Seismic Applications of Downhole DAS Lellouch, Ariel; Biondi, Biondo L
Sensors (Basel, Switzerland),
04/2021, Letnik:
21, Številka:
9
Journal Article
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Distributed Acoustic Sensing (DAS) is gaining vast popularity in the industrial and academic sectors for a variety of studies. Its spatial and temporal resolution is ever helpful, but one of the ...primary benefits of DAS is the ability to install fibers in boreholes and record seismic signals in depth. With minimal operational disruption, a continuous sampling along the trajectory of the borehole is made possible. Such resolution is highly challenging to obtain with conventional downhole tools. This review article summarizes different seismic uses, passive and active, of downhole DAS. We emphasize current DAS limitations and potential ways to overcome them.
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.
Plain Language Summary
A new seismic recording technology, called distributed acoustic sensing (DAS), turns common telecommunications fiber‐optic cables into a very long (tens of kilometers) array of single‐component seismometers. We catalog and analyze earthquake observations from three different DAS experiments and demonstrate some of the possibilities of using this technology for earthquake seismology. We compare DAS records with a conventional seismometer and then show how the array style response of DAS enables recording of additional information such as the direction of the seismic energy. We also install a fiber in a typical telecommunications conduit below Stanford University, as opposed to directly burying the fiber in soil, and show that main seismic waves are still detected despite the hypothesized decrease in coupling between the fiber and the ground.
Key Points
Distributed acoustic sensing (DAS) transforms fiber optics into dense seismic arrays (1 sensor/meter) using laser scattering
Earthquake observations indicate that DAS and seismometer sensitivity is similar at 0.8‐1.6 Hz
Fiber installed in telecommunication conduit shows sensitivity to propagating seismic waves
Low-velocity zones located deep in the subsurface can act as seismic waveguides. Traditionally, their experimental observation has been limited by the practical challenges of in situ recording. We ...use a measurement technique in which optical fibers are turned into seismic sensors. The fiber is deployed along a horizontal well drilled inside a 15-m-thin shale formation at a depth of about 2 km. Owing to the high-resolution recording of the optical fiber, we can distinctly observe three previously elusive guided wave modes over a wide frequency range. As their propagation is primarily confined to the waveguide and strongly depends on its seismic properties, such guided waves hold tremendous potential for high-resolution imaging of deep low-velocity structures, such as fault zones, saline aquifers, and hydrocarbon reservoirs.
We reviewed the literature (January 2010–June 2021) on the effectiveness of debulking strategies before venetoclax initiation in patients with chronic lymphocytic leukemia to reduce tumor burden, ...downgrade tumor lysis syndrome (TLS) risk, and avoid hospitalization. Low TLS incidence and reduced TLS risk based on tumor burden were reported following debulking in clinical trials. Real‐world observational studies reporting debulking regimens recorded no TLS events, and those without debulking strategies had greater TLS incidence. Debulking prior to venetoclax considerably reduces TLS incidence. Further clinical trials and real‐world studies may provide additional evidence on effectiveness of debulking in reducing TLS incidence and hospitalization need.
Improved management of chronic lymphocytic leukemia (CLL) has resulted in a growing population of CLL survivors; these patients have a higher risk of developing second primary malignancies (SPMs) ...versus the general population. This retrospective cohort study aims to assess the timing, frequency, incidence and types of SPMs in treated and untreated patients with CLL in the USA, using the Surveillance, Epidemiology, and End Results (SEER) Medicare database, which links a nationally representative cancer registry with Medicare claims data.
Patients aged ≥66 years with newly diagnosed CLL between 1 January 2010 and 31 December 2016, who were enrolled in Parts A and B of Medicare for ≥12 months pre-diagnosis of CLL were selected from the database. Patients were assessed for ≥36 months until the end of continuous enrollment in Medicare Parts A, B and D, a switch to a health maintenance organization, death, or end of the study period (December 2019).
Of 3053 patients included in the analyses, 620 (20.3%) were treated and 2433 (79.7%) were untreated within 36 months of diagnosis. Overall, 638 (20.9%) patients developed a SPM, 26.8% of patients in the treated cohort and 19.4% of patients in the untreated cohort. The most common SPMs for both cohorts were squamous cell carcinoma and acute myeloid leukemia. Among the 166 treated patients who developed a SPM, a greater proportion developed their first SPM after treatment initiation versus those who developed their first SPM prior to treatment initiation (p < 0.001). A significantly lower percentage of patients who received targeted therapy developed a SPM (p < 0.05) versus patients treated with anti-CD20 + chemotherapy.
Findings indicate that treatment type and timing can affect SPM development in patients with CLL. Combined with previous findings, this can help inform best practices in monitoring for SPM in patients with CLL.
With fiber-optic seismic acquisition development, continuous dense seismic monitoring is becoming increasingly more accessible. Repurposing fiber cables in telecommunication conduits makes it ...possible to run seismic studies at low cost, even in locations where traditional seismometers are not easily installed, such as urban areas. However, due to the large volume of continuous streaming data, data collected in such a manner will go to waste unless we significantly automate the processing workflow. We train a convolutional neural network (CNN) for earthquake detection using 3000 events from a publicly available catalog and data acquired over three years by fiber cables in telecommunication conduits under the Stanford University campus. We performed a hyperparameter search both on the network architecture itself (e.g., number of layers, number of parameters) and on its training parameters, showing that CNNs with a small number of layers are sufficient for performing this detection task with high accuracy. We introduce a novel method for combining the deep learning results on fiber-optic and seismometer data to improve detection accuracy, dramatically reducing the false detection rate that is often a problem when processing large time-scale noisy continuous data. Consequently, we demonstrate that enhancing two sparse seismometer stations with an urban fiber system allows for the reliable detection of small earthquakes despite a low signal-to-noise ratio. We scale this processing method over three years of continuous data and show that this system reliably detects local small-amplitude earthquakes down to magnitudes as low as 0.5, leading to the discovery of previously uncataloged events.
•We use fiber-optic cables in telecom conduits to record continuous seismic data.•We train a neural network to detect small local earthquakes with 93% accuracy.•We combine fiber-optic and seismometer data to further improve detection accuracy.•This is an extensive urban fiber seismic study with 3 years of continuous data.•Using our method, we detect small local earthquakes that were not cataloged before.
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