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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]