Search

In this study, geodetic and seismic phenomena occurring in the subduction zone were observed with two tiltmeters that were installed in seafloor boreholes drilled in the Kumano Basin in the Nankai Trough, southwest Japan. We used one electrolytic-type tiltmeter and one pendulum-type tiltmeter installed 6 and 19 m below the seafloor, respectively. The two tiltmeters have been continuously operated since 2019 and 2021, respectively. The records of the two tiltmeters showed microseisms, seismic signals of normal earthquakes, and variations that correlated well with the M2 tide (tidal response). The noise environment relative to existing seafloor observatories was assessed by comparing the power spectral densities of the tiltmeters and broadband seismometers in the seismic frequency band. The two tiltmeters and the seismometers showed similar spectral peaks in the microseism band, and at lower frequencies below 10–2 Hz, the pendulum tiltmeter had a noise level that was up to 20 dB lower than that of the broadband seismometers. The tidal responses of the tiltmeters were analyzed to reveal corresponding mechanisms because a large amplitude of the tidal response makes it difficult to detect geodetic phenomena. Considerable azimuthal dependence was observed in the NW–SE direction for both tiltmeters. The principal direction was approximately the same as the down-slope direction of the local bathymetry. This tendency was thus interpreted to be caused by the differences in the thickness of the sedimentary layer along the direction of the slope. Furthermore, the validity of the tilt measurements for the two tiltmeters was demonstrated by in-situ loading experiments, where the theoretical response of an elastic half-space medium was computed and compared with the experimental results. The results of the loading experiments can be explained using theoretical values with a set of realistic elastic moduli. [ABSTRACT FROM AUTHOR]

Due to the blockage of seawater, seafloor displacement cannot be directly measured by space geodesy. The combination of Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) has been used to overcome the electromagnetic barrier, so that a GNSS‐determined sea surface vessel's coordinates can be transformed to seafloor benchmarks in a global reference frame. Due to the high cost and science priorities, previous GNSS‐A studies mainly targeted relatively deep water and a minimum of three transponders were used to form an array, equivalent to a precision geodetic station. With recent developments in unmanned autonomous surface vessels, low cost GNSS‐A surveys are poised to become practical. Here we demonstrate that with a carefully designed surveying trajectory, Wave Glider‐based GNSS‐A surveying of a single transponder in shallow water can provide centimeter‐level accuracy on horizontal seafloor positioning, even if the sound speed model deviates from the actual value by a few meters per second. Results from a nine‐month experiment conducted at ∼54 m water depth show that the repeatability of the seafloor horizontal positioning is better than 2 cm. When conditions allow, the acoustic observations should be collected symmetrically about the transponder and data redundancies are recommended to reduce the error associated with time‐dependent variations in sound speed. Plain Language Summary: Seafloor geodesy is important for monitoring offshore strain processes. Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) uses sound signals traveling in water as rulers to measure the distances between the sea surface vessel and the seafloor sensor (transponder), so that the GNSS‐determined sea surface vessel's coordinates can be transformed to the seafloor through triangulation. Multiple transponders deployed on the seafloor are often needed to form an array so that noise caused by irregular variations in the sound signals can be reduced. In shallow water, due to the relatively short distance, the range distortion caused by a sound signal's refraction is smaller, and the time it takes to conduct each complete cycle of survey is shorter. These conditions allow the seafloor displacements to be measured with centimeter‐level accuracy using only one transponder. Key Points: For single transponder Global Navigation Satellite Systems‐acoustic ranging in shallow water, seafloor vertical position estimation is sensitive to sound speed error and variationA geometrically symmetric surveying trajectory is preferable for error reduction in horizontal seafloor positioningSeafloor horizontal positions were measured with <2 cm repeatability at ∼54 m water depth over five repeat surveys with Wave Gliders [ABSTRACT FROM AUTHOR]

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation. Plain Language Summary: The Cascadia subduction zone has produced large earthquakes and tsunamis whose potential size and interval is affected by the amount and distribution of locking between the tectonic plates. A large portion of the subduction zone is offshore, where typical land‐ and satellite‐based methods are ineffective at measuring crustal changes. Seafloor water pressure observations can be used to measure height changes, but pressure gauges inherently drift at rates typically greater than the expected vertical seafloor deformation rates. The absolute self‐calibrating pressure recorder (ASCPR) measures the true, absolute, sensor‐independent seafloor pressure by addressing and correcting sources of error caused by the internal piston gauge calibrator and recording pressure gauges. The accuracy of our measurements is about 25 ppm of the water depth, or about 2.5 cm of height per 1,000 m of water. Campaign survey‐style measurements using the ASCPR at seven benchmarks off the coast of Newport, Oregon from 2014 to 2017 establish a long‐term record of absolute measurements that can be referenced by studies decades or more in the future or can estimate and correct drift of nearby continuous pressure gauges. Continued measurements can provide insights on seafloor deformation and thus locking in Cascadia. Key Points: Campaign‐style surveys of absolute calibrated seafloor pressure measurements were made in the Cascadia subduction zone from 2014 to 2017These sensor‐independent measurements act as long‐term, absolute reference values that can be used for future vertical deformation studiesWe document and quantify the sources of error in the technique with a total uncertainty of ∼25 ppm, or ∼0.25 kPa per 1,000 m water depth [ABSTRACT FROM AUTHOR]

Axial Seamount is a seafloor volcano with frequent eruptions and periodic cycles of inflation and deflation. Seafloor pressure gauges monitor vertical deformation with time, but inherent instrumental drift complicates and biases geodetic interpretation. A drift corrected pressure recorder was deployed on Axial Seamount on 6 July 2018, at coordinates 45° 57.29′ North latitude, −130° 0.56′ East longitude, depth 1,535 m. This system includes two independent quartz‐resonant pressure gauges, which nearly continuously observe the seafloor pressure. At regular intervals, the gauges are calibrated in situ with a modified deadweight tester at a pressure within 98% of the nominal seafloor pressure. Using the calibration data, the drift of each gauge has been modeled as a simple linear plus decaying exponential function of time. The two estimated linear sensor drift rates are 0.45 ± 0.12 and 0.36 ± 0.08 kPa/year; the modeled sensor drift represents a significant error if uncorrected. The standard deviations of the drift model residuals are of order 0.06 kPa or 6 mm depth equivalent. Once calibrated, the difference between the two seafloor pressure timeseries exhibits a RMS deviation of ±6 mm at the 90% confidence limit and a linear trend less than 1 mm/year. A time series from July 2018 to December 2021 tracks the inflation of Axial Seamount with differing inflation rates over different time intervals. Plain Language Summary: The seafloor undergoes movements due to earthquakes and volcanoes, but seafloor motions are difficult to measure with conventional methods. One approach is to use pressure gauges to monitor changes in seafloor depth, but unavoidable gauge drift complicates the interpretation. This paper describes an instrument that can measure and remove gauge drift, providing a precise and unbiased measurement. This instrument also uses the power and data infrastructure of a seafloor instrumentation network, which allows for longer measurements and continuous monitoring. Axial Seamount is west of Oregon and undergoes repeated cycles of uplift due to magma injection, followed by submarine volcanic eruptions and deflation; a new cycle of magma injection and uplift begins anew. This instrument can be used, along with other sensors, to monitor the pattern and rates of deformation. Key Points: A pressure logging instrument with in situ sensor drift correction was deployed on Axial SeamountA 42‐month pressure gauge time series, converted to equivalent vertical displacement, demonstrates internal consistency at the sub‐cm levelThe drift corrected uplift time series is measuring uplift of the Axial volcanic system [ABSTRACT FROM AUTHOR]

Optical fibers, stretched between anchors attached to the Earth and buried in boreholes and shallow trenches, offer an inexpensive and robust means to observe Earth strain. We have deployed several optical fiber strainmeters (OFSs) in boreholes, where thermal stability helps mitigate the noise from temperature changes. We have also constructed horizontal OFSs in 1‐m‐deep trenches of lengths 221 and 174 m. In these, a second optical fiber having a contrasting temperature coefficient provides a means to separate true strain from thermal noise. Signals in the seismic band from the horizontal strainmeters agree well with records from collocated seismometers, and the predominant ambient noise recorded by both techniques are consistent. Solid Earth strain tides with amplitude of 30 nε (nanostrain) are observed in these horizontal OFSs that agree well with those observed by collocated vacuum laser strainmeters. Noise at periods of a few days amounts to 50 nε. The two horizontal strainmeters show drift rates of up to 1 µε per month, and occasional false signals (evident from comparisons with the vacuum laser strainmeters) can accumulate occasionally to the order of 50 nε over times from hours to weeks. Results to date indicate that this technology on land could detect slow‐slip events of amplitude 100 nε and higher. Key Points: Several styles of long‐baseline optical fiber strainmeters have been deployedThe instruments are useful for the study of slow slip events, fluid injection, earth tides, and seismic signals, including for earthquake early warning [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: It is important to learn about the rate and causes of land subsidence along low‐lying coastlines and deltas where a large fraction of the world's population lives. In the Mississippi Delta, sediment deposited during the last ∼11,000 years (Holocene) has gradually compacted under its own weight. Constraining the path of the Mississippi River with levees over the past century has prevented deposition of new sediment over most of the delta, causing the land surface to drop with respect to sea level, increasing the threat from inundation by seawater. We measured the compaction of a nearly 40 m thick sediment layer at one site in the Mississippi Delta using a combination of optical fiber strainmeters in boreholes to several depths and GPS antennas mounted near the land surface and to deeper points. We found that the subsidence at our site, which occurs at a rate of several mm per year, is caused by very shallow (of order 1 m) and very deep (below 40 m) processes and is not driven by compaction of the bulk of the 38 m thick Holocene layer. Key Points: First integration of multiple coastal subsidence monitoring techniques at one siteHigh‐resolution records from co‐located optical fiber strainmeters and GPS instruments corroborate one anotherThe site exhibits isostatic subsidence with a potential role of fluid extraction and negligible compaction [ABSTRACT FROM AUTHOR]

Conventional tide gauges are usually housed along the coast. Satellite altimetry works well in the open ocean but poorly near the coast due to signal contamination by land returns. These limitations lead to an observational gap in the transition zone between the coast and open ocean. Using data collected by a GPS installed on top of an anchored spar‐buoy in Tampa Bay, we retrieved water levels through a combination of precise positioning and interferometric reflectometry. Individual water level retrievals agree with a nearby acoustic tide gauge (19.5 km distance) at ∼15 cm level. Amplitude and phase of the major tidal constituents are well recovered by the GPS spar‐buoy measurements. Over a 2.9‐year period, agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the nearby acoustic tide gauge is 4.4 cm. When sea level data measured by the GPS spar‐buoy are included in the local coastal ocean circulation model, low‐frequency error propagated from the open boundary is significantly reduced. Plain Language Summary: GPS receivers record direct signals from satellites as well as reflected signals from local objects. The reflected signal can interfere with the direct signal, enhancing or reducing overall signal strength. This characteristic can be used to measure the height difference between the GPS antenna and the reflecting surface. We used GPS data collected by a spar‐buoy anchored in Tampa Bay to calculate water levels at different times. The calculated water levels can be used to study sea level change, ocean circulation, and tidal height predictions. Key Points: An anchored spar‐buoy seafloor geodetic system is used to measure offshore sea levels based on Global Positioning System interferometric reflectometry (GPS‐IR)Agreement of de‐tided daily mean sea levels measured by the GPS spar‐buoy and the closest acoustic tide gauge (19.5 km away) is 4.4 cmSea levels measured with the GPS spar‐buoy can help improve coastal ocean circulation models [ABSTRACT FROM AUTHOR]

We present seafloor pressure records from the Cascadia Subduction Zone, alongside oceanographic and geophysical models, to evaluate the spatial uniformity of bottom pressure and optimize the geometry of sensor networks for resolving offshore slow‐slip transients. Seafloor pressure records from 2011 to 2015 show that signal amplitudes are depth‐dependent, with tidally filtered and detrended root‐mean‐squares of <2 cm on the abyssal plain and >6 cm on the continental shelf. This is consistent with bottom pressure predictions from circulation models and comparable to deformation amplitudes from offshore slow slip observed in other subduction zones. We show that the oceanographic component of seafloor pressure can be reduced to ≤1‐cm root‐mean‐square by differencing against a reference record from a similar depth, under restrictions that vary with depth. Instruments at 100–250 m require depths matched within 10 m at separations of <100 km, while locations deeper than 1,400 m are broadly comparable over separations of at least 300 km. Despite the significant noise reduction from this method, no slow slip was identified in the dataset, possibly due to poor spatiotemporal instrument coverage, nonideal deployment geometry, and limited depth‐matched instruments. We use forward predictions of deformation from elastic half‐space models and hindcast pressure from circulation models to generate synthetic slow‐slip observational records and show that a range of slip scenarios produce resolvable signals under depth‐matched differencing. For future detection of offshore slow slip in Cascadia, we recommend a geometry in which instruments are deployed along isobaths to optimize corrections for oceanographic signals. Plain Language Summary: Slow‐slip earthquakes, a special class of earthquake in which slip along a fault occurs over periods of days to years without producing shaking, have increasingly been found to occur in the subsea segments of subduction zones worldwide. There is evidence that these offshore slow‐slip earthquakes can occur within the same region where large, damaging earthquakes are generated and that they may precede and potentially trigger these events. Offshore slow‐slip earthquakes cause the seafloor to move vertically by up to a few centimeters, which can be detected by measuring the pressure on the seafloor if the effects of ocean tides and circulation can be corrected for. In this study, we search for evidence of slow‐slip earthquakes in seafloor pressure data from the Cascadia Subduction Zone, off the U.S. and Canadian west coast, and assess how to best reduce oceanographic signals in these data in order to reliably observe slow‐slip deformation. No evidence for these events is seen in Cascadia pressure data from 2011 to 2015. Using a combination of observational data and model simulations, we show that oceanographic signals can be largely eliminated from seafloor pressure measurements if instruments are placed in rows of constant depth. A network of 22 instruments would be sufficient to monitor for large offshore slow‐slip earthquakes offshore central Oregon, but smaller events would be difficult to detect with this method. By increasing our ability to detect these slow‐slip earthquakes, we can better understand subduction zone processes and their relation to large, damaging megathrust earthquakes. Key Points: In Cascadia, differencing tidally filtered, depth‐matched seafloor pressure records can reduce oceanographic noise to less than 1‐cm RMSModels of offshore SSEs in Cascadia suggest that a network of depth‐matched sensors at 30‐km spacing can detect Mw 6.4 and larger eventsNo offshore slow‐slip earthquakes are detected in the available Cascadia seafloor pressure data from 2011 to 2015 [ABSTRACT FROM AUTHOR]

Abstract: We monitored the length of an optical fiber cable stretched between two seafloor anchors separated by 200 m at a depth of 1900 m, 90 km west of Newport, OR, near the toe of the accretionary prism of the Cascadia subduction zone. We continuously recorded length changes using an equal arm Michelson interferometer formed by the sensing cable fiber and a mandrel‐wound reference fiber. A second, nearly identical fiber interferometer (sharing the same cable and housing), differing only in its fiber's temperature coefficient, was recorded simultaneously, allowing the separation of optical path length change due to temperature from that due to strain. Data were collected for 100 days following deployment on 18 October 2015, and showed an overall strain (length change) of −10.7 με (shorter by 2.14 mm). At seismic periods, the sensitivity was a few nε; at tidal periods the noise level was a few tens of nε. The RMS variation after removal of a −79 nε/day drift over the final 30 days was 36 nε. No strain transients were observed. An unexpected response to the varying hydrostatic load from ocean tides was observed with a coefficient of −101 nε per meter of ocean tide height. [ABSTRACT FROM AUTHOR]

Fiber-based interferometers provide the means to sense very small displacements over long baselines, and have the advantage of being nearly completely passive in their operation, making them particularly well suited for geophysical applications. A new 250 m, interferometric vertical borehole strainmeter has been developed based completely on passive optical components. Details of the design and deployment at the Piñon Flat Observatory are presented. Power spectra show an intertidal noise level of -130 dB (re. 1 ε²/Hz), consistent within 1-3 dB between redundant components. Examination of its response to Earth tides and earthquakes relative to the areal strain recorded by an orthogonal pair of collocated, 730 m horizontal laser strainmeters yield a Poisson's ratio for local near surface material of 0.25 that is consistent with previous results. [ABSTRACT FROM AUTHOR]

One method to detect vertical crustal deformation of the seafloor, where Global Positioning System (GPS) surveys are not possible, is to monitor changes in the ambient seawater pressure, whose value is governed primarily by depth. Modern pressure sensors based on quartz strain gauge technology can detect the pressure shift associated with subsidence or uplift of the seafloor by as little as 1 cm. Such signals can be caused by tectonic or volcanic activity, or by hydrocarbon production from an offshore reservoir. However, most gauges undergo a slow drift having unpredictable sign and magnitude, which can be misinterpreted as real seafloor height change. To circumvent this problem, we have developed an instrument that calibrates the pressure gauges in place on the seafloor. In this autonomous system, a pair of quartz pressure gauges recording ambient seawater pressure are periodically connected to a piston gauge calibrator. In a 104 day test off the California coast at 664-m depth, the contribution to the uncertainty in depth variation from gauge drift was 1.3 cm based on calibrations occurring for 20 min every ten days. [ABSTRACT FROM AUTHOR]

Wind noise reduction (WNR) is important in the measurement of infrasound. Spatial averaging theory led to the development of rosette pipe arrays. The efficacy of rosettes decreases with increasing wind speed and only provides a maximum of ∼20 dB WNR due to a maximum size limitation. An Optical Fiber Infrasound Sensor (OFIS) reduces wind noise by instantaneously averaging infrasound along the sensor's length. In this study two experiments quantify the WNR achieved by rosettes and OFISs of various sizes and configurations. Specifically, it is shown that the WNR for a circular OFIS 18 m in diameter is the same as a collocated 32-inlet pipe array of the same diameter. However, linear OFISs ranging in length from 30 to 270 m provide a WNR of up to ∼30 dB in winds up to 5 m/s. The measured WNR is a logarithmic function of the OFIS length and depends on the orientation of the OFIS with respect to wind direction. OFISs oriented parallel to the wind direction achieve ∼4 dB greater WNR than those oriented perpendicular to the wind. Analytical models for the rosette and OFIS are developed that predict the general observed relationships between wind noise reduction, frequency, and wind speed. [ABSTRACT FROM AUTHOR]

The article discusses a study which examined the importance of linear deformation mechanisms in ice-sheet flow using observations and modeling of Siple Dome, West Antarctica. The crossover stress, which refers to the threshold value of the effective deviatoric stress below which nonlinear flow mechanisms are dominated by linear flow mechanisms, by combining ice property measurements with in situ deformation rate measurements and a finite-element ice flow model. Findings revealed that the best match between predicted and observed deformation rates is produced by a crossover stress of 0.18 bar. In addition, crystal fabric has influenced the pattern of flow near the divide at Siple Dome.

Infrasound arrays typically consist of several microbarometers separated by distances that provide predictable signal time separations, forming the basis for processing techniques that estimate the phase velocity direction. The directional resolution depends on the noise level and is proportional to the number of these point sensors; additional sensors help attenuate noise and improve direction resolution. An alternative approach is to form an array of directional line sensors, each of which emulates a line of many microphones that instantaneously integrate pressure change. The instrument response is a function of the orientation of the line with respect to the signal wavefront. Real data recorded at the Piñon Flat Observatory in southern California and synthetic data show that this spectral property can be exploited with multiple line sensors to determine the phase velocity direction with a precision comparable to a larger aperture array of microbarometers. Three types of instrument-response-dependent beamforming and an array deconvolution technique are evaluated. The results imply that an array of five radial line sensors, with equal azimuthal separation and an aperture that depends on the frequency band of interest, provides directional resolution while requiring less space compared to an equally effective array of five microbarometers with rosette wind filters. [ABSTRACT FROM AUTHOR]

Inclement weather in Florida forced the space shuttle “Atlantis” to land at Edwards Air Force Base in southern California on June 22, 2007, passing near three infrasound stations and several hundred seismic stations in northern Mexico, southern California, and Nevada. The high signal-to-noise ratio, broad receiver coverage, and Atlantis’ positional information allow for the testing of infrasound propagation modeling capabilities through the atmosphere to regional distances. Shadow zones and arrival times are predicted by tracing rays that are launched at right angles to the conical shock front surrounding the shuttle through a standard climatological model as well as a global ground to space model. The predictions and observations compare favorably over much of the study area for both atmospheric specifications. To the east of the shuttle trajectory, there were no detections beyond the primary acoustic carpet. Infrasound energy was detected hundreds of kilometers to the west and northwest (NW) of the shuttle trajectory, consistent with the predictions of ducting due to the westward summer-time stratospheric jet. Both atmospheric models predict alternating regions of high and low ensonifications to the NW. However, infrasound energy was detected tens of kilometers beyond the predicted zones of ensonification, possibly due to uncertainties in stratospheric wind speeds. [ABSTRACT FROM AUTHOR]

A new distributed sensor for detecting pressure variations caused by distant sources has been developed. The instrument reduces noise due to air turbulence in the infrasound band by averaging pressure along a line by means of monitoring strain in a long tubular diaphragm with an optical fiber interferometer. Above 1 Hz, the optical fiber infrasound sensor (OFIS) is less noisy than sensors relying on mechanical filters. Records collected from an 89-m-long OFIS indicate a new low noise limit in the band from 1 to 10 Hz. Because the OFIS integrates pressure variations at light-speed rather than the speed of sound, phase delays of the acoustical signals caused by the sensor are negligible. Very long fiber-optic sensors are feasible and hold the promise of better wind-noise reduction than can be achieved with acoustical-mechanical systems. [ABSTRACT FROM AUTHOR]

Optical Fiber Infrasound Sensors (OFIS) are long compliant tubes wrapped with two optical fibers that interferometrically measure the differential pressure variation along the length of the tube. Because each sensor averages spatially along the length of the tube, the frequency response of the recorded pressure variation is a function of the orientation of the OFIS sensor relative to the back azimuth and incidence angle of the incoming wave. We have exploited this property to investigate the ability of various OFIS geometries to determine the back azimuth of infrasound signals. We have found that an OFIS comprised of two orthogonal 89-m-long arms having their centers separated by 63 m can resolve the back azimuth of most infrasound signals with a good signal-to-noise ratio. We find a good match between the back azimuths determined with our technique and those determined for the same signals recorded on the co-located pipe array I57US with the Progressive Multichannel Cross-Correlation technique. Based on these results and additional synthetic tests, we have built and are testing a larger OFIS with several arms that will be able to resolve signals from all directions and with small signal-to-noise ratios. [ABSTRACT FROM AUTHOR]