Author(s): Lindsey, NJ; Martin, ER; Dreger, DS; Freifeld, B; Cole, S; James, SR; Biondi, BL; Ajo-Franklin, JB | Abstract: 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 20nm of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0.8–1.6nHz. 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.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2017GL075722

Fiber‐Optic Network Observations of Earthquake Wavefields

There is an ever-growing demand for measuring respiratory variables during a variety of applications, including monitoring in clinical and occupational settings, and during sporting activities and exercise. Special attention is devoted to the monitoring of respiratory rate because it is a vital sign, which responds to a variety of stressors. There are different methods for measuring respiratory rate, which can be classed as contact-based or contactless. The present paper provides an overview of the currently available contact-based methods for measuring respiratory rate. For these methods, the sensing element (or part of the instrument containing it) is attached to the subject’s body. Methods based upon the recording of respiratory airflow, sounds, air temperature, air humidity, air components, chest wall movements, and modulation of the cardiac activity are presented. Working principles, metrological characteristics, and applications in the respiratory monitoring field are presented to explore potential development and applicability for each method.

Contact-Based Methods for Measuring Respiratory Rate

The ability to measure tiny variations in the local gravitational acceleration allows, besides other applications, the detection of hidden hydrocarbon reserves, magma build-up before volcanic eruptions, and subterranean tunnels. Several technologies are available that achieve the sensitivities required for such applications (tens of microgal per hertz1/2): free-fall gravimeters1, spring-based gravimeters1, 3, superconducting gravimeters4, and atom interferometers5. All of these devices can observe the Earth tides6: the elastic deformation of the Earth’s crust as a result of tidal forces. This is a universally predictable gravitational signal that requires both high sensitivity and high stability over timescales of several days to measure. All present gravimeters, however, have limitations of high cost (more than 100,000 US dollars) and high mass (more than 8 kilograms). Here we present a microelectromechanical system (MEMS) device with a sensitivity of 40 microgal per hertz1/2 only a few cubic centimetres in size. We use it to measure the Earth tides, revealing the long-term stability of our instrument compared to any other MEMS device. MEMS accelerometers—found in most smart phones7—can be mass-produced remarkably cheaply, but none are stable enough to be called a gravimeter. Our device has thus made the transition from accelerometer to gravimeter. The small size and low cost of this MEMS gravimeter suggests many applications in gravity mapping. For example, it could be mounted on a drone instead of low-flying aircraft for distributed land surveying and exploration, deployed to monitor volcanoes, or built into multi-pixel density-contrast imaging arrays.

/pdf/measurement-of-the-earth-tides-with-a-mems-gravimeter-1836f6onmg.pdf

Measurement of the Earth tides with a MEMS gravimeter

MEMS (Micro Electro-Mechanical Systems) sensors enable a vast range of applications: among others, the use of MEMS accelerometers for seismology related applications has been emerging considerably in the last decade In this paper, we provide a comprehensive review of the capacitive MEMS accelerometers: from the physical functioning principles, to the details of the technical precautions, and to the manufacturing procedures We introduce the applications within seismology and earth sciences related disciplines, namely: earthquake observation and seismological studies, seismic surveying and imaging, structural health monitoring of buildings Moreover, we describe how the use of the miniaturized technologies is revolutionizing these fields and we present some cutting edge applications that, in the very last years, are taking advantage from the use of MEMS sensors, such as rotational seismology and gravity measurements In a ten-year outlook, the capability of MEMS sensors will certainly improve through the optimization of existing technologies, the development of new materials, and the implementation of innovative production processes In particular, the next generation of MEMS seismometers could be capable of reaching a noise floor under the lower seismic noise (few tenths of ng/ Hz ) and expanding the bandwidth towards lower frequencies (∼001 Hz)

A Review of the Capacitive MEMS for Seismology.

Precise measurement of variations in the local gravitational acceleration is valuable for natural hazard forecasting, prospecting, and geophysical studies. Common issues of the present gravimetry technologies include their high cost, high mass, and large volume, which can potentially be solved by micro-electromechanical-system (MEMS) technology. However, the reported MEMS gravimeter does not have a high sensitivity and a large dynamic range comparable with those of the present commercial gravimeters, lowering its practicability and ruling out worldwide deployment. In this paper, we introduce a more practical MEMS gravimeter that has a higher sensitivity of 8 μGal/√Hz and a larger dynamic range of 8000 mGal by using an advanced suspension design and a customized optical displacement transducer. The proposed MEMS gravimeter has performed the co-site earth tides measurement with a commercial superconducting gravimeter GWR iGrav with the results showing a correlation coefficient of 0.91. Researchers in China have developed a small, portable gravimeter based on micro-electromechanical-system (MEMS) technology with a sensitivity and dynamic range comparable with larger, commercially available gravimeters. The new device was created by a team led by Liangcheng Tu at Huazhong University of Science and Technology. The MEMS mechanism consists of a spring-mass system designed around a combination of curved and folded beams. Together, these ensure that the system is stiff under low loads but flexible at loads around 1 g. An optical component measures the displacement of the proof mass to measure gravitational acceleration. The team tested their design alongside a commercial gravimeter and found a 90% correlation in the measurements of Earth tides, demonstrating its utility for applications from oil and gas exploration to hazard detection.

/pdf/a-high-sensitivity-mems-gravimeter-with-a-large-dynamic-47whg50zuv.pdf

A high-sensitivity MEMS gravimeter with a large dynamic range.

A new generation of accelerometers based on a microelectromechanical system (MEMS) can deliver broadband (0 to 800 Hz) and high-fidelity measurements of ground motion even at a low level. Such performance has been obtained by using a closed-loop configuration and a careful design which greatly mitigates all internal mechanical and electronic noise sources. To improve the signal-to-noise ratio toward the low frequencies at which instrument noise increases, a new MEMS has been developed. It provides a significantly lower noise floor (at least −10 dB) and thus a higher dynamic range (+10 dB). This performance has been tested in an underground facility where conditions approach the minimum terrestrial noise level. This new MEMS sensor will even further improve the detection of low frequencies and of weak signals such as those that come from deep targets or from microseismic events.

Jérôme Laine

Papers

A high-sensitivity MEMS-based accelerometer