Bottom Instrument Research Articles

The continental margin off Oregon from seismic investigations

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across the continental margin using ocean-bottom seismometers (OBS) and hydrophones (OBH) as well as land recorders. Two-dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles. The subducting oceanic crust of the Juan de Fuca plate can be traced from the trench to its position some 10 km landward of the coastline. At the coastline, the Moho has a depth of 30 km. The dip of the plate changes from 1.5° westward of the trench to about 6.5° below the accretionary complex and to about 16° further eastward below the coast. The backstop forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America about 50 Ma ago, is well defined by the observations. It is located about 60 km to the east of the deformation front and has a seaward dip of 40°. At its seaward edge, the base of the Siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths. The upper oceanic crust is thinner to the east of this contact than to the west. At depths greater than 18 km, the top of the oceanic crust is the origin of pre-critical reflections observable in several land recordings and in the data of one ocean bottom instrument. These reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition.

Recurrent large‐scale landsliding on the west flank of La Palma, Canary Islands

A large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of La Palma. Multibeam bathymetry and its derivative backscatter data, Towed Ocean Bottom Instrument (TOBI) sidescan sonar images, and 3.5 kHz and airgun seismic reflection data have been used to identify at least two, and possibly as many as four, major landslide events. The youngest of the events, the Cumbre Nueva Debris Avalanche, extends onshore into the valleys bounded by the Caldera de Taburiente and Cumbre Nueva Ridge, which mark the degraded collapse scars. Radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide. The debris avalanche covers an area of 780 km2, has a maximum thickness of 500 m, and has an estimated volume of 95 km3. Older deposits, collectively referred to as the Playa de la Veta Debris Avalanche Complex, are probably, as the name indicates, an amalgamation of at least two or three events rather than the result of a single catastrophic failure. The Playa de la Veta Debris Avalanche Complex is associated onshore with an unconformity dated as late Matuyama (1 Ma to 800 ka). It covers an area of 1200 km2, has a maximum thickness of 1300 m, and may represent a total volume of up to 650 km3. The greater thicknesses and limited areas occupied by debris avalanches on the western flank of La Palma, compared to other landslides in the Canary Archipelago, suggest that the La Palma landslide masses have relatively low mobility. The different debris avalanche lobes formed by each landslide event are separated by channels 2–2.5 km wide. The clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways. The relief of the submarine flanks of the La Palma volcanoes, in the areas of island slope unaffected by landslides, is mainly the result of constructional volcanic processes. However, the older submarine slopes, such as in the northern Taburiente volcano, may also have been modified by smaller‐scale submarine mass wasting and sediment flows.

高精度デジタル海底地震計ＭＯＯＢＳ‐２４の開発

We made significant improvements for the previous digital OBS/H called MOOBS/H-I. We call the new OBS MOOBS/H-24 (Magneto Optical Ocean Bottom Seismometer and Hydrophone-24). The most significant improvements are use of a 1Gb Magneto Optical (MO) disk unit and 24 bit resolution A/D converters. The previous system uses a 320 Mb magneto optical disk unit and a 16 bit A/D converter. The 24 bit A/D converter is delta-sigma type.Mainly due to the limitation on the power consumption, we obtained 120 dB dynamic range and 20 bit effective resolution. We also increased the buffer memory size from 2 Mb to 4 Mb. Using this buffer memory, we can continuously obtain a seismic record during 1 hour and 14 minutes if we choose 100 Hz sampling rate and 3 ch. inputs. The loss time during the 4Mb-data transfer from the buffer memory to the MO disk is 1 minute and 14 seconds. If we choose careful time schedules for explosions in the seismic experiment, it might be a reasonable specification. The instrument has also logical and/or level trigger modes. It can be operated during a specified time window. The parameters for initial setting is input through a serial or parallel interface unit. Using the serial interface unit, we can start the instrument after it is enclosed inside of a glass sphere pressure case.We used ten MOOBS/H-24 units in the field experiments off Cape Muroto and off Tokai, Japan and obtained satisfactory records. When a 15 kg-dynamite explosion was detonated just at the sea surface above-an ocean bottom instrument at 2 km water depth, the acoustic water wave showed high amplitude unsaturated waveforms.

TOBI image processing-the state of the art

TOBI (Towed Ocean Bottom Instrument) is a deep-tow sidescan sonar vehicle from which sidescan sonar data are now routinely collected and archived. This paper describes the algorithms developed for detailed processing of TOBI data. Sonar imagery has a characteristic set of processing challenges and these are addressed. TOBI provides a very large sonar dataset, and to limit the difficulties of handling and processing these data, the raw data are subjected to a data reduction technique prior to further processing. Slant-range correction is improved by editing vehicle altitude data using a median filter. Noise on TOBI imagery can appear in two main forms; speckle noise and line dropouts. Speckle noise is removed by a small median difference kernel and line dropouts are removed using a ratio of two box-car filters, each with appropriate thresholding techniques. Precise geocoding of the imagery requires an accurate estimate of vehicle location, and a method of calculation is presented. Two optional processing algorithms are also; presented; deblurring of imagery to improve along-track resolution at far range, and the suppression of a surface reflection return which may occur when TOBI is operated in relatively shallow water. Several of the techniques presented can be transcribed and modified to suit other datasets. >

Real-time data acquisition from mid-ocean observatories

Recent advances in underwater acoustic communication have enabled real-time contact with ocean bottom instrumentation. This overview talk summarizes developments in acoustic, radio, and satellite communication links and in undersea networks constructed for telemetry from mid-ocean instrumentation. Acoustic communication has made major advances in terms of energy efficiency, data rate, and robustness. Data from several experiments illustrating capabilities of the acoustic modems, algorithm structure of the communication link, and its relationship to the characteristics of the acoustic channel are presented. Technology resulting from this work is incorporated in a line of underwater modems that are being used extensively by the oceanographic community. The modems are of course useful as direct point-to-point communication devices; however, incorporating modems into communication networks greatly increases their utility. Several deployed networks are discussed, in particular an under-ice system recently develoyed on Lake Winnipesaukee and in the Arctic off Pt. Barrow. The under-ice network is a part of the Autonomous Oceanographic Sampling Network (AOSN), a long-term development program aimed at improving the timeliness and cost-effectiveness of oceanographic data acquisition.

About this title

Conventional studies of oceanic structure and evolution are inevitably hampered by the difficulties with sampling the crust in the third dimension. Subsurface studies with deep seismic techniques provide some measure of understanding below the seafloor. Ophiolite studies, however, can provide a vast body of knowledge with regard to the deeper structure and crustal compositions, which is unattainable in the deep sea. In turn, many of the recent regional and detailed structures identified at mid-ocean spreading systems such as ridge segmentation patterns and ridge axis discontinuities, now form a significant contribution to recent studies of ophiolite complexes. The book will be of interest to academics from undergraduates upwards and also to industrial researchers concerned with metal sulphide and platinum group element deposits. Cover illustration: Deep-tow sidescan sonograph of the axial floor of the Mid-Atlantic Ridge around 24°N. Data were recorded using the IOSDL Towed Ocean Bottom Instrument. View is 6×8 km in a water depth of around 3500 m. Cratered seamounts, freshly mounded pillow constructs and linear fault scarps are clearly imaged. These currently active ridge processes compare directly with their ancient analogues discussed in this publication.

Isolation of source and receiver statics and scale factors in waveform inversion of marine refraction data

In this paper I present a method that uses multiple ocean bottom instruments to separate variations in seismic amplitudes and travel times caused by near‐source topography and variable source amplitudes from similar variations related to the overall one‐dimensional seismic structure. The results indicate that the standard correction for topography, sediment thickness, and source size can be improved with this method, reducing uncertainties in the velocity solution.

A current meter with intelligent data system, environmental sensors, and data telemetry

To measure oceanographic parameters such as currents, temperature, conductivity, pressure, and suspended sediment concentrations, two film-recording current meters were upgraded with microprocessor-controlled data recorders and additional sensors. Two telemetry links relay data and allow the in situ operation of the remote instrument to be checked. In one configuration, the bottom-mounted current meter communicated by a 35-m-long wire to a small surface spar buoy, and then by a packet radio link to a nearby ship. In another development, the current meter relays data to a controller and buoyant data capsule on the bottom instrument package. The controller collects and processes the data from the current meter and periodically transfers these processed data to a data capsule and releases it. When released, the capsule rises to the surface and transmits its data to shore via the ARGOS satellite, while acting as a satellite tracked drifter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the spectra shape of seismic noise and earthquakes recorded in the ocean

The shape of the spectra of ocean bottom noise and earthquakes recorded in the course of seismological observations in the ocean is analysed. The generalized spectra of bottom seismic noise are suggested as reference curves for comparison with new data. The comparison showed a similarity in the shape and level of the generalized spectra to the noise spectra obtained recently both on the bottom and under the bottom by seismographs placed in holes drilled by the Deep-sea Drilling Project. The spectral analysis of the earthquakes recorded on the bottom of the North-west Pacific basin has shown clear maxima at frequencies of 7–17 Hz formed both by site effects and by absorption of seismic energy in the lithosphere. The frequency of spectral maxima decrease with increasing epicentral distance. Comparison of the earthquake spectra with generalized spectra of bottom noise revealed (probably) incidental coincidence in the frequencies of the minimum for noise spectra (10 Hz) and the average maximum of earthquake record spectra. The sharp mechanical impacts on a bottom seismograph, usually induced by bottom displacements under the instrument, are suggested as rough analogues of the impulses for the bottom instrument system transient calibration. Test measurements have shown that in many cases this method makes it possible to distinguish spectral peaks characterising earthquakes and seismic noise wave trains from those which are caused by coupling resonances of the OBS-sediment interface.

Preliminary analysis of resolving power of existing strong-motion arrays for source inversion.

The effectiveness of existing strong-motion arrays for source inversion analysis is investigated by estimating the accuracy of the inversion solution using simplified theoretical seismograms. The resolving power is examined for different subfault sizes and for different array configurations for four existing array networks. They are for the 1979 Imperial Valley and the anticipated Parkfield, California earthquakes, and the 1968 Tokachi-Oki and the anticipated Tokai, Japan earthquakes. We use the method previously developed by Miyatake, Iida, and Shimazaki in 1986 based on the Wolberg's prediction analysis. The main results are: (1) The array used for an analysis of the 1979 Imperial Valley earthquake is not suitable for source inversion; especially the El Centro array, a linear array crossing perpendicularly the fault trace, is ineffective. (2) The array installed for the anticipated Parkfield earthquake by the end of 1985 seems to be satisfactory because of the intensive installation of many stations. However, the resolving power of the whole array will further increase by adding a few stations in the northwestern part of the fault or in distant areas for a better azimuthal coverage in the northwest of the fault. (3) Detailed source inversion analysis cannot be expected for the 1968 Tokachi-Oki, Japan, earthquake because of both the large fault area and lack of offshore stations. Strong-motion, ocean bottom instruments within the fault area are required for a further improvement of the inversion analysis for this type of earthquake. (4) An addition of several land stations on the west and north sides of the fault area is desirable to the present network for the anticipated Tokai, Japan earthquake. The resolving power of the whole array for this earthquake is strongly dependent on a rupturing direction because station coverage of the southern oceanic part of the fault tends to be poor.

Selected examples of Scholte wave propagation in deep and shallow water

The Naval Ocean Research and Development Activity recently completed a series of three experiments to measure very low‐frequency (VLF) seismo‐acoustic propagation in both shallow and deep water marine environments. A vertical hydrophone array and ocean bottom instruments were deployed in all three experiments. The ocean bottom instruments contained both hydrophone and three‐component geophone sensors. In the single shallow water experiment, measurements were also made with buried three‐component geophones. We compare the VLF propagation characteristics in deep and shallow water seen with ocean bottom and water column sensors. The importance of Scholte wave propagation in accounting for both signal and ambient noise energy at near bottom receivers is shown for selected frequencies using both measured and modeled results. Necessary environmental conditions for Scholte wave excitation are also examined.

Data retrieval from bottom instrumentation using acoustic link

Abstract Bottom instrumentation is being used to collect diverse oceanographic, seismic, and geodetic data, often over prolong periods of time. Such data can be retrieved periodically using an acoustic communication link. Typical but conflicting requirements of such a link are high transmission rates, reliable transmission over long distances, and low power consumption of bottom transmitter. In this article a steerable narrow‐beam acoustic link is suggested as the best feasible approach offering improvement over broad‐beam systems. Various beam configurations are considered and compared in terms of their ability to reject ambient noise and multipath interference. A vertically suspended linear receiving array is postulated as the best solution to multipath suppression and beam tracking.

Surface and near-surface effects on seismic waves—theory and borehole seismometer results

AbstractA simple plane wave model is adequate to explain many surface versus borehole seismometer data sets. Using such a model, we present a series of examples which demonstrate the effects of the free-surface, near-surface velocity gradients, and low impedance surface layers on the amplitudes of upcoming body waves. In some cases, these amplitudes are predictable from simple free-surface and impedance contrast expressions. However, in other cases these expressions are an unreliable guide to the complete response, and the full plane wave calculation must be performed. Large surface amplifications are possible, even without focusing due to lateral heterogeneities or nonlinear effects. Both surface and borehole seismometer site responses are almost always frequency-dependent.Ocean bottom versus borehole seismic data from the 1983 Ngendei Seismic Experiment in the southwest Pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation. The borehole seismic response to upcoming P waves is reduced at high frequencies because of interference between the upgoing P wave and downgoing P and SV waves reflected from the sediment-basement interface. However, because of much lower borehole noise levels, the borehole seismometer enjoys a P-wave signal-to-noise advantage of 3 to 7 dB over nearby ocean bottom instruments.

The Nantucket Shoals Flux Experiment (NSFE79). Part I: A Basic Description of the Current and Temperature Variability

The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability. In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed over the upper slope. The wind stress during summer was weak and only moderately correlated with the subtidal current fluctuations. In the winter period (October–March), when the seasonal thermocline was destroyed and the shelf water locally homogenized by increased surface cooling and wind mixing, the mean currents observed over the shelf were also primarily alongshelf towards the west at speeds comparable to those measured in summer. However, the low-frequency current fluctuations over the shelf were much more energetic in winter. These subtidal current fluctuations were highly coherent horizontally and vertically over the shelf and with surface wind-stress fluctuations (which increased in magnitude by a factor of 5 over the summer period). The most energetic subtidal current events observed over the shelf also tended to extend into the upper slope region. The subtidal currents observed over the upper slope in summer were dominated by three bursts of large eastward currents which correspond to the passage of anticyclonic Gulf Stream warm-core rings near or through the moored army. The effect of these rings on the current field does not appear to penetrate shoreward of the shelf break. In winter only two rings passed near the array and their influence on the observed upper slope currents was unclear owing, in part, to the increased subtidal current variability caused by the stronger synoptic wind forcing in winter. Multiple regression analysis was used to identify possible annual variations in the NSFE79 moored current and temperature data. Significant annual variations were found in the temperature field over the shelf and upper slope and in the low-frequency current variability over the shelf. No significant annual variation was observed in the alongshelf current over the shelf, however, suggesting that there is, at least on time scales of one month and more, a continuous flow of shelf water into the Middle Atlantic Bight from the Georges Bank/Gulf of Maine region. The mean westward volume flux between the 40 and 120 m isobaths observed in NSFE79 was 38.3 ± 6.9 × 104 m3 s−1.

POPUP: A Prototype Bottom-Moored Long-Term Current Profiler

Abstract A self-contained, bottom-mounted instrument for measuring high-resolution vertical profiles of horizontal current in the deep ocean over long time periods (up to a year) has been designed, constructed and successfully tested. It employs an interferometric technique to track small, expendable, buoyant probes containing continuous wave (CW) acoustic beacons which are released on a preset schedule. A carrying capacity of 60 probes is quite feasible. Tracking data are stored on cassette tape and recovered with the bottom instrument. The design of the instrument is discussed and errors analyzed. The uncertainty in the horizontal current averaged over 10 m vertical intervals increases near the surface to approximately 2 cm s−1 and is due primarily to acoustic phase noise from reflection off the rough sea surface. Considerable improvement in future design can be expected.

Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone

A first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics. As we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth's surface, it is becoming increasingly necessary to study the tectonic, volcanic, and hydro­ thermal processes within the spreading center plate boundary zone. All ocean­ ic crust bears the imprint of these processes . This review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid-ocean ridges with reference to associated volcanic and hydrothermal processes. It draws heavily on recent studies that use deeply towed instrument packages, multi-beam bathymetric mapping, ocean bottom instruments , and ALVIN (e.g. the Famous, AMAR, RISE, and Galapagos expeditions). We begin with a review of the large-scale structure of spreading centers. We then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones. Next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity , as well as the generation of magnetic anomaly stripes and their implications for crustal generation. One of our findings is that the initial two-dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

An oceanographic data recorder

The authors have designed and built a low‐power, low‐cost tape recorder capable of extended operation in either analog or digital recording modes. Its small size permits use inside 6‐inch ID pressure vessels that would normally preclude a commercial recorder of similar specifications. Data are recorded on 1800 ft of 1/4‐inch audio tape using standard 5‐inch diameter tape reels. Total continuous analog recording time at 1/40 inches/sec is 10 days. In the digital mode, total storage capacity at 2500 bits per inch is [Formula: see text] bits. This permits 14 hours continuous recording at 125-Hz sample rate or 7 hours at a 250-Hz sample rate. A single motor‐gearhead combination drives the capstan flywheel and tape take‐up fluid clutch through urethane belts. Guides are of polished stainless steel and are fixed, in keeping with simplicity. Cost to manufacture, including fabrication and parts, is $1,000.00, plus $150.00 for a four‐track analog head or $500.00 for a seven‐track head. During the last 2 years, the recorders have performed in the analog mode in eight Woods Hole Oceanographic Institution ocean bottom hydrophone systems and in the digital mode in six U.S.G.S. ocean bottom instrument packages.

NOAA monitoring deep ocean mining tests

Operational test mining of the deep ocean floor in the central Pacific Ocean is underway. Manganese nodules are being pumped from the seabed at a depth of more than 3 mi. Ocean Mining, Inc., a group of U.S., Canadian, West German, and Japanese companies, is testing a system by which the nodules are collected, separated from other ocean floor material, and lifted up to a converted ocean oil‐well drilling ship.During the early stages of the test period, the NOAA ship Oceanographer worked nearby to observe the effect of the plume of materials along the ocean floor generated as the collector draws in and separates the nodules and other materials, and the effect of a surface plume caused by shipside disposal of excess materials brought to the surface along with the nodules. Previously placed ocean bottom instruments did the former, while the latter was done by sampling the surface discharge.

A precise and continuous monitoring system of the distance between the near-bottom instruments and the sea floor

A relatively simple and inexpensive distance monitoring system (DMS) was devised to monitor continuously and accurately the position of an underwater camera with an attached pinger relative to the sea floor. Although the precision graphic recorder (PGR) or precision depth recorder (PDR) have hitherto been used to record the pinger to bottom distance, these complicated and expensive instruments are not always expected on all vessels. This newly developed system, which measures with a counting timer the interval between the direct sound pulses generated by a pinger and the bottom echoes, yielded results equal to or better than that of the PGR. The distance information thus obtained was accurate within ±2.5% under good conditions (calm weather and flat and level topography) just above the sea bed. This system simplified the photographing operations and made it possible to obtain a precise and quantitative assessment of the epibenthic fauna with the use of a single camera. The DMS is also applicable for determining the precise location of a bottom water sampler, grab sampler, or other underwater oceanographic equipment as well as that of an underwater camera.

MODE:IGPP Measurements of Bottom Pressure and Temperature

Abstract We review the MODE deployment of IGPP bottom instruments, together with preliminary tests on the Pacific seafloor and in the laboratory. Pressure and temperature were measured with quartz-crystal transducers in different configurations. Spectra of instrument noise in the laboratory and on the seafloor were estimated from duplicate transducers. These estimates are prerequisite to the forthcoming discussions of MODE tides, the bottom experiment, and internal waves. There are two puzzling features: (i) the temperature noise continuum on the seafloor is generally 20 dB above that in the laboratory, and (ii) the pressure noise spectrum has a tidal line structure. Instrument drifts during MODE are of the order of a few millibars and a millidegree Celcius, respectively.