Optical fiber infrasound sensor

Details Optical fiber infrasound sensor Optical fiber infrasound sensor

1999-04-30

2004-09-07

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

Using fiber or waveguide interferometer

C385S012000, C385S013000

Reexamination Certificate

active

06788417

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally pertains to methods for the sensing of, and sensors for the detection of, very low frequency pressure waves, particularly in the atmosphere as infrasound.
The present invention particularly concerns fiber optic pressure sensors, and the arrangement of fiber optic pressure sensors as an interferometer in order to detect infrasound with common mode rejection of (i) temperature and (ii) strain or vibration noise, and with a high signal-to-noise ratio of (iii) detected infrasound versus wind noise.
2. Background of the Invention
2.1 Infrasound
Infrasound signals are very low frequency (0.01 to 10 Hz) pressure waves that travel through the atmosphere. They have been detected using conventional pressure gauges; usually electro-mechanical barometers which measure pressure at a point.
Noise from wind makes sensing these signals difficult. Indeed, the key weakness in existing infrasound detection systems is the rapid deterioration in the signal-to-noise ratio with increasing wind velocity. To increase sensitivity researchers have attached conventional pressure gauges to long tubes perforated with variously distributed holes in attempts to average out the noise from the wind. Propagation effects in these mechanical filters limit their effectiveness.
Studies in infrasound detection often center on techniques of noise reduction. Almost all work to date consists or recording pressure at a single point while an attempt is made to force the pressure at that point to be representative of the pressure averaged along a line or over an area. As previously stated, a series of perforated pipes or permeable hoses are typically connected to a microphonic sensor. Pressure noise along the pipe's length is partially incoherent while, for pipe lengths less than the wavelength of interest (typically a few hundred meters), the signal is coherent. The aim of the mechanical filter is to add pressure variations at discrete inlets along its length so that the incoherent noise will average away while the signal is enhanced. Conceptually, one can envision the ideal case or many sensors in an array separately recorded and their signals added together electronically.
Prior art observations of infrasonic noise versus wind speed through and in a system or about 0.5 Hz bandwidth will be shown in graph form in
FIG. 1
of this specification. Reference Clauter & Blandford, 1996. A prior art observed spectrum of infrasonic noise under calm wind conditions at Warrarmunga, Central Australia from Christie, et al., 1978, will be shown in
FIG. 2
of this specification.
Significant efforts have gone into the designs of prior art mechanical filters. One of the more venerable devices is the Daniels filter (Daniels, 1959), which relics on pipes of varying diameter to create a series or acoustic impedance changes with the hope of reducing acoustic reflections in the pipe. Burridge (1971) analyzed this type of “pipe-microbarograph” and similar configurations. When added acoustically in the filter pipe, there exists a phase delay for each clement caused by the finite speed of sound. Burridge modeled pipes with varying (i) dimensions, (ii) numbers of inlets, and (iii) acoustic impedances to find the best compromise response flattening and attenuation minimization. In all cases, however, the response clearly is a compromise. The difficulty becomes greater as the frequency increases. As Burridge showed, flat response above 0.1 or 1 Hz are not attainable.
2.2 Infrasound Detection as Part of the Comprehensive Test Ban Treaty (CTBT)
As part of the international monitoring system of the Comprehensive Test Ban Treaty (CTBT), infrasound signals in the band 0.02 to 4 Hz must be detected in the presence of ambient noise generated chiefly by wind. Thus effectiveness of acoustic filters employed in standard infrasound sensors is limited by pressure propagation and attenuation characteristics within the filter. To improve the filtering characteristics, an optical fiber for sensing the integrated pressure variations along a line has been designed. The optic fiber sensor can easily average over kilometer-scale lengths of arbitrary geometry with an averaging bandwidth governed by the speed of light and thus should offer significant practical advantages in reducing the effect of wind noise and thus increasing the signal-to-noise ratio over a wide bandwidth.
2.2.1 Specific Previous Infrasound Detection
Infrasonic monitoring is an effective, low cost technology for detecting atmospheric explosions of nuclear weapons. The low frequency components of explosion signals propagate to long ranges (a few thousand kilometers) where they can be detected with arrays of infrasound sensors.
A prototype infrasound system for use under a comprehensive test ban treaty has been constructed by the United States. The system is near real time, automated and unattended.
The United States Infrasound Sensor System Prototype is consistent with a specification in a Preparatory Commission document prepared under the Comprehensive Test Ban Treaty (CTBT), The system is a four-element array in a triangular layout with an infrasound sensor element at each corner and one in the center. The prototype infrasound sensor element spacing is 1 km, however, the specifications provide for an optional spacing up to 3 km. All prototype components are exportable and operable over a wide range of environmental conditions.
System security is provided by housing the array element hardware (sensor, digitizer, authenticator, etc.) in a secure enclosure. The enclosure is buried in the ground and is protected by active (switch closure) and passive tamper detection devices.
The objectives of the United States Infrasound Sensor System Prototype are to reliably acquire and transmit near-real-time infrasonic data to facilitate the rapid location and identification of atmospheric events. The prototype system is also directed to providing documentation that could be used by the United States and foreign countries to procure infrasound systems commercially to fulfill their CTBT responsibilities.
Detail requirements for infrasound monitoring set by The Conference on Disarmament and the CTBT Preparatory Commission (PrepCom) are as follows.
A wideband microbarograph, or equivalent such as the instrument of the present invention, should exhibit a flat frequency response from 0.02 to 4.0 Hz.
An array of four such elements, with a sensor spacing from 1 to 3 km is typical. The Conference on Disarmament and the CTBT Preparatory Commission (PrepCom) recommended an equilateral triangular array, 1 to 3 km on a side. An array element is located at each corner and at the center.
Sensor noise should be at least 18 dB below the minimum acoustic noise of 5.0 mPa at 1.0 Hz.
Sensors should include acoustic filtering of wind noise. In previous sensors this is realized with noise reduction pipes.
Resolution should be better than 1 count per mPa.
Dynamic range should be at least 108 dB.
The sensor array would usefully provide a data stream at a sample rate of about 10 samples per second (sps). Data from all array elements would desirably be authenticated.
An exemplary prior art microbaragraph (above) is a 10″ diameter Chaparral Physics model 4.11. The vendor re-packaged the sensor to accommodate the above data survey features. The infrasound system includes four array elements, intra-site communications, and a host receiving station.
The components that make up the infrasound system also include an array of infrasound detector elements each containing a sensor, a digitizer with GPS, and a data authenticator. A host receiving station contains a multiplexer, data displays and state-of-health displays. The host receiving station also transmits the data to the NDC in near real time.
The array element hardware (sensor, digitizer, authenticator, etc.) is housed in an enclosure (a utility box) to provide system security. The enclosure is buried in the ground and is protected by tamper detection devices such as a switch closure. This l

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