Barbara J. Sotirin

Array element localization for horizontal arrays via Occam’s inversion

Accurate locations for the individual elements of an acoustic sensor array are required for the application of advanced array processing methods. This paper develops a general method of localizing horizontal line array (HLA) elements which overcomes bandwidth constraints of low-frequency arrays and uncertainty in the experimental configuration. Array elements are localized for two HLA’s associated with the Spinnaker Array, a three-dimensional sensor array located in the high Arctic. Recordings were made of imploding glass light bulbs deployed at a series of locations surrounding the array site. Implosion instants were not measured; hence, the data consist of relative travel times. In addition, the source locations were measured only approximately in the field, and are treated as unknown parameters. The inverse problem of determining hydrophone and source locations is nonunique and ill-conditioned. To determine the most physically meaningful solution, an iterative linearized inversion is developed which ap...

Optimal array element localization

Advanced array processing methods require accurate knowledge of the location of individual elements in a sensor array. Array element localization (AEL) methods are typically based on inverting acoustic travel-time measurements from a series of controlled sources at well-known positions to the sensors to be localized. An important issue in AEL is designing the configuration of source positions: a well-designed configuration can produce substantially better sensor localization than a poor configuration. In this paper, the effects of the source configuration and of errors in the data, source positions, and ocean sound speed are quantified using a sensor-position error measure based on the a posteriori uncertainty of a general formulation of the AEL inverse problem. Optimal AEL source configurations are determined by minimizing this error measure with respect to the source positions using an efficient hybrid optimization algorithm. This approach is highly flexible, and can be applied to any sensor configuration and combination of errors; it is also straightforward to apply constraints to the source positions, or to include the effects of data errors that vary with range. The ability to determine optimal source configurations as a function of the number of sources and of the errors in the data, source positions, and sound speed allows the effects of each of these factors to be examined quantitatively in a consistent manner. A modeling study considering these factors can guide in the design of AEL systems to meet specific objectives for sensor localization.

Large aperture digital acoustic array

A digital array of 120 acoustic channels 900 m in length has been constructed to study low-frequency (20-200 Hz) ambient noise in the ocean. The array may be deployed vertically or horizontally from the research platform FLIP and the array elements are localized with a high-frequency acoustic transponder network. The authors describe the instrumentation, telemetry, and navigation systems of the array during a vertical deployment in the northeast Pacific. Preliminary ambient noise spectra are presented for various array depths and local wind speeds. Ambient noise in the frequency band above 100 Hz or below 25 Hz increases with local wind speed. However, in the frequency band 25-100 Hz, ambient noise is independent of wind speed and may be dominated by shipping sources. >

Shallow‐water receptions from the transarctic acoustic propagation experiment

In April 1994 a long‐range, low‐frequency acoustic propagation experiment took place in the Arctic to test the feasibility of using acoustic methods to observe large‐scale thermal variability. Here the characteristics of the received signals at a vertical hydrophone array and a horizontal geophone array located at the edge of the continental shelf in the Lincoln Sea, about 1 Mm from the source, are discussed with respect to the ‘‘forward’’ problem of understanding and correctly modeling propagation. Both cw (tonal) and tomographic M‐sequence transmissions are analyzed. It is found that phase stability is very good, and that phase changes are almost entirely due to source/receiver motions. Travel times are not quite as stable, but are consistent with the phase observations, showing that phase can be used to measure travel time changes very accurately. Modal decomposition of M‐sequence transmissions received by the vertical array shows an arrival structure in rough agreement with predictions from a coupled‐...

An Arctic acoustic array system: Installation, in situ, repair, and preliminary data analysis

Under Project Spinnaker a joint Canadian/U.S. large multiple aperture array was deployed and cabled back to shore under the Arctic ice pack during April 1996 to acquire long‐term measurements of Arctic ambient noise. The data collected have been documented and are being distributed to investigators for analysis. As well, the system is planned as a receive station for the joint U.S./Russian Arctic Climate Observations Using Underwater Sound (ACOUS) project. Although batteried for a 3‐to‐5‐year life, in June 1996 the signal from the array terminated abruptly. After detailed analysis of the last recorded data, a repair effort was planned for the Spring of 1997. This paper will describe the anaysis, methods, and outcome of the in situ repair and future use of the deployed acoustic array system. [Work supported by ONR, SPAWAR, and CRAD.]

Array element localization for vertical hydrophone arrays in the Arctic ocean

This paper describes array element localization (AEL) for two vertical line arrays (VLA’s) of hydrophones, which comprise part of the Spinnaker Array, a 3‐D sensor array in the Arctic Ocean. The AEL system associated with each VLA monitors the location of three high‐frequency acoustic receivers, positioned at intervals along the array cable, by measuring travel times from five seafloor transponders. The transponders are comprised of a controller unit at the base of the array and four remote units at ranges of approximately 500 m. The accuracy to which the AEL receivers can be located depends directly on the knowledge of transponder positions. Two steps were used to accurately determine the transponder positions. First, travel‐time measurements were made to an independent set of acoustic sensors suspended just below the ice. Second, use was made of the AEL travel‐time data themselves. Both data sets were inverted using an iterative regularized inversion that applied a priori information to overcome the non...

Earthquake studies using under‐ice hydrophone data (‘‘Spinnaker’’)

Preliminary studies indicate that the Spinnaker array, a network of hydrophones currently deployed in the Arctic Ocean, may be used to monitor seismicity of the Arctic Basin, as well as long distance teleseismic arrivals from the Southern Hemisphere. The Arctic Basin is a tectonically complex region that has seen few seismic studies, and the Spinnaker array has the potential to monitor some of its most interesting features, such as the Nansen–Gekkel Ridge (the slowest spreading ridge in the world), in near real time. In addition, teleseismic arrivals at the array may have raypaths that coincide closely with the Earth’s spin axis. These data may prove useful to current studies of the crystalline alignment and differential spin of the Earth’s inner core.

Array element localization for a horizontal hydrophone array in the Arctic Ocean

In 1996, an acoustic research array with both horizontal and vertical components was deployed through the polar ice pack north of Ellesmere Island. The horizontal components consist of a 2.4‐km horizontal line array (HLA) of 80 hydrophones on the seafloor and a 240‐m secondary HLA of 8 hydrophones perpendicular to the main HLA. To accurately localize these hydrophones, a series of recordings were made immediately after deployment. Glass light bulbs were imploded at 50‐m depth at eight locations surrounding the array. Source locations were known to within approximately 2 m; however, to invert the travel time data to within their estimated uncertainties, it was necessary to include corrections to the source locations as unknowns. Source instants were not measured, and were also included as unknowns. An iterative inversion algorithm was formulated by expanding the ray‐tracing equations and neglecting second‐order terms to yield a linear system of equations. The system was solved by minimizing the HLA curvatu...

Characteristics of low‐frequency Arctic ambient noise determined using the SPINNAKER array

The ambient noise field at low frequencies in the Arctic is dominated by sounds resulting from pressure ridging caused by convergences in the surface ice field. These convergences are driven by currents in the ocean and by winds on the surface, and are modulated by the ice strength and compactness. Analysis of long time series of ambient noise might therefore be a useful tool in studying the spatial and temporal changes in these factors. The SPINNAKER array recently deployed at the edge of the continental shelf north of Ellesmere Island and cabled to shore provides a unique opportunity for measuring the structure and variability of the ambient noise field. Preliminary analysis shows a number of interesting features, which will be correlated with environmental factors to monitor long‐term changes in the Arctic oceanography.

Long‐distance low‐frequency modal propagation into the Lincoln Sea

One‐Mm transmissions from the April 1994 trans‐Arctic acoustic propagation (TAP) experiment recorded by a vertical line array deployed from an icecamp at the edge of the continental shelf in the Lincoln Sea are analyzed. The received phase is astonishingly stable, and appears to vary mainly with source/receiver motions. Travel times determined from M‐sequence transmissions are less stable and are not consistent with the phase measurements. Modal decomposition shows that bottom effects strip out the higher‐order modes as sound propagates onto the shelf. The amplitude of the surface‐trapped first mode is also weaker than predicted by standard loss mechanisms. Some implications of this analysis for the design of future Arctic low‐frequency tomography experiments are discussed.