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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.