Sound Fixing Research Articles

Infrasound from underwater sources

Atmospheric low-frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here, we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.

Evanescent wave coupling in a geophysical system: Airborne acoustic signals from the Mw 8.1 Macquarie Ridge earthquake

AbstractAtmospheric low‐frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water‐air interface making it almost fully reflective. Here we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.

Remote hydroacoustic sensing of large icebergs in the southern Indian Ocean: Implications for iceberg monitoring

Hydroacoustic signals generated by drifting icebergs that crack, disintegrate, and collide were identified on two hydrophone arrays in the Indian Ocean. These hydrophone arrays are deployed in the Sound Fixing and Ranging channel, enabling the detection of small sources over ranges of several thousand kilometers due to the low attenuation. Source locations estimated from the signal bearings at the arrays are used to monitor two very large icebergs, C20 and B17B. Spatial and temporal correlation of the location estimates with satellite observations confirm that the icebergs can be hydroacoustically resolved. Hydroacoustic generation rates at both C20 and B17B are highest at times of observed breakup. For C20, which underwent continuous breakup, clusters of events to the southeast of the main iceberg show that hydroacoustic observations can identify trails of icebergs that calved from the main berg whose dimensions are less than that easily resolved by moderate resolution satellite monitoring.

A Novel Energy Efficient Joint Dynamic Emissive Location-Based Routing Scheme for SOFAR Channel Underwater Sensor Networks

The Underwater Sensor Network (UWSN) could be recorded by hydrophones suspended in the channel as earthquake monitors by seismic events on or below the seafloor generated energy. In this paper, we present a novel position of sensor devices and routing protocol towards short for Sound Fixing and Ranging channel (SOFAR) for Sound Fixing and Ranging channel technologies, which could save transmission power. The sound speed also increases towards the warmer sea surface with temperature. Thus we use the oceanography to find more efficient ways of utilizing routing protocol and in to obtain more effective detection and identification capabilities for underwater sources. We present an adaptive location-based routing protocol which can overcome the location based UWSN without Global Positioning System (GPS) position. It reduced costs of deployment in long range propagation. The performances are measured according to the energy consumption per bit in which the result of simulator is better than other routing protocols such as FBR.

Observations of Earthquake-Generated T-Waves in the South China Sea: Possible Applications for Regional Seismic Monitoring

We present a detailed study of T-waves originating from earthquakes in the South China Sea region, near the Indochina Peninsula and Luzon islands which were recorded by a broadband seismic station at Nansha Island. Most of these T-waves appear to have been the source originating from earthquakes with epicentral distances greater than 600 km from this station. The T-waves in this region were identified via their apparent stable measured velocities of about 1.45 km s^(-1), and represent the first reported T-waves and the first T-waves observed from an island station in the South China Sea. However, during the period of analysis (November 2004 to December 2005) additional earthquakes also occurred beyond the South China Sea region, but in these instances, any associated T-waves were not picked up by the station at Nansha Island. An analysis of Twave travel times reveals the possible locations of the P-wave to T-wave transitions at the ocean to crust interface were presumably situated near the earthquake source side. Our results indicate that the Sound Fixing and Ranging (SOFAR) channel is well developed in the South China Sea region. Ultimately, developing a solid understanding of the effective transmission of T-waves through the ocean may provide new opportunities for detecting and locating small earthquakes which would be useful for both seismic monitoring and in helping to predict and reduce the damaging effects of earthquakes and tsunamis in the South China Sea region.

On Possible Plume-Guided Seismic Waves

Abstract Hypothetical thermal plumes in the Earth’s mantle are expected to have low seismic-wave speeds and thus would support the propagation of guided elastic waves analogous to fault-zone guided seismic waves, fiber-optic waves, and acoustic waves in the oceanic SOund Fixing And Ranging channel. Plume-guided waves would be insensitive to geometric complexities in the wave guide, and their dispersion would make them distinctive on seismograms and would provide information about wave-guide structure that would complement seismic tomography. Detecting such waves would constitute strong evidence of a new kind for the existence of plumes. A cylindrical channel embedded in an infinite medium supports two classes of axially symmetric elastic-wave modes, torsional and longitudinal-radial. Torsional modes have rectilinear particle motion tangent to the cylinder surface. Longitudinal-radial modes have elliptical particle motion in planes that include the cylinder axis, with retrograde motion near the axis. The direction of elliptical particle motion reverses with distance from the axis: once for the fundamental mode, twice for the first overtone, and so on. Each mode exists only above its cut-off frequency, where the phase and group speeds equal the shear-wave speed in the infinite medium. At high frequencies, both speeds approach the shear-wave speed in the channel. All modes have minima in their group speeds, which produce Airy phases on seismograms. For shear wave–speed contrasts of a few percent, thought to be realistic for thermal plumes in the Earth, the largest signals are inversely dispersed and have dominant frequencies of about 0.1–1 Hz and durations of 15–30 sec. There are at least two possible sources of observable plume waves: (1) the intersection of mantle plumes with high-amplitude core-phase caustics in the deep mantle; and (2) ScS -like reflection at the core-mantle boundary of downward-propagating guided waves. The widespread recent deployment of broadband seismometers makes searching for these waves possible.

Southeast Indian Ocean-Ridge earthquake sequences from cross-correlation analysis of hydroacoustic data

SUMMARY Parameters of earthquake sequences, for instance location and timing of foreshocks and aftershocks, are critical for understanding dynamics of mid-ocean ridge and transform faults. Whole sequences including small earthquakes in the ocean cannot be well recorded by land-based seismometers due to large epicentral distances. Recent hydroacoustic studies have demonstrated that T waves are very effective in detecting small submarine earthquakes because of little energy loss during propagation in Sound Fixing and Ranging channel. For example, an MW 6.2 (2006 March 6, 40.11 ◦ S/78.49 ◦ E) transform-fault earthquake occurred at the Southeastern Indian Ocean Ridge, but National Earthquake Information Center only reported three aftershocks in the first following week. We applied cross-correlation method to hydroacoustic data from the International Monitoring System arrays in the Indian Ocean to examine the whole earthquake sequence. We detected 14 aftershocks and none foreshock for the earthquake and locations of these aftershocks show an irregular pattern. From the observation, we suggest that the feature could be caused by complicated transcurrent plate-boundary dynamics between two overlapped spreading ridges that is possibly explained by the bookshelf faulting model.

Generation of hydroacoustic signals by oceanic subseafloor earthquakes: a mechanical model

For more than 15 yr, the recording of hydroacoustic signals with hydrophones moored in a minimum sound-velocity channel, called the SOFAR (SOund Fixing And Ranging) channel, has allowed for detection and localization of many small-magnitude earthquakes in oceanic areas. However, the interpretation of these hydroacoustic signals fails to provide direct information on the magnitudes, focal mechanisms, or focal depths of the causative earthquakes. These limitations result, in part, from an incomplete understanding of the physics of the conversion, across the seafloor interface, from seismic waves generated by subseafloor earthquakes to hydroacoustic T waves. To try and overcome some of these limitations, we have developed a 2-D finite-element mechanical model of the conversion process. By computing an exact solution of the velocity field of the waterborne T waves, our model shows that a double-couple source mechanism of a subseafloor earthquake generates T waves, whose take-off angles are adequate to allow penetration into the SOFAR channel and efficient trapping by this waveguide. Furthermore, our model confirms that a double-couple source with a high S-wave content produces higher-amplitude T waves than a simple explosive source, which only generates P waves.

Sound fixing and ranging beneath sea-ice

Sound fixing and ranging techniques, the so-called SOFAR technique, is widely used in the field of oceanography for tracking neutrally buoyant floats underwater over large distances (several thousands of kilometres) and for long periods of time (several years). The range propagation depends largely on the existence or not of the SOFAR sound channel. Twenty years ago we investigated SOFAR acoustic propagation under sea-ice over large distances and at various frequencies from 80 Hz up to 1560 Hz. We discovered the range for acoustic propagation was much reduced (100 kms to 150 kms) no matter the frequency used. This was due to large scattering induced by under sea-ice topography since there is no way to avoid acoustic rays to bounce back to the surface towards sea-ice and create multiple reflections. More recently during the Tara Damocles transpolar drift across the Arctic Ocean, we reinvestigated the SOFAR acoustic range propagation under sea-ice for longer periods of time in various conditions (summer and winter conditions). Our results confirmed those obtained 20 years ago and indicated a high sensitivity to the ambient conditions. We are presenting the results obtained during the late fall- early winter months of the Tara transpolar drift across the Arctic Ocean.

Hydroacoustic blockage prediction and measurement at Diego Garcia using the Adiabatic Mode Parabolic Equation Model

Underwater explosion monitoring with sparse sensors at long ranges relies on the efficient propagation of acoustic energy in the sound fixing and ranging (SOFAR) channel. When sound traveling in this channel encounters an island or seamount, it will either diffract, scatter, or be converted into seismic energy. Signals observed on the opposite side of these obstructions have been affected by some combination of these processes, and models of global detection and localization depend on knowing these effects. We present a study using the Adiabatic Mode Parabolic Equation (AMPE) model to predict these processes in three dimensions at the Chagos Archipelago. Predictions at 5, 10, and 20 Hz are compared with measurements of approximately 300 T-wave signals from six years of earthquakes on either side of the Chagos Archipelago. These have been recorded at the hydrophone arrays around Diego Garcia. The result of this 360-degree analysis, and the agreement with observed data, demonstrate the utility of the model in understanding the physical effects of these obstructions.

Sound propagation in two-axis underwater channel based on beam-displacement ray-mode theory

Sound propagation in a deep ocean two-axis underwater channel is often complex and difficult to simulate between surface channel and sound fixing and ranging (SOFAR) channel. The beam-displacement ray-mode (BDRM) theory is a normal mode method for propagation modeling in horizontally stratified shallow water. An improved method for computing the upper boundary reflection coefficient in the BDRM is proposed and applied to calculate the acoustic fields of a two-axis underwater channel. Transmission losses in the two-axis underwater channel are calculated in the new BDRM. The corresponding results are in good agreement with those from the Kraken code, and furthermore the computed speed of the new BDRM excels the other methods.

Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone array

The seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002–09/2003), the northern ‘SIRENA’ network, deployed between latitudes 40° 20′N and 50° 30′N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0–5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35–4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of mainshock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.

Hydroacoustic Constraints on the Rupture Duration, Length, and Speed of the Great Sumatra-Andaman Earthquake

On 26 December 2004, one of the largest recorded earthquakes occurred, triggering a devastating tsunami that killed an estimated 300,000 people. The event was initially classified as Mw 9.0 based on the analysis of seismic body and surface waves (Nettles and Ekstrom, 2004; Ji, 2005; Park et al. , 2005). Classical methods of magnitude calculation are hampered by the long duration of the event, however, since late-arriving phases from the earliest portion of the rupture may obscure first-arriving energy sourced from other portions of the rupture. This same phenomenon also limits our ability to constrain the earthquake's duration, length, and speed of propagation. To work around these limitations, Stein and Okal (2005) studied Earth's low-order normal modes to estimate the size of the earthquake. Their results indicated a moment of 1.3 × 1030 dyne-cm ( Mw 9.3), which is ∼2.5 times larger than initial reports, and a centroid position near 7°N. In contrast with previous analyses, which had suggested displacement occurred primarily along the southern third of the aftershock region (Ji, 2005; Yagi, 2005; Yamanaka, 2005), these results argue that significant slip occurred along the entire aftershock zone. For shallow submarine earthquakes, hydroacoustic recordings of tertiary ( T ) waves can provide additional constraints on rupture length, as well as velocity and direction of rupture propagation ( e.g. , Bohnenstiehl et al. , 2004). T waves are formed when seismically generated energy is scattered at the seafloor-ocean interface and becomes trapped in the ocean's low-velocity wave guide, which is known as the SOund Fixing And Ranging (SOFAR) channel (Tolstoy and Ewing, 1950). Locating the source of a T wave, therefore, does not necessarily provide an estimate of the earthquake epicenter, but rather the position of the T -wave radiator. This is particularly true for the subduction zone …

Effects of Source Pulse Shape on Low-Frequency-Sound Propagation

We studied the degradation of the source pulse form in long-range propagation of low-frequency sound in the sound fixing and ranging (SOFAR) channel in the ocean. We made the numerical simulation of the source pulse propagation with changing the parameters of the simulation; carrier frequency, source pulse width, and propagation range. In long-range propagation such as 1000 km, 2000 km and 3000 km, there are a limited number of combination of the frequency and the pulse width for minimizing the consequent pulse degradation. These results are applicable to the design of the actual ocean propagation experiment itself and also of sound source equipment for the experiments.

Zones ofT-wave excitation in the NE Indian ocean mapped using variations in backazimuth over time obtained from multi-channel correlation of IMS hydrophone triplet data

Ground‐coupled hydro‐acoustic waves (so called T waves) observed at International Monitoring System (IMS) hydrophone triplet stations in the Indian ocean were analysed for backazimuth and apparent wave speed by applying a progressive multi‐channel correlation method (PMCC). For T waves, which propagate along topographically unblocked geodesic paths, the three triplet channels generally show good signal correlation over sliding windows and various frequency bands. A characteristic change in backazimuth over time, in some cases combined with signs of dispersion, is observed for many T waves generated by earthquakes from the margin of the Indo‐Australian Plate between the Sunda Islands to the southeast and the Andaman Islands to the northwest. Backazimuths and arrival times of those PMCC detections are used to map point‐by‐point the areas where ground‐to‐water coupling takes place. Such areas are mostly linked to large gradients in the bathymetry, predominantly at approximately the depth of the axis of the SOund Fixing And Ranging (SOFAR) channel, along island arcs, edges of continental shelves and at comparatively small features like isolated islands or sea mountains. Six examples are discussed in this paper. For the strongest events, the area of detectable ground‐to‐water coupling often spans distances of more than 15°, in one case even more than 26°, and a range in backazimuths of up to 35°. Information on the accuracy of backazimuth determination, on the average hydro‐acoustic wave speed along the wave path and on topographic blockage of waves can also be inferred from the results owing to the consistency and great detail of the mapped coupling zones. For the study area, it can be shown that it is predominantly seismic energy from P and/or Pn waves that couples efficiently into the SOFAR channel. Particularly for deep focus events, the backazimuths determined for the highest amplitude segments of the signals can deviate considerably from the great‐circle azimuth between source and receiver, and may therefore lead to inaccurate epicentre locations based on backazimuth measurements of T waves. In some cases, laterally reflected T waves are observed.

P- and T-Wave Detection Thresholds, Pn Velocity Estimate, and Detection of Lower Mantle and Core P-Waves on Ocean Sound-Channel Hydrophones at the Mid-Atlantic Ridge

Since 1999 six Sound Fixing and Ranging (SOFAR) hydrophones have been moored along the Mid-Atlantic Ridge (MAR) (15-35 N). These hydrophones (8-bit data resolution) are designed for long-term monitoring of MAR seismicity using the acoustic T waves of seafloor earthquakes. The completeness level of the MAR T- wave earthquake catalog estimated from size-frequency constraints is mb 3.0, a significant improvement in detection compared to the mb 4.6 completeness level estimated from National Earthquake Information Center magnitude-frequency data. The hydrophones also detect the acoustic phase of converted upper mantle P arrivals from regional earthquakes at epicentral distances of 374-1771 km and from events as small as mb 3.6. These regional P waves are used to estimate a Pn velocity of 8.0 0.1 km sec 1 along the east and west MAR flanks. An unexpected result was the identification of P arrivals from earthquakes outside the Atlantic Ocean basin. The hydrophones detected P waves from global earthquakes with magnitudes of 5.8-8.3 at epicentral distances ranging from 29.6 to 167.2. Examination of travel times suggests these teleseismic P waves constitute the suite of body-wave arrivals from direct mantle P to outer- and inner-core reflected/refracted phases. The amplitudes of the teleseismic P waves also exhibit the typical solid-earth wave field phenomena of a P shadow zone and caustic at D 144. These instruments offer a long-term, relatively low-cost alternative to ocean-bottom seismometers that allows for obser- vation of Pn velocities and mantle/core phases arriving at normally inaccessible deep- sea locations.

Comparison of Teleseismically and Hydroacoustically Derived Earthquake Locations along the North-central Mid-Atlantic Ridge and Equatorial East Pacific Rise

This letter examines earthquake locations derived using independent teleseismic and hydroacoustic data sets for events along the north-central (15°-35°N) Mid-Atlantic Ridge (MAR) and equatorial (10°S-10°N) East Pacific Rise (EPR). It represents the first large-scale comparison of such data, providing ground-truth information for the teleseismic locations and insight into the process of T -wave generation. An assessment of location accuracy and catalog completeness in these remote ocean settings is critical for explosion monitoring efforts associated with the Comprehensive Test-Ban Treaty (CTBT), as well as seismotectonic studies at mid-ocean ridges (MOR's). During the last decade, the value of hydroacoustic studies in monitoring MOR seismicity has been demonstrated through work using the U.S. Navy's SOund SUrveillance System (SOSUS) ( e.g., Fox et al., 1994) and arrays of moored autonomous underwater hydrophones (AUH's) ( e.g., Fox et al., 2001; Smith et al., 2002). These studies utilize seismically generated tertiary (7) waves that propagate within the ocean's SOund Fixing And Ranging (SOFAR) channel (Tolstoy and Ewing, 1950). In locating submarine earthquakes, the main advantages of the hydroacoustic method stem from the efficiency of this low-velocity wave guide, which allows for the detection of much smaller events at longer ranges relative to that possible with waves that travel through the solid earth, as well as from the existence of a well defined ocean sound-speed model ( e.g., Teague et al., 1990). Although T waves may be produced within regions of shallow sloping bathymetry near oceanic islands, continental shelves, and subduction zones ( e.g., Shurbet and Ewing, 1957; Johnson and Norris, 1968; Talandier and Okal, 1998), here we examine their generation in association with shallow hypocenter events within a deeper-ocean or abyssal environment ( e.g., Johnson, et al., 1968; Fox et al., 1994). In this setting, the scattering of energy from a rough seafloor has emerged as the …

SOFAR Floats Reveal Midlatitude Intermediate North Atlantic General Circulation. Part I: A Lagrangian Descriptive View

Quasi-Lagrangian trajectories of 26 sound fixing and ranging (SOFAR) floats have been collected near a depth of 700 m in the Central North Atlantic between 1983 and 1989, aiming at studying the influence of the Mid-Atlantic ridge on the large-scale intermediate circulation. Launched as tight clusters (18 km near neighbor distance) on either side of the Mid-Atlantic ridge, the floats dispersed quickly over a few months, jumping from one mesoscale eddy to the next. By and large, cyclonic and anticyclonic eddy motions are equipartitioned. Apparently the Mid-Atlantic ridge remains a barrier even at that shallow depth, since only one float from either side drifted across the ridge. After a few years, floats have circulated through most of the western basin (west of the Mid-Atlantic ridge), between 30° and 45°N; while east of the ridge and south of the Azores Plateau, floats advected east of the Great Meteor et al. Seamounts by the Azores current wandered more sluggishly. On this timescale, float dispersion is much more efficient zonally than meridionally, an anisotropy mainly seen west of the ridge, where floats spread westward over 30° longitude, while no float penetrated south of 30°N and only two crossed 45°N northward.

Estimation of the Stability of Acoustic Multipath in Long-Range Propagation

We present the stability estimation results of the data collected during the sound transmission experiment performed by Japan Marine Science and Technology Center (JAMSTEC) in 1996 in the western equatorial Pacific Ocean. The combined information of the amplitude and the phase was effective for identifying a ray and observing the ocean-structure change. The phases of correlated signals were very stable within 50 s not only for the paths propagated near the sound fixing and ranging (SOFAR) axis but for ones turning at about 2500 m depth. The travel time could be estimated within an error of less than 0.3 ms.

Numerical Analysis of Acoustical Propagation Characteristics in Central Equatorial Pacific Ocean with Ridges of Seabed Used by Three-Dimensional Parabolic Equation Method

Ocean acoustic tomography (OAT) using sound propagation times in the ocean is a useful observation system to determine the actual temperature of water. OAT needs an adequate numerical method of sound propagation to obtain the prior information about the ocean. We reported about the wide-angle parabolic equation (PE) calculation method for long-range propagation. However, if the ridges on a seabed exist near the SOund Fixing And Ranging (SOFAR) axis, it blocks sound pulses passing near the SOFAR axis. Sound pulses are reflected not only in the vertical direction such as sea surface, but also in the horizontal direction because of ridges of seabed. Two-dimensional (2-D) analysis of the sound pressure distribution many have a slight error behind ridges or islands. Therefore, it is necessary to consider the sound propagation characteristics using the three-dimensional parabolic equation (3-DPE) calculation method but the 2-DPE calculation method. We obtained accurate results for sound pressure distribution on the SOFAR axis using 3-DPE calculation method, when the sound pulse was blocked by the seabed. The calculation results of underwater characteristics by 3-DPE calculation were compared with those of the 2-DPE calculation method.