Search

Land-surface subsidence is a major contributor to land loss in many river deltas. New approaches yielding high-resolution data are needed to parse the relevant driving forces. In 2016, we established a novel “subsidence superstation” ∼2 km from the Mississippi River in coastal Louisiana (USA) to measure compaction in a global reference frame as a function of depth in Holocene sediments and deeper subsidence. The site features three borehole optical fiber strainmeters to obtain continuous records of displacement between ∼1.3 m below the surface and depths of ∼11, 25, and 38 m. These data are complemented by an adjacent station providing hydrologic data and near-surface compaction. We also installed three GPS antennas, one of which is mounted to a rod cemented into the Pleistocene basement. A core from one of the boreholes provides insight into the sediment properties of the entire Holocene succession. Five years of records reveal the compaction rate in the material between 1.3 and 38 m is likely less than 0.25 mm/yr. The GPS records yield a subsidence rate of 2.5 mm/yr regardless of the depth of the anchor, thus corroborating the low compaction rates observed by the strainmeters. The new instrumental records show that current subsidence at this location is governed mostly by deformation of the Pleistocene or underlying strata rather than compaction of Holocene material, with the exception of the uppermost meter. The methodology represents an important new approach to mapping subsidence rates at varying depths, providing insight into the mechanisms governing delta subsidence.

The methods and tools available to make geodetic measurements offshore are still limited but increasingly important for studies of underwater subduction zones, volcanoes, and more. One method is to use ocean bottom pressure sensors, which can be used to measure vertical seafloor motion. While most pressure sensors inherently drift at rates that can exceed tectonic signals, some drift correction methods have been developed.Self-calibrating pressure recorders (SCPR) correct sensor drift using a piston-gauge calibrator (PGC) that produces a reference pressure whose value can be determined with a high level of accuracy. Changes in the reference pressure observed by the pressure sensors are attributed to sensor drift and are removed from the seafloor pressure. The resulting drift-free pressure record can be used to infer seafloor height. We successfully demonstrated an SCPR at Axial Seamount between 2013 and 2015, and later connected it to real-time cabled infrastructure in 2018.We also used an SCPR in campaign-style surveys to make absolute pressure measurements on seafloor benchmarks. In this absolute SCPR (ASCPR) we make accurate determinations of the parameters that affect the reference pressure to determine its absolute value. We use the true, absolute reference pressure and the difference between the observed reference and seafloor pressures to determine the absolute seafloor pressure. The measurements provide instrument-independent, fiducial values for calibrating nearby sensors or as individual points in long-term time series. We developed the campaign calibrated pressure measurements and conducted four surveys between 2014 and 2017 in the Cascadia subduction zone to measure the vertical deformation occurring offshore. The results serve as initial reference values that will be useful far into the future for marine geodesy and future surveys could be used to estimate the secular vertical deformation rate.I investigated additional methods, models, and avenues relat

Under the auspices of the International Association for Physical Sciences of the Ocean (IAPSO) a committee was established to identify the oceanographic and geophysical requirements for fixing Tide Gauge Bench Maries (TGBM's) in an absolute terrestrial coordinate system; to evaluate the technology for fixing TGBM's; and to malce recommendations to the Commission on Mean Sea Level and Tides (IAPSO) of a strategy for coordinated global fixing ofTGBM's and for making the results centrally·available. To meet these goals, the committee met for a several day session at the Woods Hole Oceanographic Institution in November, 1988. From this workshop came a series of technical conclusions, plus specific recommendations to achieve the goals of the committee. Included in these discussions were Very Long Baseline Interferometry, Satellite Laser Ranging, Lunar Laser Ranging, Global Positioning System, and Absolute Gravity Meters, as well as mechanisms for logging and distributing the results from these systems, perhaps via the Permanent Service for Mean Sea Level (PSMSL) in Britain., Funding was provided by the International Association for Physical Sciences of the Ocean (IAPSO), the National Oceanic and Atmospheric Administration (NOAA), and the Sea Grant Program through the Woods Hole Oceanographic Institution.

An optical fiber infrasound sensor (OFIS) is a long compliant tube wrapped with two optical fibers that integrate pressure variation along the length of the tube at the speed of light via laser interferometry. Here we only present the initial results of research conducted under this contract due to delays in the construction of the Camp Elliott OFIS array. All the field work has been completed, but the remainder of the data analysis will be completed in the next two months and presented in the associated poster. In this paper, we show that M-sequences transmitted via an array of 18-inch subwoofers comprise a portable infrasound sensor calibrator, which also permits in situ evaluation of wind noise reduction filters. For example, a comparison of a 60-m OFIS with a Bruel & Kjaer (B&K) microphone at a signal frequency of 50 Hz for a source ~240 m away shows that the OFIS reduced wind and other types of undesirable noise by 18 dB over the B&K microphone with a standard sponge wind screen. An 8-subwoofer array at a range of ~240 m yields a reliable calibration signal that smoothly decreases in power by a rate of 2.5 dB/Hz with decreasing frequency to 8 Hz, which is fairly remarkable given the 20 Hz rolloff frequency of the amplifier and speakers., Presented at Proceedings of the Monitoring Research Review (30th): Ground-Based Nuclear Explosion Monitoring Technologies, held at Portsmouth, VA on 23-25 Sep 2008. Published in Proceedings of the Monitoring Research Review (30th): Ground-Based Nuclear Explosion Monitoring Technologies, p902-911, 2008. The original document contains color images. Proposal no. BAA06-41.

Optical fiber infrasound sensors (OFIS) are long compliant tubes wrapped with two optical fibers that integrate pressure variation along the length of the tubes via laser interferometry at the speed of light. We have previously shown that several OFIS arms can resolve the back azimuth and elevation angle of infrasound signals due to the spectral fingerprint of the instrument response in the recorded signal and the time separation provided by separating the centers of each OFIS arm. In this paper, we present the recent results of calibration, coherence, wind-noise reduction, and fine-tuning experiments that we accomplished in route to completing our grant objectives. Specifically, we show that the sensitivity of the OFIS is temperature dependent, and develop a real-time calibration system that allows us to compensate for this variation. We also show that M-sequences transmitted via subwoofers can be used as portable calibration tools in the low-frequency audible range and can test wind noise reduction technologies for some sensors. Comparison of a 60-m OFIS with a B&K microphone at a signal frequency of 50 Hz for a source tilde 300 m away shows that the OFIS reduces wind and other types of undesirable noise by 12 dB over the B&K microphone with an attached sponge wind filter. We also present the layout of a six-arm OFIS array at Camp Elliott (CEL) in north San Diego county where our investigations are continuing. This array has an aperture of 150 m, and is collocated with a digital video camera that will provide confirmation of the back azimuth and elevation angle of aircraft infrasound signals that are tracked by a passive infrasonic radar system., Presented at the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies held in Denver, Colorado on 25-27 Sep 2007. Published in the Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, p884-893, 2007. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). The original document contains color images.

The detection of infrasound in the presence of wind is challenging. Increasing wind speeds lead to higher noise floors. While the exact nature of the noise is still a subject of investigation, several technologies and array configurations have been developed to maximize the signal to noise ratio and lower the detection threshold. Optical fiber infrasound sensors (OFIS) are long compliant tubes wrapped with two optical fibers that integrate pressure change along the length of the tube with laser interferometery. Spatially incoherent wind noise is naturally attenuated relative to spatially coherent infrasound. Because the signal pressure variation is integrated along the length of the tube, the instrument response is a function of the orientation of the OFIS arms relative to the orientation of the wavefront. We show with theory and real infrasound data recorded at Pinon Flat Observatory in southern California that this spectral property can be exploited with multiple OFIS arms in different ways to determine the phase velocity of infrasound signals. Based on these findings and unpublished results, we propose two OFIS configurations as alternatives to rosette pipe array infrasound stations., Presented at the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies held in Denver, Colorado on 25-27 Sep 2007. Published in the Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, p894-904, 2007. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). The original document contains color images.

We have designed a new type of infrasonic sensor using optical fibers as distributed sensing elements. The design addresses the limitations of mechanical spatial filters used to average wind-generated, turbulent pressure fluctuations. We have built two styles of prototype. In the first design of an OFIS (optical fiber infrasound sensor), we rely on the change in optical path that results from an optical fibers strain-optic coefficient (i.e., the index of refraction is pressure dependent). In the second design, the optical fiber is attached to and strained by a compliant tube, which deforms under pressure. In both types, the optical fiber path length is monitored interferometrically. The second type has proven to have a better signal-to-noise ratio and is currently the primary focus of our developments. These devices lend themselves to deployment in long, continuous lines, effectively integrating the pressure variations along their lengths. The response over the length of the sensor is governed by the speed of light rather than the speed of sound (as is the case for conventional infrasound filters). Since our sensors can readily be made kilometers long and deployed as spatial filters in arbitrary geometries, they potentially are a significant improvement over conventional systems., Proceedings of the 22nd Annual DoD/DOE Seismic Research Symposium: Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on 13-15 September 2000. U.S. Government or Federal Rights. The original document contains color images.

The infrasound element of the global International Monitoring System (IMS) will consist of 60 stations. Each station in the global network will consist of an array of sensors. As the local surface wind strongly influences ambient infrasonic noise levels in the band of interest to the nuclear treaty monitoring community (between 0.05 and 4 Hz), each of these sensors will include a spatial filter designed to reduce the effects of wind noise. There are, however, many different design suggestions for spatial filtering system. In order to maximize the signal-to-noise ratio of signals of interest, it is essential that the spatial noise reduction systems be optimized to provide the most significant noise power reduction over the broadest band possible. We believe that substantial progress in these areas requires the comparative testing of the major noise reducing systems exposed to the same meteorological conditions. Under our new DTRA contract we will construct and test several spatial filtering systems by simultaneous operation at the Pinon Flat Observatory (PFO) in southern California. These systems include the wind fence, microporous pipes, and a recent design in which a large number of open ports are connected to a summing manifold, and then to the microbarometer, by solid pipes. We will also compare these mechanical noise suppression schemes with a new design based on light-speed pressure averaging via optical fibres. The observatory is an ideal test site as it is subjected to winds that come from all azimuths at speeds ranging from 0 to over 10 m/s., Proceedings of the 22nd Annual DoD/DOE Seismic Research Symposium: Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on September 13-15, 2000, U.S. Government or Federal Rights.. The original document contains color images.

Gravity can be measured on the sea surface and on the seafloor. Surface gravity suffers from lack of resolution over deep seas, because the perturbing source masses are far from the meter. Bottom measurements recover this resolution, but suffer from poor coverage because of the time needed for individual measurements. Our proposed solution to this problem is a towed deep-ocean gravity meter (TOWDOG), which combines the rapid data collection of shipboard measurements with resolution approaching ocean bottom measurements. The near-bottom gravity measurements have the advantages of both existing techniques while avoiding the problems inherent in each one. While gravity methods have been used on land for several decades to further our understanding of earth structure, we are just beginning to bring the power of gravity field quantification to constructing detailed models of the crust underlying the ocean. Our work focuses on the development of gravity measuring and interpretation tools that can be applied to undersea structures such as seafloor volcanoes and mid-ocean ridges. Rather than relying on the standard marine gravimetric methods, we wish to lengthen the list of tools available to the marine geophysicist for applying gravity measurements to understanding the makeup of the crust beneath the oceans. Gravity can be measured on the sea surface and on the seafloor. Surface gravity suffers from lack of resolution over deep seas, because the perturbing source masses are far from the meter. Bottom measurements recover this resolution, but suffer from poor coverage because of the time needed for individual measurements. Our proposed solution to this problem is a towed deep-ocean gravity meter (TOWDOG), which combines the rapid data collection of shipboard measurements with resolution approaching ocean bottom measurements.

An atomic wavelength standard has been acquired and a single-mode optical fiber network fabricated for the Pinon Flat Geophysical Observatory. These enhancements have greatly increased the precision of this unique facility.

The Institute of Geophysics & Planetary Physics (IGPP) has completed a two-phase three-year program of absolute gravity research and development. The development phase included modifications to an existing absolute gravity meter. Our goal was to incorporate state-of-the-art technology and portability while maintaining meter accuracy. The research phase included both field testing of the modified meter and the establishment of new absolute gravity sites. In particular, we were able to survey absolutely a series of closely spaced sites along existing calibration lines. Keywords: Absolute gravity meters; Gravity; Interferometers; Acceleration; Falling bodies; California; Free fall retroreflectors.