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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.

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.

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.

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, MA, Elsberg DH, Harrison WD, Husmann E, Morack JL, Pettit EC, Waddington ED.  2002.  Measurement of vertical strain and velocity at Siple Dome, Antarctica, with optical sensors. Journal of Glaciology. 48:217-225.   10.3189/172756502781831421   AbstractWebsite

As part of a larger program to measure and model vertical strain around Siple Dome on the West Antarctic ice sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-wall stainless-steel tubes, frozen into holes drilled with hot water, and stretched from the surface to various depths (up to 985 m) in the ice sheet. An optical system, connected annually to the fibers, reads out their absolute lengths with a precision of about 2 mm. Two sets of five sensors were installed in the 1997/98 field season: one set is near the Siple Dome core hole (an ice divide), and a second set is on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m). The optical-fiber length observations taken in four field seasons spanning a 3 year interval reveal vertical strain rates ranging from -229 +/- 4 ppm a(-1) to -7 +/- 9 ppm a(-1). In addition to confirming a non-linear constitutive relationship for deep ice, our analysis of the strain rates indicates the ice sheet is thinning at the flank and is in steady state at the divide.

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.

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.

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]

Ander, ME, Zumberge MA, Lautzenhiser T, Parker RL, Aiken CLV, Gorman MR, Nieto MM, Ferguson JF, McMechan GA.  1989.  A New Field Experiment in the Greenland Ice Cap to Test Newton Inverse Square Law. Annals of the New York Academy of Sciences. 571:672-680.   10.1111/j.1749-6632.1989.tb50553.x   AbstractWebsite

Recent experimental evidence suggests that Newton’s law of gravity may not be precise. There are modern theories of quantum gravity that, in their attempts to unify gravity with other forces of nature, predict non-Newtonian gravitational forces that could have ranges on the order of 102-105 m. If they exist, these forces would be apparent as violations of Newton’s inverse square law. A geophysical experiment was carried out to search for possible finite-range, non-Newtonian gravity over depths of 213-1673 m in the glacial ice of the Greenland ice cap. The principal reason for this choice of experimental site is that a hole drilled through the ice cap already existed and the uniformity of the ice eliminates one of the major sources of uncertainty arising in the first of earlier namely, the heterogeneity of the rocks through which a mine shaft or drill hole passes. Our observations were made in the summer of 1987 at Dye 3, Greenland, in the 2033-m-deep borehole, which reached the basement rock.

Parker, PR, Zumberge MA, Parker RL.  1995.  A New Method for Fringe-Signal Processing in Absolute Gravity Meters. Manuscripta Geodaetica. 20:173-181. AbstractWebsite

In all modern absolute gravity meters, an interferometer illuminated with a stabilized laser tracks the motion of a freely falling retroreflector. The value of gravity is measured by timing the passage of interference fringes. Typically, the sinusoidal fringe signal is converted to a series of pulses, a subset of which are input to a time digitizer. In our new system, the fringe signal is digitized with a fast analog-to-digital converter and fit to an increasing-frequency sine wave. In addition to being smaller and less expensive, the system should eliminate some potential systematic errors that may result from imperfect zero-crossing discrimination and pulse pre-scaling.

Zumberge, M, Faller JE, Rinker RL.  1984.  A new portable, absolute gravimeter. Precision Measurements and Fundamental Constants II. ( Taylor BN, Philips WD, Eds.).:405-409. Abstract
Sasagawa, GS, Crawford W, Eiken O, Nooner S, Stenvold T, Zumberge MA.  2003.  A new sea-floor gravimeter. Geophysics. 68:544-553.   10.1190/1.1567223   AbstractWebsite

A new reservoir management application uses precise time-lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3-5-year period. We have constructed and deployed a new instrument-the ROVDOG (Remotely Operated Vehicle-deployed Deep-Ocean Gravimeter) system-for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea-floor bench marks. We have built an instrument based upon the Scintrex CG-3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties. The instrument prototype was deployed in the North Sea during June 1998; 75 observations were made at 32 stations. The standard deviation of repeated gravity measurements was 0.026 mGal; the standard deviation of pressure-derived heights, for repeated measurements, was 1.4 cm. A refined instrument was deployed in August 2000 in a three-sensor configuration. Multiple sensors improved the precision by averaging more samples without incurring additional survey time. A total of 159 measurements were made at 68 station. The standard deviation of repeated measurements was 0.019 mGal; the standard deviation of pressure-derived heights was 0.78 cm. A ROVDOG pressure case rated to 4500 m depth has also been constructed. This system was deployed with the Alvin manned submersible in November 2000 to a depth of 2700 m.

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.

Stevenson, JM, Hildebrand JA, Zumberge MA, Fox CG.  1994.  An Ocean-Bottom Gravity Study of the Southern Juan-De-Fuca Ridge. Journal of Geophysical Research-Solid Earth. 99:4875-4888.   10.1029/93jb02076   AbstractWebsite

We use seafloor and sea surface gravity data to model density structure along the southern Juan de Fuca Ridge. We determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models. Of 63 seafloor gravity measurements obtained, 42 observations were distributed over the 3-km left-lateral overlapping rift zone (ORZ), located at latitude 45-degrees-03'N, and separating the Juan de Fuca ridge into the Cleft and Vance segments. A 21-measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44-degrees-52'N, a region of linear ridge geometry on the Cleft segment. These seafloor gravity measurements, which are sensitive to shallow crustal density variations, were augmented by roughly 800 km of sea surface gravity measurements. Using a bathymetry-Bouguer anomaly analysis of the seafloor gravity data, we determined the average density of the shallow (roughly upper 2 km) oceanic crust at these two locations to be 2630 kg m-3 +/- 50 kg m-3. Within the uncertainties of the density determinations, there is no difference between the average shallow oceanic crustal density at the linear Cleft segment and at the Cleft-Vance ORZ. Using the seafloor measurements, we modeled the study area's underlying density to provide constraints on its fine-scale structure. Localized porosity of up to 17% within the upper 500 m of oceanic crust (layer 2A) provide a possible explanation for the observed gravity anomalies at the Cleft segment. Two-dimensional density models of the crust underlying the linear Cleft segment (44-degrees-52'N) show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis. For the Cleft-Vance ORZ, three-dimensional modeling suggests low-density material between the rifts, in agreement with thickened layer 2A from seismic observations. In contrast, layer 2A variations do not explain the low density inferred at the northern end of the Cleft segment since seismic observations suggest a thin layer 2A. The magmatic activity recently observed in this region may have a distinct, deeper source.

Zumberge, MA, Canuteson EL, Parker PR.  1995.  Optical fiber gravity meter. , U.S.A.: The Regents of the University of California Abstract
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.

Zumberge, MA, Wyatt FK.  1998.  Optical fiber interferometers for referencing surface benchmarks to depth. Pure and Applied Geophysics. 152:221-246.   10.1007/s000240050152   AbstractWebsite

We have developed and operated optical fiber interferometers for monitoring displacements within boreholes, as part of a program of crustal deformation measurement. These optical tiber strainmeters-a total of twelve instruments at two sites in southern California-were installed to sense the motion of the end-monuments of much longer baseline strainmeters and tiltmeters, allowing correction for any near-surface ground movement. One of the installations was specifically designed to investigate the distribution of deformation with depth, measuring over several borehole length-intervals from 5 m to 50 m. The displacements recorded over year-long time scales along these length intervals range up to 6 mm and show internal consistency and stability at the 50 mu m level. The use of these interferometers to provide correction signals for kilometer-scale crustal strain measurements has resulted in greatly improved records.

Zumberge, MA, Wyatt FK, Yu DX, Hanada H.  1988.  Optical Fibers for Measurement of Earth Strain. Applied Optics. 27:4131-4138.   10.1364/AO.27.004131   AbstractWebsite

We report on laboratory experiments on single-mode optical fibers for use in measuring earth strain. We have monitored the long-term stability of 25-m long tensioned fibers and found their rates of fractional change in optical path lengths to be no more than 2 × 10-6/yr. The optical temperature coefficients for several fibers whose physical lengths were held constant were found to be within 4% of 1.17 × 10-5 apparent strain/°C. The strain sensitivity (the ratio of observed optical path change to physical path change) was determined to be within 1% of 1.16 for all the fibers tested. Initial field tests indicate that fibers are suitable for earth strain measurements of moderate precision.

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.

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.

Faller, JE, Rinker RL, Zumberge MA.  1979.  Plans for the Development of a Portable Absolute Gravimeter with a Few Parts in 109 Accuracy. Tectonophysics. 52:107-116.   10.1016/0040-1951(79)90212-9   AbstractWebsite

Successful development of a few parts in 109 portable g apparatus (which corresponds to a height sensitivity of about 1 cm) would have an impact on large areas of geodynamics as well as having possible application to earthquake prediction. Furthermore, the use of such an instrument in combination with classical leveling or extraterrestrially determined height data would yield information on internal mass motions. The plans for the development of such an instrument at JILA using the method of free fall will be given. The proposed interferometric method uses one element of an optical interferometer as the dropped object. Recent work has resulted in substantial progress towards the development of a new type of long-period (T > 60 sec) suspension for isolating the reference mirror (corner cube) in the interferometer. Improvements here over the isolation methods previously available, together with state-of-the-art timing and interferometric techniques, are expected to make it possible to achieve a few parts in 109 accuracy with a field-type instrument.

Faller, JE, Rinker RL, Zumberge M.  1978.  Plans for the development of a portable absolute gravimeter: A tool for studying non-tidal variations in gravity. Boll. Geofis.Teor. Appl. 20:355-362. Abstract
Chave, AD, Zumberge MA, Ander ME, Hildebrand JA, Spiess FN.  1987.  Polar Ice Test of the Scale Dependence of G. Nature. 326:250-251.   10.1038/326250b0   AbstractWebsite
Zumberge, MA, Rinker RL, Faller JE.  1982.  A Portable Apparatus for Absolute Measurements of the Earths Gravity. Metrologia. 18:145-152.   10.1088/0026-1394/18/3/006   AbstractWebsite

We have developed a new and portable apparatus for making absolute measurements of the acceleration due to the Earth's gravity. We use the method of free fall, and interferometrically determine the acceleration of a freely falling cube corner. In the design and development of this instrument, particular attention was paid to those aspects which would affect its performance in the field. The resulting instrument, we believe, provides a viable new tool for the study of tectonic motions. The system is very small; it can be transported in a small van and requires only two hours for assembly. A high rate of data acquisition is available; if necessary, a single measurement can be made every two seconds. Further, we have made a concerted effort to detect and (we hope) eliminate systematic errors. The results of extensive tests indicate that the achievable accuracy for g is about six parts in 109. This instrument therefore provides a sensitivity to vertical motions (e.g., of the Earth's crust) as small as 2 cm.

Zumberge, MA.  1997.  Precise optical path length measurement through an optical fiber: Application to seafloor strain monitoring. Ocean Engineering. 24:531-542.   10.1016/s0029-8018(96)00029-7   AbstractWebsite

An optical tiber strainmeter intended for measuring tectonic strains on the seafloor is under development. In this instrument, an optical fiber is stretched between two points fixed to the ocean bottom; relative displacement of these points causes a change in the elongation of the fiber. This associated change in optical path length is monitored by an electronic distance meter. The dominant sources of noise in determining the optical path length of the fiber stem from the dependence of the fiber's index of refraction on both wavelength and temperature. In a 50 day long experiment performed in the shallow ocean, a test fiber was installed along a 210 m long baseline on the bottom. The RMS Variation in length was 5 mm except for two displacements of order 10 cm caused by known effects. (C) 1997 Elsevier Science Ltd.