File Name: wave propagation and scattering in random media creator.zip
- Internationaler Tag der Eisbären
- Waves and Imaging through Complex Media
- Guided wave photonics saunders college publishing electrical engineering
- Optics Express
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Internationaler Tag der Eisbären
Acoustical methods are playing an increasingly important role in the measurement of small-scale processes in the ocean. Most of the applications to date are based on backscatter sonar in various forms; and the techniques of Doppler and echo sounder measurement of ocean currents, internal waves, surface waves, bubble fields, mixing processes, and turbulence, as well as biological phenomena, are in a state of rapid development. An alternative and less well developed technology exploits a bistatic system transmitter and receiver separately located to measure the influence of the medium on signals traveling along wholly refracted paths.
Properties of the medium are then recovered through an inversion of the detected signals. This, of course, is also the goal of acoustic tomography Munk et al. We refer to this approach as acoustical scintillation. Many reports on these developments have appeared in the acoustic and oceanographic literature using fixed-bottom-mount frames that are cabled to shore.
Here we describe a self-contained instrument that can be used in diverse deployments to emphasize its potential for oceanographic measurement.
The acoustic scintillation method is sensitive to spatial structure the choice of acoustic frequency and range ensures that the scales within the inertial subrange of turbulence are sampled , and sampling is carried out fast enough 10—20 Hz to obtain adequate temporal statistics. A crucial requirement of the scintillation method for mean flow measurements is that there is sufficient coherence between the signals detected at two spatially separated receivers, and, for turbulence measurements, that the acoustic scattering remains weak.
While recognizing the importance of larger-scale phenomena e. In fact, the measurement of turbulence is a valuable application of the scintillation technique in boundary layer dynamics.
The advantage of the acoustical scintillation technique is that path-averaged properties over a baseline several times that of the turbulent eddies being measured are obtained. The space—time averaging properties of the scintillation method can be an advantage because geophysical turbulence is both patchy and intermittent.
Another advantage of the acoustic method is that there are no probes that are deployed within the turbulent flow being measured and, thus, it is well suited for the hydrothermal vent application described here. This paper reports the design and results of a self-contained in situ acoustic scintillation instrument that was used in two different oceanographic environments.
The measurement concept and theoretical background describing acoustic propagation is described next in section 2. The design of the instrument is detailed in section 3. Section 4 describes the results from a deployment at the Main Endeavour vent field of the Juan de Fuca Ridge, and section 5 describes the results from a deployment in the bottom boundary layer of Mediterranean flow through the Bosporus Strait. The concept of acoustic scintillation analysis has its origin in studies of wave optical, radio, or acoustic propagation through a turbulent atmosphere Tatarskii , Fluctuations in acoustical signals in the ocean have long been known, but the application of scintillation analysis methods has been more recent.
As conceived, for example, by Lee and Waterman in the determination of the speed of the wind, a single transmitter, for example, a distant radio source, is detected at two locations, such that a radial velocity component of the perturbing medium is orthogonal to the two nearly parallel paths joining the transmitter with the two receivers.
Refractive perturbations in the medium pass through the two paths, creating a correlation in the detected signals, the time lag of which is inversely proportional to the wind velocity component. Refractive variability will produce the perturbations, but a crucial requirement is that there be sufficient coherence between the signals detected at the two locations to allow for a useful measurement to be made.
The application of this concept to the ocean was first demonstrated in a simple set of measurements that were acquired with a sound source and pair of hydrophones towed through the water Clifford and Farmer ; Farmer and Clifford Moreover, in many interesting coastal environments it is possible to fix the source-and-receiver system to the seafloor so as to provide path stability, but this is not essential and will be shown with the system described in this paper.
The application of space—time coherence of the fluctuating field distinguishes this approach from most other inversions of propagation data. Analysis of these signals can provide information on the path-averaged current that is resolved perpendicular to the path and turbulent refractive index intensity and related phenomena.
Our survey includes the slowly varying properties, in addition to the fluctuating properties, and is illustrated with results acquired in two very different environments. In its simplest configuration Fig. Both transducers are perpendicular to the acoustic path and aligned with the direction of the main flow being measured. Effective refractive index perturbations passing through the parallel paths create fluctuating signals, the time-lagged cross correlation of which can be solved to determine the flow speed.
The parallel path configuration is easy to use, and path-dependent weighting is essentially uniform. To solve the wave equation in 1 , we make use of the weak scattering theory of Rytov outlined in Tatarskii The scintillation instrument for flow and turbulence has been previously described in F. Rowe and D. Lemon , unpublished manuscript.
It consists of a transmitter and receiver as two separate, independent modules, each contained in its own pressure case and powered independently by its own battery pack. Figure 2 shows the exterior view of the transmitter module, which is externally identical to the receiver module. In this Bosporus Strait deployment the two-transducer array is mounted horizontally above the end cap; a vane, together with swivels, forced the transducer array parallel with the mean flow.
For the hydrothermal vent measurement program the transducers were separated vertically to measure the buoyancy-driven flow. Because the system was intended to be deployed in a taut-line mooring, the pressure cases were enclosed within a load-bearing frame that was included as an in-line portion of the mooring.
Special consideration was given to the design and fabrication of the acoustic transducers for both the transmitter and receiver. The toroidal transducer design was chosen to provide horizontal omnidirectionality, thereby preventing any sensitivity to twisting by the mooring and removing any requirement for array alignment.
The main part of the transducer is a toroidal EC piezoelectric ceramic. EC material was chosen for its low sensitivity to large changes in hydrostatic pressure, which tends to shift the center frequency.
The toroidal shape was chosen to obtain a uniform response in the horizontal plane, while maintaining some directivity in the vertical. Figure 3 shows the piezoelectric ceramic mounted on an epoxy cup, covered with a thin layer of urethane.
A smaller toroid of lead shot—filled urethane is mounted on the cup inside the ceramic as a sound barrier. It absorbs part of the acoustic energy radiated by the inner wall of the ceramic to prevent the formation of unwanted interference patterns.
A hole in the middle of the cup permits the mounting of the transducers on a support rod; another hole permits the passage of the cable when the transducers are mounted vertically. A screw on the side of the cup is used to secure the transducer at the desired position along the rod. The transducers connect to the electronics pressure cases with the desired length of cable and a three-pin underwater connector.
The transmitter was designed to run automatically, without a processor or any other intelligence built into it. The transmitter operating parameters are selected via dip-switch settings.
The timing controller contains the system master clock and generates the transmit waveforms for the power amplifier. It also controls the operation of the power amplifier and the multiplexer, which switches the power amplifier output between the two projectors. The master clock is generated by dividing a 2. The clock signal is then divided by 4 to produce the carrier frequency of kHz.
The transmitter is fitted with a single-power amplifier, whose output is switched to the transducers via a field effect transistor FET switch multiplexer and tuning network. There is no control over the transmitter power level, which is determined by the power amplifier supply voltage. Once the timing switches have been set and the system has been turned on, the transmitter will operate until it is switched off or the battery is exhausted.
In operation, the average current consumption is 20 mA. The system is powered by batter packs consisting of either 24 alkaline D cells connected in series to form a V supply, or of 12 lithium double-D cells to form a V supply. In either case, two packs can be connected in parallel, resulting in a or A-h supply, respectively. Alkaline batteries will yield a transmitter endurance of approximately 27 days, as compared to days for lithium batteries.
Figure 4b shows the main modules of the scintillation receiver. The main computer is a single-board MC microprocessor computer. This computer performs all of the required computations and control functions in the system, including communications with an external computer through an RS interface, control of the acoustic signal sampling and detection, data compression and recording, and power consumption and optimization. Raw amplitude, time of arrival, and phase data, together with system time, are recorded onto a Mbyte recorder that is implemented with highly reliable flash Erasable Programmable Read-Only Memory EPROM chips.
Power is supplied by up to two packs of lithium or alkaline batteries in the same configuration as those that are used in the transmitter. To conserve battery power and extend the data collection time, the system has the capability to power down between sampling intervals.
Either an alarm in the real-time clock or the arrival of a character through the RS interface will wake the system. The tuning circuitry adjusts the frequency response of the transducer so that it is centered at the operating frequency of kHz, with the required bandwidth of 30 kHz. The hard-limiter receiver is a two-stage amplifier. The first stage provides a fixed dB gain and is optimized for low noise. This signal is further processed for the phase measurement.
It is filtered with a fourth-order LC bandpass filter centered on the carrier to remove higher harmonics of the carrier frequency. The second-stage amplifier is implemented with an intermediate frequency IF frequency modulation FM receiver with a dynamic range of 80—90 dB, which provides a hard-limited signal output with a voltage output that is proportional to the logarithm of the input signal amplitude.
A two-pole RC low-pass filter is then applied to remove the carrier frequency. To increase the amplitude resolution prior to digitization, a fixed voltage is subtracted from the log-amplitude signal and then the difference is amplified by a factor of 5. This scheme permitted a 0. The offset voltage is periodically adjusted from the stored statistics of the amplitude to compensate for long-term fluctuations. This sampling rate is essentially twice that of the phase.
Over the width of the arrival peak the phase is slowly varying and, thus, is permitted a reduced sampling rate. A timing circuit provides sampling windows that can be moved in time under control of the computer, which would permit acquiring samples only around the times of arrival of the pulses.
Two sampling windows are provided that correspond to the signal from the two transmitters. The computer controls the spacing between the windows that is set by the transmitter pulse separation and the pulse cycle, as well as their size determined by the pulse width recall Fig. Once full, an interrupt is signaled to the computer to begin processing the signals for the peak amplitude, phase, and arrival time.
The operating software consists of a number of processes that run in a sequential manner and can operate in manual or stand-alone mode. The processes are synchronized by the real-time operating system RTOS , which gives highest priority to signal locking and data acquisition. The software was written in C with some Motorola MC assembler language. The operating software consists of four main processes: communications manual mode , control stand-alone mode , signal locking and data acquisition, and data storage.
In manual mode the system is under user control with commands from the RS port for setting system parameters and testing system functions. In the stand-alone mode of operation, the system is run by the control process. The control process is capable of continuous data acquisition or can be programmed to go to sleep to save power and wake up at specific time intervals to collect the acoustic data for a short period of time.
As well as saving power it allows subsampling of the data over a longer period. When the system wakes up from sleep mode or on power up, it reads the system parameters that were set by the user, checks the clock, and compares with the data acquisition start time. If the time has been reached the process tells the signal-locking and data acquisition process to lock on the acoustic signal.
Waves and Imaging through Complex Media
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Acoustical methods are playing an increasingly important role in the measurement of small-scale processes in the ocean. Most of the applications to date are based on backscatter sonar in various forms; and the techniques of Doppler and echo sounder measurement of ocean currents, internal waves, surface waves, bubble fields, mixing processes, and turbulence, as well as biological phenomena, are in a state of rapid development.
Nishizawa, G. Effects of small-scale heterogeneities on seismic waveform fluctuations were studied by physical model experiments. Using a laser Doppler vibrometer, we recorded elastic waves propagating through a granite block at observation points that were arranged as an equally spaced circular array. A disc-shaped PZT source was attached on the other side surface of the circular array for realizing equivalent positions with respect to both source radiation pattern and travel distances of waves. Waveform pairs were selected out from the waveforms, and cross spectra of time-windowed partial waveforms were calculated by applying the multivariate AR model.
These vortex beams have helical wave front and their Poynting. Few of them are an astigmatic mode converter, computer generated We show that the propagation characteristics depend only on width of the host Gaussian Keywords: Optical vortices, Scattering, Random media, Speckles, Astigmatic.
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