Accounting for Doppler in moving ship tomography

Early attempts at understanding scintillations in sound transmission through the ocean assumed that ocean fine structure is homogeneous and isotropic. It is, in fact, dominated by internal waves, which are neither homogeneous nor isotropic. Sound Transmission Through a Fluctuating Ocean (Flatté et al., Cambridge University Press, Cambridge, England 1979) combined recently developed internal-wave models with path-integral methods to predict the fluctuations of resolved acoustic multipaths. At that time, Ocean Acoustic Tomography (Munk et al., Cambridge University Press, Cambridge, England 1995), which uses ray travel times to determine large-scale ocean structure, had just been proposed. The wideband acoustic sources and receivers required to make travel-time measurements for tomography also provided the technology needed to quantify acoustic fluctuations. Conversely, internal-wave-induced scattering limits travel-time measurement precision. Tomography experiments at 25-km range provided some of the earliest tests of path-integral predictions. Subsequent tomography experiments provided data at ever-increasing ranges and decreasing frequencies. The Acoustic Thermometry of Ocean Climate project made measurements at megameter ranges with 1400-m vertical line array receivers. The next step is a modular distributed VLA capable of spanning the full water column that is under development to enable separation of acoustic modes using spatial filtering and to fully characterize deep-water acoustic time fronts.

Accurate knowledge of the spatial and temporal evolution of the three-dimensional ocean sound speed profile (SSP) is crucial for underwater source localization. Ocean acoustic tomography (OAT) methods aim at reconstructing SSPs variations based on acoustic measurements between multiple source-receiver pairs (e.g., eigenrays arrival times). This study investigates the estimation of range-dependent SSPs using a classical model-based OAT method (i.e., ray-based ocean acoustic tomography), for various configurations of source and receiver configurations using autonomous platforms in a highly-dynamic upper ocean environment. A regional oceanographic simulation of the De Soto Canyon circulation in the Gulf of Mexico is used to construct 3D sound speed variations spanning a month long which exhibits significant sub-mesoscale variability. Two main aspects affecting OAT performance in the presence of high 3D SSPs variability are investigated: (1) The influence of the input acoustic data (i.e., source-receivers configuration and platform motion, arrival-times accuracy...) and (2) the actual implementation of the OAT scheme (i.e., selection of complexity reduction basis, linearized forward model assumptions as well as the use of iterative solvers) on SSPs estimations errors. Practical implications for the design of OAT experiments in the dynamic upper ocean will be discussed. [Work supported by the Office of Naval Research.]

On the basis of the stream function field computed using a diagnostic model in the South China Sea in the summer 1998, the acoustic ray travel time tomography algorithm for reconstruction of two-dimensional flow field was tested by the numerical simulation in the region east to Vietnam. Error or noise contaminates the coefficients of both transfer matrix and measure data of the inverse equation of ocean acoustic tomography (OAT). Previous work on ill-posed problem in OAT commonly used Tikhonov regularization technology considering error only in observation data. Here, the truncated total least squares method (TTLS), which simultaneously considers error and noise on both sides, is applied for inverse solutions and its performances is compared with conventional Tikhonov method. The investigation suggests that the truncated total least squares method can substantially improve reconstruction of stream function field and is a promising alternative method for the solution of ocean acoustic tomography inverse problem.

The mathematical basis for a method to invert modal ocean acoustic tomography measurements without explicitly assuming an initial sound-speed profile is described. The method is based on determining the group and phase travel times for each vertical slice through the tomographic region. The group travel times are determined directly as the measurements of modal pulse travel time. The phase travel times are determined by resolving the cycle ambiguities in the phase measurements with constraints connecting the group and phase travel times. Standard tomographic techniques then determine the modal group and phase speeds within the tomographic region, and Abel transforms can be used to determine the symmetric part (the difference between the upper and lower profiles) of the sound channel. As with ray tomography, modal tomographic measurements supply no information about the antisymmetric part of the sound channel (the average of the upper and lower profiles), but determining the lower part of the sound channel can supply the missing information. The lower part of the sound channel can be estimated from climatology or by using arrays to determine local sound-speed profiles from local modal properties and interpolating into the interior of the tomographic region.

A substantial number of underground formations penetrated during oil and gas well drilling operations consist of carbonate rocks, limestones and dolomites. Knowledge of their strength can help drill bit and mud weight selection, drilling performance prediction, wellbore stability analysis, and even casing point selection. Physical samples that allow direct destructive measurement of the strength of underground formations are frequently not available. Instead their strength is usually estimated from in-situ measurements of physical properties that are correlated with the rock's strength. The rock's unconfined compressive strength (UCS) is typically estimated from wireline or LWD (logging while drilling) measurements of the rock's acoustic travel time. Systematic physical, mechanical, and mineralogical measurements were made on a number of carbonate rocks collected from outcrops and representing a wide range of compositions and properties. This paper presents these data and uses them to show that carbonate rocks' acoustic properties are not necessarily well correlated with their strength. It describes an improved method for estimating the strength of underground carbonate formations developed from the data and illustrates that method with wireline and core data. The new strength estimation method, which recognizes and accounts for the impact of grain size on carbonate rock strength, should prove particularly valuable for bit selection and drilling performance prediction. Introduction In many instances it is useful to know the strength of underground rock formations penetrated during oil and gas well drilling operations. This information can help drill bit and mud weight selection, drilling performance prediction, wellbore stability analysis, and even casing point selection. Very often physical samples, which would allow direct destructive measurement of the strength of underground formations, are not available. Instead their strength is usually estimated from insitu measurements of physical properties that are correlated with the rock's strength properties. The strength property most usually estimated in this way is the rock's unconfined compressive strength (UCS), which is the peak compressive stress the rock can support under uniaxial loading. Typically, UCS is estimated from wireline measurements of the rock's acoustic travel time, which is the reciprocal of the acoustic velocity. A substantial number of underground formations contain carbonate rocks, limestones consisting primarily of calcite and aragonite (calcium carbonate), and dolomites consisting primarily of dolomite (calcium magnesium carbonate) and calcite. To evaluate methods for estimating the strength of carbonate rocks, we made systematic physical, mechanical, and mineralogical measurements on a number of carbonate rocks collected from outcrops and representing a wide range of compositions and properties. This paper presents these data and uses them to investigate the relationships between the UCS and other properties of carbonate rocks. The results are used to develop an improved method for estimating the strength of carbonate rocks, which recognizes and accounts for the impact of grain size. We show how the method can be modified to allow its use when grain size data are not available, and we illustrate its application using wireline and core data from a test well drilled in Oklahoma, USA.

A design procedure for an amplitude-modulated and nonlinear frequency-modulated (AM-NLFM) signal is introduced. The designed signal can drive a given transducer to its peak power to produce a sound pressure waveform into the water with a desired power spectrum and maximum possible energy. The signal can be formed either in the time domain or in the frequency domain. The frequency domain approach gives an output power spectrum precisely identical to a preferred shape. Therefore, the sidelobe levels after matched filtering are not raised by unwanted spectral magnitude ripples which exist when a time domain method is adopted. The absence of spectral ripples is desirable for applications requiring long range transmission and good multipath discrimination capability. An acceptable tradeoff between time resolution and sidelobe levels is achieved by properly choosing the desired power spectral shape. As the time resolution is usually the most critical specification for precision travel-time measurements, it is shown that by sacrificing some of the transducer's output power capability, a waveform with a considerably wider bandwidth can be transmitted, resulting in a significantly enhanced time resolution. A quasi-steady-state (QSS) approximation is used in the signal design, leading to a manageable and intuitive design procedure. >

A tomographic network of seven moored transceivers was deployed in the western Mediterranean basin in early 1994 by IfM (Kiel, Germany), IFREMER (Brest, France), and WHOI (Woods Hole, USA) in the framework of the THETIS-2 project. Signals emitted by the transceivers were also utilized in the 8-week moving ship tomography experiment, MOST. Acoustic measurements were made with a single hydrophone and an eight-element vertical array deployed from a drifting vessel and were complemented by CTD casts. The set of observation points was chosen to provide detailed CTD and acoustic data along existing propagation paths of the THETIS-2 network for comparison of traditional and moving ship tomography under well-controlled environmental conditions as well as to create numerous additional propagation paths to improve resolution in the horizontal plane. By precise compensating the Doppler shift due to vessel drift, the data processing technique used made it possible to resolve eigenray arrivals up to 400–600 km range for different sources. Preliminary results of the MOST data processing, analysis, and interpretation will be presented at the meeting. [Work supported by ISF and INTAS.] a)On leave from P. P. Shirshov Oceanography Institute, Moscow.

An ocean acoustic tomography (OAT) experiment was conducted during May to September 2008 in the Luzon Strait where the Kuroshio sporadically intrudes and strong internal tides and waves occur. Although the reciprocal sound transmissions between two acoustic stations were remarkably disturbed by these waves, ocean current signals proportional to the travel time differences, were successfully retrieved by low‐pass filtering out the high frequency wave components under these conditions. The drift of internal clock occurring during the observation is corrected through the procedure that the travel time differences for rays passing the upper 500 m layer are determined relative to those for rays passing near the underwater sound channel (USC) axis of depth about 1000 m. The observed currents relative to currents near the USC axis agree with the moored upward looking ADCP (acoustic Doppler current profiler) data. This study suggests a potential ability of OAT for the operational measurement of ocean currents.

Over the decade 1996–2006, acoustic sources located off central California (1996–1999) and north of Kauai (1997–1999, 2002–2006) transmitted to receivers distributed throughout the northeast and north central Pacific. The acoustic travel times are inherently spatially integrating, which suppresses mesoscale variability and provides a precise measure of ray‐averaged temperature. Daily average travel times at 4‐day intervals provide excellent temporal resolution of the large‐scale thermal field. The interannual, seasonal, and shorter‐period variability is large, with substantial changes sometimes occurring in only a few weeks. Linear trends estimated over the decade are small compared to the interannual variability and inconsistent from path to path, with some acoustic paths warming slightly and others cooling slightly. The measured travel times are compared with travel times derived from four independent estimates of the North Pacific: (1) climatology, as represented by the World Ocean Atlas 2005 (WOA05); (2) objective analysis of the upper‐ocean temperature field derived from satellite altimetry and in situ profiles; (3) an analysis provided by the Estimating the Circulation and Climate of the Ocean project, as implemented at the Jet Propulsion Laboratory (JPL‐ECCO); and (4) simulation results from a high‐resolution configuration of the Parallel Ocean Program (POP) model. The acoustic data show that WOA05 is a better estimate of the time mean hydrography than either the JPL‐ECCO or the POP estimates, both of which proved incapable of reproducing the observed acoustic arrival patterns. The comparisons of time series provide a stringent test of the large‐scale temperature variability in the models. The differences are sometimes substantial, indicating that acoustic thermometry data can provide significant additional constraints for numerical ocean models.

Two-point correlation functions of diffuse wave fields are known to approximate the deterministic Green’s functions that describe wave propagation between the two points. The deterministic ray travel times retrieved from cross-correlations of ambient noise recorded on vertical line arrays of hydrophones serve as the input data for passive acoustic thermometry and tomography of the water column. Accuracy of the sound-speed and water temperature inversions is largely controlled by the accuracy of the ray travel time measurement. Achieving oceanographically relevant results requires the relative error of the passive travel time measurements to be less than 0.1%. Often, the absolute error of the travel time measurement needs to be small compared to the acoustic wave period. In this paper, stationary-point analysis of an asymptotic, high-frequency integral representation of the noise cross-correlation function is used to determine ways for accurate retrieval of ray travel times. Limitations on the accuracy of the travel time retrieval due to noise anisotropy, medium non-stationarity, source and receiver motion, and finite noise averaging times are discussed. [Work supported by ONR.]

This paper presents a study on the applicability of a method for the statistical characterization of seismic signals which is based on the statistics of their wavelet sub-band coefficients using [Formula: see text] stable distributions. The method was originally applied to underwater acoustic signals of the type used in ocean acoustic tomography and seabed classification applications. The same protocol was applied to seismic signals representing the vertical displacement, measured on a traditional seismograph. The study showed that this process can indeed provide a means of characterizing a seismic signal and be used to estimate similarities between seismic signals, which is an important factor in studying seismic activity in an area.

We use data observed near the solar disk center by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) to mimic observations at high-latitude areas after applying geometric transform and projection. These data are then used to study how foreshortening affects the time–distance measurements of acoustic travel times. We find that foreshortening reduces the measured mean travel-times through altering the acoustic-power weighting in different harmonic degrees, but the level of reduction and the latitude dependence are not as strong as those measured from the observation data at the same latitude. Foreshortening is not found to be accountable for the systematic center-to-limb effect in the measured acoustic travel-time differences, which is an essential factor for a reliable inference of the Sun’s meridional-circulation profile. The differences in the acoustic power spectrum between the mimicked data and the observation data in high-latitude areas suggest that the optical spectrum-line formation height or convection cells in these areas may be the primary cause of the center-to-limb effect in helioseismic analyses.

Gulf stream and wind-induced current variability on the northeastern Florida continental shelf

Travel time measurements from an acoustic array interrogating a fluid within the cross-section of a pipe include systemic errors: from mispositioning of array elements, group delays from the transceiver circuits at each element, and acoustic propagation effects between transmitter/receiver pairs. An enhanced calibration model is described which includes the influence of these error sources, and a recursive least squares solution is applied to find calibration travel-time corrections that minimize systemic error. This enhanced calibration model is applied in-situ to an acoustic transducer array, resulting in increased accuracy and precision of acoustic travel-time measurements.

An acoustic measurement system detects solids within a fluid includes a sample cell 10 for containing the fluid, a first transducer 20 for outputting a first acoustic detection signal at a first frequency, a first acoustic receiver 20 responsive to the first acoustic detection signal, and a second acoustic transducer 20 for outputting an acoustic second mixing signal at a second frequency to mix the solids in the fluid within the sample cell. Controller 40 may vary the “on” time of the second transducer with respect to the measurement of the first acoustic signal. The fluid within the cell may be tested under a wide range of temperatures and pressures. The acoustic measurement system may also be used to measure the bulk modulus of the liquid. The method of the invention allows for the detection of crystallization for a portion of the fluid in the sample cell under widely varying conditions.