Publications

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2018
Sasagawa, GS, Zumberge MA, Cook MJ.  2018.  Laboratory simulation and measurement of instrument drift in quartz-resonant pressure gauges. Ieee Access. 6:57334-57340.   10.1109/access.2018.2873479   AbstractWebsite

Seafloor pressure gauges are used in marine geodesy to detect vertical displacement of the seafloor. Instrumental gauge drift is often larger than the sought after geophysical and oceanographic signals. We performed a 12 month laboratory test on two new methods that aim to reduce pressure gauge drift in Paroscientific Digiquartz and other pressure transducers. In one method, a reference quartz oscillator (RQO) is installed adjacent to but isolated from the Bourdon tube whose stress is measured by a vibrating quartz force transducer. In another method, the pressure gauge is periodically connected to accurately measured atmospheric pressure as a reference to allow drift calculation. We found that the RQO is not a good predictor of gauge drift. However, determining drift by periodic exposure to atmospheric pressure is effective. These drift estimates were compared to estimates determined with an absolute piston gauge calibrator; the average difference between drift rates of the two methods is 0.00 +/- 0.05 kPa/year. Finally, we tested the stability of the quartz clocks used in the Paroscientific electronics and found that they are not a significant contributor to drift.

Zumberge, MA, Hatfield W, Wyatt FK.  2018.  Measuring seafloor strain with an optical fiber interferometer. Earth and Space Science. 5:371-379.   10.1029/2018ea000418   AbstractWebsite

We monitored the length of an optical fiber cable stretched between two seafloor anchors separated by 200m at a depth of 1900m, 90km 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 100days following deployment on 18 October 2015, and showed an overall strain (length change) of -10.7 epsilon (shorter by 2.14mm). At seismic periods, the sensitivity was a few n epsilon; at tidal periods the noise level was a few tens of n epsilon. The RMS variation after removal of a -79n epsilon/day drift over the final 30days was 36n epsilon. No strain transients were observed. An unexpected response to the varying hydrostatic load from ocean tides was observed with a coefficient of -101n epsilon per meter of ocean tide height.

2017
Landro, M, Zumberge M.  2017.  Estimating saturation and density changes caused by CO2 injection at Sleipner - Using time-lapse seismic amplitude-variation-with-offset and time-lapse gravity. Interpretation-a Journal of Subsurface Characterization. 5:T243-T257.   10.1190/int-2016-0120.1   AbstractWebsite

We have developed a calibrated, simple time-lapse seismic method for estimating saturation changes from the CO2-storage project at Sleipner offshore Norway. This seismic method works well to map changes when CO2 is migrating laterally away from the injection point. However, it is challenging to detect changes occurring below CO2 layers that have already been charged by some CO2. Not only is this partly caused by the seismic shadow effects, but also by the fact that the velocity sensitivity for CO2 change in saturation from 0.3 to 1.0 is significantly less than saturation changes from zero to 0.3. To circumvent the seismic shadow zone problem, we combine the time-lapse seismic method with time-lapse gravity measurements. This is done by a simple forward modeling of gravity changes based on the seismically derived saturation changes, letting these saturation changes be scaled by an arbitrary constant and then by minimizing the least-squares error to obtain the best fit between the scaled saturation changes and the measured time-lapse gravity data. In this way, we are able to exploit the complementary properties of time-lapse seismic and gravity data.

2015
DeWolf, S, Wyatt FK, Zumberge MA, Hatfield W.  2015.  Improved vertical optical fiber borehole strainmeter design for measuring Earth strain. Review of Scientific Instruments. 86   10.1063/1.4935923   AbstractWebsite

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 Pinon Flat Observatory are presented. Power spectra show an intertidal noise level of -130 dB (re. 1 epsilon(2)/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. (C) 2015 AIP Publishing LLC.

2014
Berger, J, Davis P, Widmer‐Schnidrig R, Zumberge M.  2014.  Performance of an optical seismometer from 1 μHz to 10 Hz. Bulletin of the Seismological Society of America. 104:2422-2429.   10.1785/0120140052   AbstractWebsite

We compare the performance of four different instruments that measure the vertical component of motion of an inertial mass—an STS1 seismometer, an STS2 seismometer, a superconducting gravity meter, and an optical seismometer—operating inside the mine at the Black Forest Observatory near Schiltach in southwest Germany. Simultaneous, collocated operation of these sensors offers an opportunity to test the calibration, response, and performance of each instrument. We estimate noise floors from the tidal bands to 10 Hz. We note small nonlinearities in the suspension of the STS1, which are normally suppressed by analog signal processing and feedback or, in the optical version, by digital signal processing alone. The results demonstrate that the optical seismometer utilizing an STS1 suspension can provide observatory‐quality data over a bandwidth from tidal frequencies to at least 10 Hz and over a large dynamic range.

2013
Wielandt, E, Zumberge M.  2013.  Measuring seismometer nonlinearity on a shake table. Bulletin of the Seismological Society of America. 103:2247-2256.   10.1785/0120120325   AbstractWebsite

We have measured the nonlinear distortion in six broadband seismometers on the vertical shake table at the Institute of Geophysics and Planetary Physics La Jolla: a vertical STS1, three STS2s, a CMG-3T, and a Trillium 240. In each case, low-frequency intermodulation of a two-tone signal was observed for six frequency pairs near 0.25, 0.5, 1, 2, 4, and 8 Hz at a beat frequency of 0.02 Hz. The peak velocity amplitude was 6: 3 mm/s, which is about half of the operating range of an STS2 or CMG-3T. We found similar distortion levels in all seismometers: The average over all distortion ratios is -96 dB +/- 7 dB (standard deviation) in terms of equivalent ground acceleration, with a tendency to higher distortion at higher frequencies. When the same signals are expressed as electric output voltages or equivalent ground velocities, ratios are much higher and increase rapidly with frequency: around -65 dB at 1 Hz and around -40 dB at 8 Hz. The distortion of seismic signals cannot be predicted from the distortion of electrical signals fed into the calibration coil, and the electrical distortion is about 30 dB lower in one of the STS2s. Low-frequency distortion of the table motion has a level of -140 dB at 1 Hz in terms of acceleration, which is far below that of all seismometers. This number does not indicate a super-linear table motion but results from expressing the distortion present in the table displacement at -72 dB as a ratio of accelerations. What may seem to be a trivial conversion has a very practical implication: The linearity of seismometers can be tested on moderately performing shake tables.

Sasagawa, G, Zumberge MA.  2013.  A self-calibrating pressure recorder for detecting seafloor height change. IEEE Journal of Oceanic Engineering. 38:447-454.   10.1109/joe.2012.2233312   AbstractWebsite

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.

DeWolf, S, Walker KT, Zumberge MA, Denis S.  2013.  Efficacy of spatial averaging of infrasonic pressure in varying wind speeds. Journal of the Acoustical Society of America. 133:3739-3750.   10.1121/1.4803891   AbstractWebsite

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 similar to 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 18m 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 270m provide a WNR of up to similar to 30 dB in winds up to 5m/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 similar to 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.

2011
Pettit, EC, Waddington ED, Harrison WD, Thorsteinsson T, Elsberg D, Morack J, Zumberge MA.  2011.  The crossover stress, anisotropy and the ice flow law at Siple Dome, West Antarctica. Journal of Glaciology. 57:39-52.   10.3189/002214311795306619   AbstractWebsite

We used observations and modeling of Sip le Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Sip le Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n = 3) term. The pattern of flow near the divide at Sip le Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

Zumberge, M.  2011.  Gravity Measurements, Absolute. Encyclopedia of Solid Earth Geophysics. ( Gupta HK, Ed.).:494-497., Dordrecht: Springer Abstract
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2010
Blum, J, Igel H, Zumberge M.  2010.  Observations of Rayleigh-Wave Phase Velocity and Coseismic Deformation Using an Optical Fiber, Interferometric Vertical Strainmeter at the SAFOD Borehole, California. Bulletin of the Seismological Society of America. 100:1879-1891.   10.1785/0120090333   AbstractWebsite

We present observations from a vertical, optical fiber interferometric strainmeter in the San Andreas Fault Observatory at Depth borehole near Parkfield, California. The sensor detects both teleseismic earthquakes and local events, along with coseismic strain steps consistent with theoretical dislocation models. For teleseismic events, we investigate the possibility of determining local Rayleigh-wave phase velocities beneath the borehole by comparing the ratio of vertical ground acceleration from a nearby seismometer to vertical strain. While similar studies have used horizontal components and rotations, this is the first such attempt utilizing vertical measurements. We show that at periods from around 16-40 seconds, we can recover general dispersion characteristics that are within a few percent of models of realistic local structure.

Blum, JA, Chadwell CD, Driscoll N, Zumberge MA.  2010.  Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data. Geophysical Research Letters. 37   10.1029/2010gl043293   AbstractWebsite

Seafloor slope instability in the Santa Barbara Basin, California, poses risk to the region. Two prominent landslides, the Goleta and Gaviota slides, occupy the northern flank, with a scarp-like crack extending east from the headwall of the Gaviota slide towards the Goleta complex. Downslope creep across the crack might indicate an imminent risk of failure. Sub-bottom CHIRP profiles with <1 m accuracy across the crack exhibit no evidence of internal deformation. Daily seafloor acoustic range measurements spanning the crack detected no significant motion above a 99% confidence level of +/- 7 mm/yr over two years of monitoring. These disparate data over different timescales suggest no active creep and that the crack is likely a relict feature that formed concomitantly with the Gaviota slide. Citation: Blum, J. A., C. D. Chadwell, N. Driscoll, and M. A. Zumberge (2010), Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data, Geophys. Res. Lett., 37, L13308, doi: 10.1029/2010GL043293.

Zumberge, M, Berger J, Otero J, Wielandt E.  2010.  An Optical Seismometer without Force Feedback. Bulletin of the Seismological Society of America. 100:598-605.   10.1785/0120090136   AbstractWebsite

We are developing a new vertical seismometer, motivated by a desire to have an instrument whose performance is similar to that of observatory sensors yet can operate within a borehole without electronics. This has led us to an all-optical seismometer consisting of a spring-suspended mass whose position is monitored interferometrically. We use a Michelson interferometer illuminated with a 1 mW laser that can be linked to the seismometer with optical fibers only. A digital signal processor samples the interference fringe signal and produces a 400 samples/sec record of the seismometer mass displacement with a root mean square noise per octave band that varies from about 4 x 10(-12) m at 0.001 Hz to 4 x 10(-13) m at 1 Hz. The maximum displacement is limited by mechanical issues to a few millimeters at present, providing a dynamic range of at least 109, equivalent to 30 bits (180 dB). Experiments to test this idea have been performed on a modified STS1 vertical seismometer whose electronics have been replaced with an optical system. Comparisons with other seismometers show that, in terms of both noise and signal fidelity, the optical approach is quite viable.

2008
De Groot-Hedlin, CD, Hedlin MAH, Walker KT, Drob DP, Zumberge MA.  2008.  Evaluation of infrasound signals from the shuttle Atlantis using a large seismic network. Journal of the Acoustical Society of America. 124:1442-1451.   10.1121/1.2956475   AbstractWebsite

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. (C) 2008 Acoustical Society of America.

Walker, KT, Zumberge MA, Hedlin MAH, Shearer PM.  2008.  Methods for determining infrasound phase velocity direction with an array of line sensors. Journal of the Acoustical Society of America. 124:2090-2099.   10.1121/1.2968675   AbstractWebsite

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 Pinon 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. (C) 2008 Acoustical Society of America. [DOI: 10.1121/1.2968675]

Eiken, O, Stenvold T, Zumberge M, Alnes H, Sasagawa G.  2008.  Gravimetric monitoring of gas production from the Troll field. Geophysics. 73:WA149-WA154.   10.1190/1.2978166   AbstractWebsite

Four surveys of relative gravity and depth measurements have been conducted over the Troll field since gas production began in 1996. Precision in gravity (intrasurvey repeatability measured as standard deviation) has improved from 26 mu Gal to 4-5 mu Gal in two surveys in 2002 and 2005. The scatter of 74 station depth differences (standard deviation) has improved to 1.0 cm (with modeled subsidence removed) for the same two surveys. Subsidence of up to 3 cm is observed above the thickest reservoir of Troll East between 2002 and 2005. Time-lapse gravity shows a general slight average increase in Troll East (95% confidence for the interval centered at 5.2 +/- 4.4 mu Gal), which is likely caused by edgewater influx. One gravity station close to Troll A shows a 9-mu Gal gravity increase, indicating 2.2-m water rise. Repeated logging in a nearby well indicates 2.8-m rise. These two observations agree within uncertainty bounds. Gravity decrease in Troll West is explained by oil production and a downward-moving gas-oil contact. Gravity increases in two areas in the easternmost part appear statistically significant and are likely to have been caused by edgewater influx. This has not been confirmed by other data, but aquifers located near these areas make the results plausible. The gravity and subsidence data give estimates of total mass influx and pore compaction; these data have been used in and agree with material-balance calculations.

Sasagawa, G, Zumberge M, Eiken O.  2008.  Long-term seafloor tidal gravity and pressure observations in the North Sea: Testing and validation of a theoretical tidal model. Geophysics. 73:WA143-WA148.   10.1190/1.2976778   AbstractWebsite

Seafloor gravity and pressure measurements for 4D reservoir monitoring require precise models of the time-varying tidal signals. Current seafloor instrumentation can resolve 0.003 mGal in time-lapse gravity differences and 0.05 kPa (5 mm) in pressure. To verify model accuracy, a seafloor gravimeter and pressure gauge were operated continuously for 446 days next to the Troll A gas platform in the North Sea (60.64227 degrees north, 3.72417 degrees east) at a depth of 303 m. The seafloor gravity and pressure time series were filtered and corrected with estimates from the tidal model, which predicts the solid earth tide, ocean loading, and direct gravitational attraction of the varying water level. The rms difference between the observed tidal gravity signal and the prediction is about 0.0013 mGal during periods when there are no surface storms. A slight difference is observed for the direct attraction of the water overhead as computed from the tidal prediction versus that computed from direct seafloor pressure measurements when the entire 446-day record is analyzed; it shows an rms difference of 0.708 kPa, equivalent to 7 cm of water-height variation, yielding a gravity effect of 0.003 mGal. We conclude that existing theoretical tide models in combination with in situ pressure records are sufficiently precise for correcting time-lapse gravity observations.

Zumberge, M, Alnes H, Eiken O, Sasagawa G, Stenvold T.  2008.  Precision of seafloor gravity and pressure measurements for reservoir monitoring. Geophysics. 73:WA133-WA141.   10.1190/1.2976777   AbstractWebsite

Changes with gravity over time have proven to be valuable for inferring subsurface density changes associated with production from oil and natural gas reservoirs. Such inferences allow the monitoring of moving fluid fronts in a reservoir and provide an opportunity to optimize production over the life of the reservoir. Our group began making time-lapse seafloor gravity and pressure measurements in 1998. To date, we have surveyed six fields offshore Norway; we have made three repeat surveys at one field and one repeat survey at another. We incorporated a land-gravity sensor into a remotely operated seafloor housing. Three such relative gravity sensors mounted in a single frame are carried by a remotely operated vehicle (ROV) to concrete benchmarks permanently placed on the seafloor. Reference benchmarks sited outside the reservoir boundaries are assumed to provide stable fiducial points. Typical surveys last from a few days to a few weeks and cover from 8 to 80 benchmarks, with multiple observations of each. In our earliest surveys, we obtained an intrasurvey repeatability of approximately 20 mu Gal, but recently we have been achieving 3-mu Gal repeatability in gravity and approximately 5 mm in benchmark depth (deduced from simultaneously recorded ambient seawater pressure). We attribute the improved precision to several operational factors, including the use of multiple gravity sensors, frequent benchmark reoccupation, precise relocation and orientation of the sensors, repeated calibrations on land, and minimization of vibrational and thermal perturbations to the sensors. We believe that high-precision time-lapse gravity monitoring can be used to track changes in the height of a gas-water contact in a flooded reservoir, with a precision of a few meters.

Blum, JA, Nooner SL, Zumberge MA.  2008.  Recording Earth strain with optical fibers. IEEE Sensors Journal. 8:1152-1160.   10.1109/jsen.2008.926882   AbstractWebsite

Optical fibers are well suited to measure Earth strain because they can be stretched over long distances to average strain over a large interval. This is important to reduce disturbances to the measurement from very local effects. We have installed optical fibers ranging in length from a few 10s of meters to 2 km in vertical boreholes on land and in an icesheet, and horizontally along the sea floor. Due to the high sensitivity of optical fibers to temperature change, an environment of stable temperature is important-this is often available in boreholes or on the sea floor. Longevity of fiber cables and the means to protect the glass fibers from environmental effects and the rigors of deployment are critical issues. Our experiences cover a broad range of success in this regard, with some deployments lasting for more than four years and others failing immediately.

2007
Nooner, SL, Eiken O, Hermanrud C, Sasayawa GS, Stenvold T, Zumberge MA.  2007.  Constraints on the in situ density of CO2 within the Utsira formation from time-lapse seafloor gravity measurements. International Journal of Greenhouse Gas Control. 1:198-214.   10.1016/s1750-5836(07)00018-7   AbstractWebsite

At Sleipner, CO2 is being separated from natural gas and injected into an underground saline aquifer for environmental purposes. Uncertainty in the aquifer temperature leads to uncertainty in the in situ density of CO2. In this study, gravity measurements were made over the injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to constrain the in Situ CO2 density. The gravity measurements have a repeatability of 4.3 mu Gal for 2003 and 3.5 mu Gal for 2005. The resulting time-lapse uncertainty is S.3 mu Gal. Unexpected benchmark motions due to local sediment scouring contribute to the uncertainty. Forward gravity models are calculated based on both 3D seismic data and reservoir simulation models. The time-lapse gravity observations best fit a high temperature for-ward model based on the time-lapse 3D seismics, suggesting that the average in Situ CO2 density is about to 530 kg/m(3). Uncertainty in determining the average density is estimated to be 65 kg/m(3) (95% confidence), however, this does not include uncertainties in the modeling. Additional seismic surveys and future gravity measurements will put better constraints on the CO2 density and continue to map out the CO2 flow. (C) 2007 Elsevier Ltd. All rights reserved.

2006
Stenvold, T, Eiken O, Zumberge MA, Sasagawa GS, Nooner SL.  2006.  High-precision relative depth and subsidence mapping from seafloor water-pressure measurements. SPE Journal. 11:380-389. AbstractWebsite

A method to accurately measure seafloor subsidence away from platform locations is presented. The method is based on seafloor water pressure, which is measured on top of predeployed benchmarks visited one after another using a remotely operated vehicle (ROV) and is at the same time measured continuously throughout the survey at one or more reference locations. Because no significant subsidence is expected during a few days of data acquisition, high-precision relative depths representative for the average time of the survey can be obtained. Accurate subsidence estimates between seafloor surveys are found assuming negligible subsidence at benchmarks located outside the field. Results from six seafloor surveys performed at two gas fields in the North Sea are presented. For an area of 1 km(2) at 80 m water depth, single-measurement relative depth precision (standard deviation) of 0.4 cm was obtained. Correspondingly, for an area of 700 km(2) at 295 to 345 m water depth, 0.6 cm was obtained. Single-station subsidence accuracy down to 1 cut is achieved from the two most recent pressure surveys at the large field. A subsidence signal is seen for this difference, and it is compared with modeled subsidence. Error budgets for depth precision and subsidence, incorporating instrumental and environmental errors, are discussed.

Chadwick, WW, Nooner SL, Zumberge MA, Embley RW, Fox CG.  2006.  Vertical deformation monitoring at Axial Seamount since its 1998 eruption using deep-sea pressure sensors. Journal of Volcanology and Geothermal Research. 150:313-327.   10.1016/j.jvolgeores.2005.07.006   AbstractWebsite

Pressure measurements made on the seafloor at depths between 1500 and 1700 m at Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge in the northeast Pacific Ocean, show evidence that it has been inflating since its 1998 eruption. Data from continuously recording bottom pressure sensors at the center of Axial's caldera suggest that the rate of inflation was highest in the months right after the eruption (20 cm/month) and has since declined to a steady rate of similar to 15 cm/year. Independent campaign-style pressure measurements made each year since 2000 at an array of seafloor benchmarks with a mobile pressure recorder mounted on a remotely operated vehicle also indicate uplift is occurring in the caldera at a rate up to 22 +/- 1.3 cm/year relative to a point outside the caldera. The repeatability of the campaign-style pressure measurements progressively improved each year from +/- 15 cm in 2000 to +/- 0.9 cm in 2004, as errors were eliminated and the technique was refined. Assuming that the uplift has been continuous since the 1998 eruption, these observations suggest that the center of the caldera has re-inflated about 1.5 +/- 0.1 m, thus recovering almost 50% of the 3.2 m of subsidence that was measured during the 1998 eruption. This rate of inflation can be used to calculate a magma supply rate of 14 x 10(6) m(3)/year. If this rate of inflation continues, it also suggests a recurrence interval of similar to 16 years between eruptions at Axial, assuming that it will be ready to erupt again when it has re-inflated to 1998 levels. (c) 2005 Elsevier B.V. All rights reserved.

2004
Zumberge, MA, Berger J, Dzieciuch MA, Parker RL.  2004.  Resolving quadrature fringes in real time. Applied Optics. 43:771-775.   10.1364/ao.43.000771   AbstractWebsite

In many interferometers, two fringe signals can be generated in quadrature. The relative phase of the two fringe signals depends on whether the optical path length is increasing or decreasing. A system is developed in which two quadrature fringe signals are digitized and analyzed in real time with a digital signal processor to yield a linear, high-resolution, wide-dynamic-range displacement transducer. The resolution in a simple Michelson interferometer with inexpensive components is 5 X 10(-13) m Hz(-1/2) at 2 Hz. (C) 2004 Optical Society of America.

Elsberg, DH, Harrison WD, Zumberge MA, Morack JL, Pettit EC, Waddington ED, Husmann E.  2004.  Depth- and time-dependent vertical strain rates at Siple Dome, Antarctica. Journal of Glaciology. 50:511-521.   10.3189/172756504781829684   AbstractWebsite

As part of a project to investigate the flow of ice at low effective stress, two independent strain-gauge systems were used to measure vertical strain rate as a function of depth and time at Siple Dome, Antarctica. The measurements were made from January 1998 until January 2002 at the ice divide and a site 7km to the northeast on the flank. The strain-rate profiles place constraints on the rheology of ice at low stress, show the expected differences between divide and flank flow (with some structure due to firn compaction and probably ice stratigraphy), and suggest that the flow of the ice sheet has not changed much in the last 8.6kyr. The strain rates show an unexpected time dependence on scales ranging from several months to hours, including discrete summer events at the divide. Time dependence in strain rate, water pressure, seismicity, velocity and possibly basal motion has been seen previously on the Siple Coast ice streams, but it is especially surprising on Siple Dome because the bed is cold.

2003
Zumberge, MA, Berger J, Hedlin MAH, Husmann E, Nooner S, Hilt R, Widmer-Schnidrig R.  2003.  An optical fiber infrasound sensor: A new lower limit on atmospheric pressure noise between 1 and 10 Hz. Journal of the Acoustical Society of America. 113:2474-2479.   10.1121/1.1566978   AbstractWebsite

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. (C) 2003 Acoustical Society, of America.