First Observation of the Earth's Permanent Free Oscillations on Ocean Bottom Seismometers

Deen, Martha; Wielandt, Erhard; Stutzmann, É.; Crawford, Wayne; Barruol, Guilhem; Sigloch, Karin

doi:10.1002/2017gl074892

Cited by 26 publications

(27 citation statements)

References 47 publications

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“…We note that the SIO phase 2 instruments produced data that contained a long‐period transient that occurred approximately every 3,625 s and lasted for about 10 min. While effective techniques have been developed to remove these transients (e.g., Deen et al, ), here we simply examined 30‐min transient‐free windows for these stations, which was sufficient to observe signals with periods <200 s.…”

Section: Datamentioning

confidence: 99%

“…We note that the SIO phase 2 instruments produced data that contained a long-period transient that occurred approximately every 3,625 s and lasted for about 10 min. While effective techniques have been developed to remove these transients (e.g., Deen et al, 2017), here we simply examined 30-min transient-free windows for these stations, which was sufficient to observe signals with periods <200 s. (c) Kernels computed for a typical crust with 1,000 m of fast, volcanic sediments overlying the crust. Notice that the vertical scale only covers 2 km for the two lower sets of kernels.…”

Section: Datamentioning

confidence: 99%

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Seismic Structure of Marine Sediments and Upper Oceanic Crust Surrounding Hawaii

Doran

Laske

2019

JGR Solid Earth

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We present models of compressional and shear velocity structure of the oceanic sediments and upper crust surrounding the Hawaiian islands. The models were derived from analysis of seafloor compliance data and measurements of Ps converted phases originating at the sediment‐bedrock interface. These data were estimated from continuous broadband ocean bottom seismometer acceleration and pressure records collected during the Plume‐Lithosphere Undersea Mantle Experiment, an amphibious array of wideband and broadband instruments with an aperture of over 1,000 km. Our images result from a joint inversion of compliance and Ps delay data using a nonlinear inversion scheme whereby deviation from a priori constraints is minimized. In our final model, sediment thickness increases from 50 m at distal sites to over 1.5 km immediately adjacent to the islands. The sedimentary shear velocity profiles exhibit large regional variations. While sedimentary structure accounts for the majority of the compliance signal, we infer variations in shear velocity in the uppermost bedrock on the order of ±5%. We also require relatively high values of Poisson's ratio in the uppermost crust. Lower crustal velocities are generally seen to the north and west of the islands but do not appear well correlated with the Hawaiian Swell bathymetry. A region of strong low velocity anomalies to the northeast of Hawaii may be associated with the Molokai fracture zone.

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Section: Datamentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Seismic Structure of Marine Sediments and Upper Oceanic Crust Surrounding Hawaii

Doran

Laske

2019

JGR Solid Earth

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“…In order to enhance the signal to noise ratio, we further select the Rayleigh wave train windows on the stack of auto-correlograms by setting to 0 the rest of the signal as in Deen et al (2017). We keep signal around 0-time lag and around each surface wave train R1 to R3.…”

Section: Normal Modes: Detection and Seismometer Performancementioning

confidence: 99%

Mars’ Background Free Oscillations

et al. 2019

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Observations and inversion of the eigenfrequencies of free oscillations constitute powerful tools to investigate the internal structure of a planet. On Mars, such free oscillations can be excited by atmospheric pressure and wind stresses from the Martian atmosphere, analogous to what occurs on Earth. Over long periods and on a global scale, this phenomenon may continuously excite Mars' background free oscillations (MBFs), which constitute the so-called Martian hum. However, the source exciting MBFs is related both to the global-scale atmospheric circulation on Mars and to the variations in pressure and wind at the planetary boundary layer, for which no data are available. To overcome this drawback, we focus herein on a global-scale source and use results of simulations based on General Circular Models (GCMs). GCMs can predict and reproduce long-term, global-scale Martian pressure and wind variations and suggest that, contrary to what happens on Earth, daily correlations in the Martian hum might be generated by the solar-driven GCM. After recalling the excitation terms, we calculate MBFs by using GCM computations and estimate the contribution to the hum made by the global atmospheric circulation. Although we work at the lower limit of MBF signals, the results indicate that the signal is likely to be periodic, which would allow us to use more efficient stacking theories than can be applied to Earth's hum. We conclude by discussing the perspectives for the InSight SEIS instrument to detect the Martian hum. The amplitude of the MBF signal is on the order of nanogals and is therefore hidden by instrumental and thermal noise, which implies that, provided the predicted daily coherence in hum excitation is present, the InSight SEIS seismometer should be capable of detecting the Martian hum after monthly to yearly stacks.

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“…Unfortunately, most of them are deployed on land and observations are strongly limited by the poor instrumental coverage in ocean basins. Ocean-bottom seismometers (OBSs) have been considerably improved to fill this gap: (i) their autonomy can exceed 1 yr; (ii) the use of threecomponent, broad-band sensors is becoming standard; (iii) they are now easy to deploy and (iv) their performance are catching up with those of terrestrial stations, enabling them to measure faint signals such as Earth's free oscillations (Deen et al 2017). Despite those impressive advances, the way OBSs are deployed has not changed much since their invention.…”

Section: Introductionmentioning

confidence: 99%

Orienting and locating ocean-bottom seismometers from ship noise analysis

Trabattoni

Barruol

Dreo

et al. 2019

Geophysical Journal International

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Summary Breakthroughs in understanding the structure and dynamics of our planet will strongly depend upon instrumenting deep oceans. Progress has been made these last decades in ocean-bottom seismic observations, but ocean-bottom seismometer (OBS) temporary deployments are still challenging and face set-up limitations. Launched from oceanographic vessels, OBSs fall freely and may slightly drift laterally, dragged by currents. Therefore, their actual orientation and location on the landing sites are hard to assess precisely. Numerous techniques have been developed to retrieve this key information, but most of them are costly, time-consuming or inaccurate. In this work, we show how ship noise can be used as an acoustic source of opportunity to retrieve both the orientation and the location of OBSs on the ocean floor. To retrieve the OBS orientation, we developed a first method based on a combination of seismic and pressure data through the use of the acoustic intensity. This latter can be used to quantify the OBS orientation from the ship noise direction of arrival (DOA), which can then be compared with known ship trajectories obtained from the automatic identification system (AIS). To accurately relocate OBSs, we also developed a second method based on the hydrophone data which computes distances of acoustical sources by measuring time differences of arrival (TDOA) between direct and reverberated phases. The OBS location is then retrieved by fitting measured ship distances with known ship trajectories. In this study, a full network of OBSs deployed in the SW Indian Ocean was reoriented and a test station was relocated. We demonstrate that our new methods may quantify the OBS orientation with an accuracy of about one degree, and its location with an accuracy of a few tens of metres, depending on the number of ships used in the analysis.

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First Observation of the Earth's Permanent Free Oscillations on Ocean Bottom Seismometers

Cited by 26 publications

References 47 publications

Seismic Structure of Marine Sediments and Upper Oceanic Crust Surrounding Hawaii

Seismic Structure of Marine Sediments and Upper Oceanic Crust Surrounding Hawaii

Mars’ Background Free Oscillations

Orienting and locating ocean-bottom seismometers from ship noise analysis

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