundary can be used to obtain an estimate of the width of the neovolcanic zone of 4–10 km. This width is significantly larger than the present width of the neovolcanic zone at 19°30′ S as documented from near-bottom bathymetric and photographic data (Bicknell et al., 1987), and also larger than the width of the neovolcanic zone at 21° N on the EPR as inferred by the three-dimensional inversion of near-bottom magnetic data (Macdonald et al., 1983). The eruption of positively magnetized lava flows over negatively magnetized crust from the numerous volcanoes present in the survey area and episodic flooding of the flanks of the ridge axis by extensive outpourings of lava erupting from a particularly robust magma chamber may result in a widened neovolcanic zone. We studied the relationship between spreading rate and polarity transition widths obtained from 2-D inversions of the near-bottom magnetic field over various spreading centers. The mean transition width corrected for the time necessary for the reversal to occur decreases with increasing spreading rate but our data set is still too sparse to draw firm conclusions from these observations. Perhaps more interesting is the fact that the range of the measured transition widths also decreases with spreading rate. In the light of these results, we propose a new model for the spreading rate dependency of polarity transition widths. At slow spreading centers, the zone of dike injection is narrow but the locus of crustal accretion is prone to small lateral shifts depending on the availability of magmatic sources, and the resulting polarity transition widths can be narrow or wide. At intermediate spreading centers, the zone of crustal accretion is narrow and does not shift laterally, which leads to narrower transition widths on the average than at slow spreading centers. An intermediate, or even a slow spreading center, may behave like a fast or hot-spot dominated ridge for short periods of time when its magmatic budget is increased due to melting events in the upper mantle. At fast spreading centers, the zone of dike injection is narrow, but the large magmatic budget of fast spreading centers may result in occasional extensive flows less than a few tens of meters thick from the axis and off-axis volcanic cones. These thin flows will not significantly contribute to the polarity transition widths, which remain narrow, but they may greatly increase the width of the neovolcanic zone. Finally the gabbro layer in the lower section of oceanic crust may also contribute to the observed polarity transition widths but this contribution will only become significant in older oceanic crust (≈50–100 m.y.).

Inference of manganese nodule coverage from Sea Beam acoustic backscattering data

Normal incidence reflectivity from a manganese nodule field was measured with a 12 kHz Sea Beam multibeam echo‐sounding system, aboard the R/V Thomas Washington and used to infer nodule coverage. A reflectivity map of the area was produced using the intensity of the specular return from each ping. The patchiness of the nodule coverage is evidenced by definite highs and lows in the reflectivity pattern. Ground truth was provided by near‐bottom acoustic measurements and photographs taken with the Deep Tow instrument package of the Marine Physical Laboratory. Agreement between the simple nodule coverage predictions from Sea Beam acoustic data and the bottom photographs taken throughout the area is 98 percent. Although the Sea Beam system is limited in its dynamic range, this paper shows that it can be used very effectively to determine both topography and nodule coverage in potential mining areas.

Erosional troughs in deep-sea carbonates and their relationship to basement structure

A detailed survey of a small area in the eastern equatorial Pacific with the Marine Physical Laboratory's Deep-Tow instrument package reveals erosionally excavated troughs in an area of relatively rapid accumulation of carbonate. The troughs lie on top of a basement high implying a relationship between the troughs and the basement feature. Differential accumulation and compaction may explain the origin of the troughs. Sediment deposited around the basement high preferentially accumulates on gentle slopes and in troughs. This results in a thick sequence of sediment on the flanks of the basement high and a thin sequence above on top of the hill. Unstable sediment masses and differential compaction generate a tensile-stress field above the peak of the basement high which causes slumping and faulting. The steep slopes generated by faulting and slumping may then serve to intensify tidal currents or act as spillways and locally generate increased turbulence. This increased turbulence results in local erosion by dissolution. A trough will form above a basement high, however, only when a combination of (1) basement steepness, (2) high sedimentation rate, (3) sediment thickness, and (4) sediment erodibility are present.

Deep-sea carbonates: Physical property relationships and the origin of high-frequency acoustic reflectors

A detailed survey of a small area in the eastern equatorial Pacific (136°W,4°N) was conducted with the S.I.O. Marine Physical Laboratory's Deep-Tow instrument package. A detailed examination of the relationships between acoustic, stratigraphic, and physical properties in cores from this area reveals that variations in acoustic impedance are almost entirely controlled by changes in saturated bulk density (porosity). Velocity changes are very small. Saturated bulk density changes are highly correlated with variations in percent calcium carbonate. High-carbonate samples are dominated by high-density platy carbonate material while low-carbonate material is dominated by low-density spiny siliceous microfossils. Variations in saturated bulk density (porosity) that account for changes in acoustic impedance are ultimately caused by climatic changes. Warm periods were times of enhanced dissolution which caused low percent carbonate and low saturated bulk density (and therefore low impedance) values. The opposite is true for cold periods. These climatic cycles can be matched with glacials and interglacials for the past one million years, but have also occurred before Pleistocene glaciation. The acoustic properties were compared with the high-resolution subbottom profiles collected at the survey site. A reflection coefficient log calculated from the cores' physical properties, reveals no interfaces with large reflection coefficients that correlate with the reflectors seen on the Deep-Tow 4-kHz seismic profile. The calculated reflection coefficients are very low (typically 10 −3–10 −5) and varied about a wavelength that is of the order of the outgoing 4-kHz pulse. Convolving the outgoing 4-kHz pulse with the reflection coefficient log generates a synthetic seismogram that very closely resembles the 4-kHz reflection profile. Varying the frequency of the outgoing pulse changes the amplitude and position of the reflectors seen on the synthetic seismograms. Thus we conclude that the reflectors seen on the 4-kHz seismic profile are not caused by discrete geologic horizons, but rather are the result of the interference of many small layers.

Possible Geometries of Sandstone Bodies as Reflected by Geomorphic Features on Modern Submarine Fans: ABSTRACT

Understanding the growth processes and geomorphic features of modern submarine fans will aid in the exploitation of potential hydrocarbon resources in deep-water turbidities. Surveys on four fans in the northeast Pacific using the deep-tow instrument package show a wide variety and size range of relief features. At least three types of channels can be recognized; (1) leveed valleys, common on the upper fan; (2) distributary channels that branch from the active valley; and (3) headless channels found at the basin slope and fan margin or along the edges of depositional lobes. All the channel types are associated with areas of active sand deposition. An abandoned distributary channel would likely become the site of deposition of pelagic and hemipelagic muds and occasional hin-bedded turbidities. A connection to the main channel may be maintained by End_Page 722------------------------------ the sandy filling of the distributary channel. The distributary channels and their associated depositional lobes may serve as collectors and feeders of hydrocarbons to the main channel system. Isolated depressions ranging from a few tens of meters to more than a kilometer across are seen on all fans that have been studied with the deep tow. On both Monterey and Navy submarine fans, large scour-shaped depressions are observed. In cross sections perpendicular to the fan gradient, these depressions resemble channels. Cores and reflection data show that these features are floored by sand and could be filled by sand or mud. Contrasting the interconnected nature of buried channel systems, the limited extent and continuity of sand-filled and buried depressions would suggest little potential as hydrocarbon reservoirs. Depositional lobes are also common on submarine fans and range in size from small (1 km wide) features associated with distributary channels to large fan lobes occupy ng tens of square kilometers. End_of_Article - Last_Page 723------------

The validity of the geological interpretations of marine magnetic anomalies

Summary. In order to urlderstand the relative importance of the various phenomena which contribute to marine magnetic anomalies, an analysis was performed on magnetic profiles measured simultaneously at the sea surface and near the sea floor. The data come from three long geophysical surveys made over spreading centres in the eastern Pacific with the Deep Tow instrument package. At low spatial frequencies the two sets of observations agree very closely when related by the mathematical model of upward continuation from magnetic sources below, which are assumed to be perfectly heated, i.e. ‘two-dimensional’. However, spectral analysis reveals the presence of a discrepancy at high spatial frequencies. In all three surveys at wavelengths shorter than about 4km the observed surface field power exceeds the upward continued power by several orders of magnitude. Several causes for this discrepancy are examined, including deviations from twodimensionality , numerical problems, and the noise background of the ocean environment, and the most likely candidate is shown to be ionospheric activity. Spectra of magnetograms from the nearest land observatories intersect the upward continued spectra at about 4-km wavelength, and exhibit power at high spatial frequency which agrees with the extra power in the surface data in each survey. For the observed surface data, the regime of valid geological interpretation is limited by ionospheric noise to wavelengths longer than about 4km. The near-bottom data do not suffer from such a limitation primarily because they were obtained near the geological sources, not just because of the attenuation of the ionospheric noise by the conducting sea water above. The interpretation regime of the near-bottom data is much wider, extending to wavelengths of a few hundred metres where the observed power nears the quantization noise level and where the validity of the assumption of perfectly heated sources probably breaks down. The behaviour of the observed spectra suggests an analytical model which portrays how the bandwidth of the interpretation regime of the surface field depends on the towing speed, source depth and ionospheric activity level. It is shown that a faster towing speed reduces the effect of the ionospheric noise.

Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package

Research Article| May 01, 1977 Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package BRUCE P. LUYENDYK; BRUCE P. LUYENDYK 1Department of Geological Sciences, University of California, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar KEN C. MACDONALD KEN C. MACDONALD 2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar Author and Article Information BRUCE P. LUYENDYK 1Department of Geological Sciences, University of California, Santa Barbara, California 93106 KEN C. MACDONALD 2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (5): 648–663. https://doi.org/10.1130/0016-7606(1977)88<648:PASOTI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation BRUCE P. LUYENDYK, KEN C. MACDONALD; Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package. GSA Bulletin 1977;; 88 (5): 648–663. doi: https://doi.org/10.1130/0016-7606(1977)88<648:PASOTI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A deep-towed instrument survey was made of the floor of the rift valley in the Mid-Atlantic Ridge near lat 37 °N (FAMOUS project). Near-bottom bathymetry, side-looking sonar (SLS), and wide-angle photography are among the data brought to bear on the definition of the American-African plate boundary and the intrusion-extrusion zone between these plates. The valley floor is 1 to 4 km wide and contains eight elongate shield volcanoes that occupy 40% of the axis. Where the volcanoes — called central highs — are absent, there are sometimes shallow depressions — called central lows. Photographic data show three components to the near-bottom environment: massive pillow lavas, well-sorted rock fragments, and sediment cover. Sediment is ubiquitous in all the photos but is scarcest along the axis of the valley floor. Pillows appear the freshest on the central highs except for one locality found at the extreme east side of the valley floor. The rock fragments are evidently pillow joint blocks and are associated with spalling off steep flow fronts and fracturing from faulting. Less well sorted fragments are associated with the large-throw step faults at the edges of the valley floor. Pillow elongation approaches 10:1 and possibly indicates flow directions both across and along contour. Numerous fissures and small-throw step faults were also seen. The SLS records show that the smoothest areas are the well-sedimented regions off the central highs that are yet to be broken by faulting at the floor edges. More than 700 faults were mapped in the valley-floor region. They trend parallel to valley strike (N17 °E) and are generally absent within 500 m of the axis. Fault density is highest at the east inner floor edge, reaching 35/km2. Fissures are present in photos but are not readily recognized with SLS data. Flow edges or ridges also align near N17 °E, but some trend across strike. SLS point targets 25 m high and about 50 by 50 m represent volcanic conelets (haystacks). These align in groups and often are associated with faults. They are not detected away from the axis.The floor is intensely fractured with fissures and small-throw vertical faults as are parts of the inner walls; this is evidence that the region of tensional faulting is at least 5 km wide. The absence of faults near the axis due to volcanic burial suggests that the extrusion zone is as much as 1 km wide. The construction process in the floor is virtually entirely volcanic, but much of this relief is obscured by the intensive shear (normal) faulting found beyond the inner walls. Many geomorphic features of the floor have direct analogy with those in the Icelandic rift valley and the East Pacific Rise, although there are differences in scale. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N

A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

Abyssal Furrows and Hyperbolic Echo Traces on the Bahama Outer Ridge

Near-bottom investigations with a deep-towed instrument package show that steep-sided, flat-floored furrows (1 to 100 m wide by 0.5 to 20 m deep) eroded into Holocene and Pleistocene hemipelagic mud are responsible for the characteristic hyperbolic echo traces on surface-ship echograms recorded over the Bahama Outer Ridge (water depth, 4 to 5 km). This echogram character, previously interpreted as being caused by side reflections from depositional wave forms, has been typically recorded in abyssal regions of contour-current activity. It is suggested that these remarkably straight longitudinal furrows are formed by secondary helical circulations in the bottom-boundary mixed layer.

Growth Patterns of Deep-Sea Fans

The growth pattern of a deep-sea fan relates events in and around the fan-valleys to the structure and morphology of the open fan. The growth pattern cannot be determined without knowledge of the origin and recent history of the fan-valley system. The mapping of La Jolla and San Lucas deep-sea fans with the deep-towed instrument package developed at Marine Physical Laboratory of the Scripps Institution of Oceanography details the fine-scale morphology, structure, and internal fill of the fan-valleys and suggests the growth patterns of these fans. The La Jolla fan, 20 km west of Scripps Institution, has one meandering fan-valley that extends across the entire fan. Except on the toe of the fan, the deeply incised valley has terraced walls with steeper walls on the outside of meanders. Very low-relief levees border the fan-valley in some localities. The present erosional valley bypasses the partly buried remnants of an older distributary system on the lower fan. The San Lucas fan, off the southern tip of the peninsula of Baja California, shows a depositional lobe of sediment, or suprafan, below the short, leveed fan-valley extending from San Jose Canyon. The suprafan appears as a convex-upward bulge on a radial profile of the fan. The surface of the suprafan has a series of discontinuous depressions up to 55 m deep and 1 km wide. The depressions are generally asymmetric in cross section, commonly have terraced walls, and are underlain by coarse sand and gravel. They are interpreted to be channel remnants. A model for deep-sea fan growth, based on this study, predicts that deposition on a fan will be localized in a suprafan at the end of large, leveed valleys commonly found on, and generally confined to, the upper reaches of deep-sea fans. The suprafan normally is on the midfan and is characterized by numerous smaller distributary channels. Rapid aggradation in the suprafan coupled with migration and meandering of the channels produces a surface marked by isolated depressions or channel remnants. Uniform deposition, producing a symmetrical half-cone morphology, results from the shifting through time of fan-valleys across the area of the fan.

Deep-Sea Fan-Valleys, Past and Present

Research Article| September 01, 1969 Deep-Sea Fan-Valleys, Past and Present W. R NORMARK; W. R NORMARK Marine Physical Laboratory of the Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California Search for other works by this author on: GSW Google Scholar D. J. W PIPER D. J. W PIPER Department of Geology, University of Cambridge, Cambridge, England Search for other works by this author on: GSW Google Scholar Author and Article Information W. R NORMARK Marine Physical Laboratory of the Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California D. J. W PIPER Department of Geology, University of Cambridge, Cambridge, England Publisher: Geological Society of America Received: 24 Jan 1969 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1969, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1969) 80 (9): 1859–1866. https://doi.org/10.1130/0016-7606(1969)80[1859:DFPAP]2.0.CO;2 Article history Received: 24 Jan 1969 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. R NORMARK, D. J. W PIPER; Deep-Sea Fan-Valleys, Past and Present. GSA Bulletin 1969;; 80 (9): 1859–1866. doi: https://doi.org/10.1130/0016-7606(1969)80[1859:DFPAP]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Comparison of ancient and modern marine sedimentary environments can yield better interpretations of both. Extensive exposures of modern sediments are not available, but detailed time correlation over large areas and much environmental data are absent from ancient sediments. The development of deep sea fan-valleys and the processes leading to their eventual filling have been studied through detailed comparison of two contemporary fan-valleys with one of Miocene age now exposed on land. Results from narrow-beam near-bottom echo soundings and seismic reflection profiles from a deep-towed instrument package were coordinated with information from bottom sampling on La Jolla and San Lucas fans, off southern California and Baja California respectively. A Miocene fan-valley exposed in sea cliffs near Dana Point, southern California, was used for the comparison.In all three cases, a fan-valley has been cut into horizontally bedded muds and silts of the surrounding fan. Terraces form on resistent horizons cropping out on the valley walls. Irregularly bedded sands with mudlumps and conglomerates in some sections fill the main parts of channels. Later deposits within channels are finer grained and more regularly bedded. La Jolla fan is cut by a single fan-valley; San Lucas fan has a braided system of valleys. These two different patterns may result either from differences in the rate of deposition and grade of sediment or from different amounts of incision in the feeding canyons following the post-Pleistocene rise in sea level. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.