Jon Berger

Earth Strain Measurements with a Laser Interferometer: An 800-meter Michelson interferometer monitors the earth's strain field on the surface of the ground.

The development of the laser as a source of coherent optical radiation has permitted the application of interferometric techniques to the problem of earth strain measurement. By use of this technology, an 800-meter laser strain meter has been developed which operates above the surface of the ground. The instrument has a strain least count of 10-10, requires no calibration, and has a flat and linear response from zero frequency to 1 megahertz. The linearity and large dynamic range of the laser strain meter offer unprecedented versatility in the recording of seismic strains associated with earthquakes and nuclear blasts. The extremely wide bandwidth opens new areas of the strain spectrum to investigation. A key to the understanding of the state of stress of the earth and the association phenomona of tectonic activity and earthquakes is a knowledge of the spatial distribution of the earth strain. Measurements of secular strain and earth tides indicate that, even at these long periods, surface strain measurements are valid representations of earth strain at depth. The LSM thus provides a means of making crustal strain measurements at points selected for maximum geophysical interest and ultimately allow the mapping of strain field distributions.

A Laser Earth Strain Meter

A laser interferometer for the study of earth strain is described. Changes in the length of an 800 m arm are measured by counting fringes in the interference pattern with a least count of 4×10−10. The output is linear and has a flat response from dc to 1 MHz and a dynamic range of 106. The laser wavelength is controlled by reference to a passive optic resonator contained in a stable environment. The wavelength stability is a few parts in 1010 for periods up to a laser lifetime (∼3000 h).

Application of Laser Techniques to Geodesy and Geophysics

Publisher Summary The chapter describes two distinct types of laser employed. The first type is used in the distance-measuring devices that employ a modulated light beam and in the interferometric strain meters, has a continuous low power output and is usually a He-Ne gas laser. The other type, used for time-of-flight measurements, such as the Lunar Ranging Experiment, is a pulsed laser of extremely high power and short pulse length. The chapter discusses terrestrial laser ranging devices (LRD) including electro-optic light modulators, LRD instrumental description, LRD accuracy, two-color techniques and description of two-color LRD. The chapter also discusses extraterrestrial laser ranging devices, lunar laser ranging experiment and laser ranging to artificial satellite. The applications of laser technology to geophysical instrumentation over the past decade resulted in significant improvements in the instrumental capabilities. The data from geodetic surveys using LRDs contribute to the understanding of the workings of the Sen Andreas fault system in particular and to the understanding of tectonic processes in general. The data combined with the measurements will have great bearing on the problem of earthquake prediction provides an opportunity to check the theories of continental drift and plate tectonics.

A deep ocean acoustic noise floor, 1–800 Hz

The ocean acoustic noise floor (observed when the overhead wind is low, ships are distant, and marine life silent) has been measured on an array extending up 987 m from 5048 m depth in the eastern North Pacific, in what is one of only a few recent measurements of the vertical noise distribution near the seafloor in the deep ocean. The floor is roughly independent of depth for 1-6 Hz, and the slope (∼ f-7) is consistent with Longuet-Higgins radiation from oppositely-directed surface waves. Above 6 Hz, the acoustic floor increases with frequency due to distant shipping before falling as ∼ f-2 from 40 to 800 Hz. The noise floor just above the seafloor is only about 5 dB greater than during the 1975 CHURCH OPAL experiment (50-200 Hz), even though these measurements are not subject to the same bathymetric blockage. The floor increases up the array by roughly 15 dB for 40-500 Hz. Immediately above the seafloor, the acoustic energy is concentrated in a narrow, horizontal beam that narrows as f-1 and has a beam width at 75 Hz that is less than the array resolution. The power in the beam falls more steeply with frequency than the omnidirectional spectrum.

Surveying infrasonic noise on oceanic islands

An essential step in the establishment of an international monitoring system (IMS) infrasound station is the site survey. The Provisional Technical Secretariat recommends infrasonic noise be recorded with meteorological data at four separate, promising, locations in the vicinity of the nominal station location to identify a secure site which would give the good signal‐to‐noise while offering easy access to power and telecommu‐nications. A good infrasound station should be sheltered from the prevailing winds by gentle topography, trees, and ground cover and located as far as possible from other natural and cultural noise sources. As the IMS network has been designed to give uniform global coverage, some of the stations will be located on oceanic islands. Noise surveys are being conducted at three locations in the Atlantic: Sao Miguel, Azores; Santiago, Cape Verde; and Ascension Island. These island locations are very important as they provide coverage in a region of great monitoring interest. However, they...

New Buoy Designs for Ocean Observatories

The Ocean Research Interactive Observatory Networks (ORION - www.orionprogram.org) is a federally funded program that focuses the science, technology, education and outreach of an emerging network of science driven ocean observing systems. The objective of this initiative is to provide the scientific community with ocean observatories around the globe that will provide the infrastructure necessary to collect, among other things, time series data of oceanographic and geophysical phenomena which can’t be collected from vessel expeditions alone. The program calls for placing buoys in all water depths and a full range of metocean domains, from calm equatorial sites to high latitude deepwater southern ocean locations. Conventional oceanographic buoys will not meet all the requirements of this program, so new buoy designs are being developed. The authors have developed a variation of the spar buoy design for the more robust requirements, and are currently looking into other options. This paper will review the more demanding criteria for the Orion buoys, and the proposed solutions for new buoy designs.

Optical fiber infrasound sensor arrays: Signal detection and characterization capabilities in the presence of wind noise

Optical fiber infrasound sensors (OFIS) are compliant tubes wrapped with two optical fibers that integrate pressure variation along the length of the tube with laser interferometry. The coherent signals traveling at acoustic speed are averaged along the length of the OFIS at the speed of light, while spatially incoherent noise traveling at wind speed is attenuated. We present new wind noise measurements, which augment initial work that suggested an OFIS has a lower noise floor above 1 Hz than a traditional microbarometer with a rosette pipe or hose filter in light wind. We also discuss OFIS configurations and techniques to determine the signal phase velocity direction. The instrument response is a function of the orientation of the OFIS relative to that of the wavefront. That property can be exploited to help determine the phase velocity of infrasound signals. This type of instrument‐response‐dependent beamforming is theoretically an improvement to standard beamforming for the same number of sensors when the signals are immersive or continuous in nature. When the signals are more impulsive in nature, the success of these techniques mostly depends on how successful the OFIS is in reducing wind noise and the signal bandwidth.

Global infrasound monitoring—Research issues

The International Monitoring System being installed to support monitoring compliance with the Comprehensive Nuclear Test Ban Treaty provides scientists with a unique opportunity for research. There are still a number of problems which limit the full exploitation of the system. These include limitations on signal‐to‐noise imposed by wind noise and the absence of well defined, internationally accepted calibration standards for sensors. But perhaps the major research challenges lie in the area of source characterization and definition. Most of the signals recorded at the few sites now operating come from unidentified sources. There has been some effort devoted to identifying local and regional sources but the unidentified category still exceeds 50% of all distinct events. There are a number of infrasound sources that occur naturally. These include volcanic eruptions, bolides, microbaroms, mountaintop/wind interactions, severe storms, and earthquakes. Manmade sources include most any energetic activity. After...