Distributed fiber-optic sensing technology coupled to existing subsea cables (dark fiber) allows observation of ocean and solid earth phenomena. We used an optical fiber from the cable supporting the Monterey Accelerated Research System during a 4-day maintenance period with a distributed acoustic sensing (DAS) instrument operating onshore, creating a ~10,000-component, 20-kilometer-long seismic array. Recordings of a minor earthquake wavefield identified multiple submarine fault zones. Ambient noise was dominated by shoaling ocean surface waves but also contained observations of in situ secondary microseism generation, post–low-tide bores, storm-induced sediment transport, infragravity waves, and breaking internal waves. DAS amplitudes in the microseism band tracked sea-state dynamics during a storm cycle in the northern Pacific. These observations highlight this method’s potential for marine geophysics.

We present one of the first case studies demonstrating the use of distributed acoustic sensing deployed on regional unlit fiber-optic telecommunication infrastructure (dark fiber) for broadband seismic monitoring of both near-surface soil properties and earthquake seismology. We recorded 7 months of passive seismic data on a 27 km section of dark fiber stretching from West Sacramento, CA to Woodland, CA, densely sampled at 2 m spacing. This dataset was processed to extract surface wave velocity information using ambient noise interferometry techniques; the resulting VS profiles were used to map both shallow structural profiles and groundwater depth, thus demonstrating that basin-scale variations in hydrological state could be resolved using this technique. The same array was utilized for detection of regional and teleseismic earthquakes and evaluated for long period response using records from the M8.1 Chiapas, Mexico 2017, Sep 8th event. The combination of these two sets of observations conclusively demonstrates that regionally extensive fiber-optic networks can effectively be utilized for a host of geoscience observation tasks at a combination of scale and resolution previously inaccessible.

A new characterization approach is employed in this study that enables the measurement of the surface area of each reactive mineral located within the connected pore network of a sandstone from a carbon sequestration pilot site in Cranfield, Mississippi. The mineral distribution is measured in 2D by chemical mapping using Energy Dispersive X-ray Spectroscopy-Scanning Electron Microscopy (SEM-EDX) coupled with an image segmentation technique. The pore structure is mapped at high resolution using a pixel contrast thresholding technique applied to 2D Backscattered Electron Microscopy (BSE-SEM) images. After merging the mineral distribution and porosity maps, the accessibilities of each mineral present in the rock sample are quantified. These quantifications require characterizing in advance the connected pore network in the merged maps, which is done considering the permeability of chlorite measured at the nano-scale in three dimensions by Focus Ion BeamScanning Electron Microscopy (FIB-SEM). The accessible surface area of each reactive mineral is finally determined by multiplying the fraction of each reactive mineral next to the connected pore network, measured in 2D, with the surface area of the connected pore network in the rock, which is measured in 3D from X-ray based micro tomography (μ-CT) images and subsequently refined with a correction factor that accounts for the missing pore connectivity. This is necessary since μ-CT voxel resolution (880 nm) is lower than the pixel resolution achieved with BSE-SEM (330 nm). The accessible surface areas of the reactive minerals present in the sandstone rock can be used to accurately scale the rate constants for quantitative prediction and ultimately control of CO 2 injection in the subsurface at the Cranfield pilot site.

Nonreflection seismic and inversion of surface and guided waves 936 The Leading Edge June 2013 Nonreflection seismic and inversion of surface and guided waves Field testing of fiber-optic distributed acoustic sensing (DAS) for subsurface seismic monitoring

Our understanding of subsurface processes suffers from a profound observation bias: seismometers are sparse and clustered on continents. A new seismic recording approach, distributed acoustic sensing (DAS), transforms telecommunication fiber‐optic cables into sensor arrays enabling meter‐scale recording over tens of kilometers of linear fiber length. We analyze cataloged earthquake observations from three DAS arrays with different horizontal geometries to demonstrate some possibilities using this technology. In Fairbanks, Alaska, we find that stacking ground motion records along 20 m of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0.8–1.6 Hz. Using an L‐shaped DAS array in Northern California, we record the nearly vertically incident arrival of an earthquake from The Geysers Geothermal Field and estimate its backazimuth and slowness via beamforming for different phases of the seismic wavefield. Lastly, we install a fiber in existing telecommunications conduits below Stanford University and show that little cable‐to‐soil coupling is required for teleseismic P and S phase arrival detection.

Ambient-noise-based seismic monitoring of the near surface often has limited spatiotemporal resolutions because dense seismic arrays are rarely sufficiently affordable for such applications. In recent years, however, distributed acoustic sensing (DAS) techniques have emerged to transform telecommunication fiber-optic cables into dense seismic arrays that are cost effective. With DAS enabling both high sensor counts (“large N”) and long-term operations (“large T”), time-lapse imaging of shear-wave velocity (V S) structures is now possible by combining ambient noise interferometry and multichannel analysis of surface waves (MASW). Here we report the first end-to-end study of time-lapse V S imaging that uses traffic noise continuously recorded on linear DAS arrays over a three-week period. Our results illustrate that for the top 20 meters the V S models that is well constrained by the data, we obtain time-lapse repeatability of about 2% in the model domain—a threshold that is low enough for observing subtle near-surface changes such as water content variations and permafrost alteration. This study demonstrates the efficacy of near-surface seismic monitoring using DAS-recorded ambient noise.

Distributed Acoustic Sensing (DAS) is a novel tool in array seismology that measures the phase of backscattered laser pulses traveling in a fiber‐optic cable, and relates this measurement to the axial strain induced on the cable by a propagating seismic wavefield. Combining DAS with telecommunications optical fiber networks has begun to address a range of earth science questions where cost and field logistics have historically hindered observations. Unlike classic inertial seismometers, DAS instrument response is presently unquantified. This topic includes a variable sensing element—the fiber, including packaging and installation—which changes between experiments. Ignoring this element, one DAS record should approximate a fixed‐length strain gauge, which exactly measures Earth's motion down to quasi‐static frequencies relevant to geodesy. In this paper, we test this hypothesis using seismological observations of teleseismic earthquakes and microseism noise spanning the 1 to 120 s period range. We use a commercial DAS interrogator unit connected to an optical fiber previously used for telecommunication and a colocated broadband seismometer to estimate the DAS transfer function. We find a 1:1 correspondence with actual ground motion from 10–120 s. At shorter periods (1–10 s), DAS amplitude response is enhanced by 3–11 dB. Phase response is flat over this range of periods. We interpret the recovered DAS response function in terms of hypothesized fiber coupling and photonic effects. We propose this calibration methodology for future DAS experiments where seismic amplitude information is desired.

The upscaling of calcite precipitation rates in porous media from the pore (micron) to continuum (centimeter) scale is evaluated with an integrated experimental and modeling approach. Experiments using cylindrical cores packed with glass beads and calcite (Iceland spar) crystals were injected over a period of 28 days with a supersaturated mixture of CaCl 2 and NaHCO 3 to induce calcite growth. Bulk rates of precipitation based on the change in aqueous chemistry over the length of the column are compared with spatially resolved determinations of carbonate precipitation using X-ray synchrotron microtomographic imaging at the micron scale. These data are supplemented by continuously-stirred reactor experiments using the same calcite seed material so as to minimize differences in the effects of reactive site density of the seed material, and to evaluate the rate of precipitation in the absence of transport or "porous medium" effects. Calcite precipitation rates determined in the stirred flowthrough reactor in this study are considerably slower than rates determined at similar supersaturation in unseeded batch experiments by Tang et al. [2008a], although these rates are compatible with those reported in Nehrke et al. [2007] for precipitation on Iceland spar when the same normalization to reactive surface area is used. The nearly linear dependence of the rates on supersaturation cannot be attributed to a diffusion control in the case of the stirred reactors and is likely the result of multi-sourced spiral growth.Integrated precipitation rates based on column effluent chemistry from a higher supersaturation experiment are in good agreement with determinations of total carbonate precipitated based on determination of pre-and post-experiment mass in the column using X-ray microtomography. Using the rates of precipitation determined in the well-stirred flowthrough reactors, it is possible to match the spatially-resolved microtomographic and aqueous data with a coarser resolution continuum model using volume-averaged flow and reactive surface area if the generation of new reactive surface area is accounted for. A nucleation or surface roughening event, which is most pronounced within two millimeters of the column inlet where the supersaturation is highest, is recorded by both BET analysis, which indicates an increase in specific surface area from 0.012 m 2 •g -1 to a value of 0.21 m 2 •g -1 for neoformed calcium carbonate, and by X-ray microtomography. The best fit of the column X-ray microtomography data is provided by simulating calcite precipitation either as a single nearly linear rate for multi-sourced spiral growth, or as two parallel rates that include the spiral growth model and a 2D heterogeneous nucleation model modified by a partial diffusion control, although nucleation needs to be relatively minor on a mass basis in order to match the data. The combined experiments and modeling indicate that it is possible to develop upscaled quantitative models for porous media reactivity, but changes in such propert...