Export 7 results:
Sort by: [ Author  (Asc)] Title Type Year
A B C D E F G H I J K L M N [O] P Q R S T U V W X Y Z   [Show ALL]
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, 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.

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, 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.