Microstrain Resolution Research Articles

Feasibility of using in situ deformation to monitor CO2 storage

Deformation during CO2 injection can lead to problems, like seismicity or fluid leaks, but small strains have the potential to be a useful signal for monitoring. The objective of this paper is to evaluate the possible evolution of the strain field during injection, and then assess existing and emerging techniques for measuring the strain field. Poroelastic analyses show that normal strains caused by injection into a reservoir are tensile in the vicinity of the well, but everywhere else at least one strain component is compressive. The vertical strain is compressive in the confining unit, and the radial strain decreases and changes sign from tensile to compressive with distance from the well. Tilting is away from the injection well at the ground surface, but it is towards the well overlying the reservoir. Methods for measuring in-situ strain include instruments that are grouted in the annulus between casing and wall rock (∼ 0.1 microstrain resolution), portable strain sensors that are temporarily clamped to the borehole wall (∼ 0.01 microstrain resolution), and strainmeters that are grouted in place (∼0.001 microstrain resolution). Instruments for measuring in-situ normal strains at the magnitudes and rates expected during injection are emerging, but they have yet to be fully evaluated in applications related to CO2 storage. In-situ strain data measured with emerging instruments promises to fill an important gap between the episodes of fast strain rates measured by seismic data, and the slow strains measured over relatively long periods of time by InSAR and GPS.

Shallow bore-hole three-axial fiber Bragg grating strain sensor for Etna volcano monitoring.

We present the realization, installation, and first results of a three-axial Fiber Bragg Grating (FBG) strain sensor prototype. This sensor has been developed in the framework of the Mediterranean supersite volcanoes (http://www.med-suv.eu, 2013) project and, in particular, with the aim at contributing to the study and monitoring of Etna volcano. The FBG sensor was installed in the facilities of the Serra La Nave Astrophysical Observatory (Catania, Italy) about 7 km south-west from the summit craters, at an elevation of about 1740 m. The three-axial device showed a dynamic range of some hundreds of microstrains with microstrain resolution (submicrostrain concerning the vertical component). That is a good trade-off among performances, cost, and power consumption. The sensor structure and its read-out system are innovative in their assembly and offers practical advantages in comparison with traditional strain meters. As a demonstration of the performances of our device, the data of about 28 months of operation are presented together with the records of some local, regional, and teleseismic events. The sensor along the vertical axis showed to be the best performing one, having a power spectral density of about -90 dB re. 1ε2/Hz around one day period.

Open pit slope deformation monitoring by fiber Bragg grating sensors

With microstrain resolution and the capability to sample at rates of 2000 Hz or higher, fiber Bragg grating (FBG) strain sensor offers exciting new possibilities for in situ deformation monitoring induced by blasting load in an open pit slope. Here, we are developing a new technology for measuring deformation in real time on the microstrain in an open pit slope during the blasting. A fiber optically instrumented rock mass strain sensor measured strain at 100-cm intervals along a two anchor rock bolt grouted in the slope intact rock mass. In field testing, a number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the slope stability and blasting cumulative effects. Therefore, FBG sensors are a useful tool for measuring in situ strain in intact rock masses.

Noncontact Inspection Method to Determine the Transfer Length in Pretensioned Concrete Railroad Ties

The traditional experimental method to determine the transfer length in pretensioned concrete members consists of measuring concrete surface strains before and after detensioning with a mechanical strain gauge. This method is prone to significant human error and inaccuracies. In addition, because it is a time-consuming and tedious process, transfer lengths are seldom if ever measured on a production basis for product quality assurance. A rapid noncontact method for determining transfer lengths in pretensioned concrete railroad ties has been developed. The new method uses laser-speckle patterns that are generated and digitally recorded at various points along the pretensioned concrete member. A prototype was fabricated as a portable self-contained unit for field testing, which incorporates a unique modular design concept that has several preferable features. These include flexible adjustment of the gauge length, easy upgradability to automatic operation, robustness, and higher accuracy. A laser-speckle strain sensor was applied to the transfer length measurements of typical pretensioned concrete railroad ties in a railroad tie plant. These prestressed concrete tie members are expected to withstand repeated axle loadings of 290 kN, totaling 250 million gross tons annually and occurring at speeds in excess of 110 km/h. The technique achieved a microstrain resolution comparable to what could be obtained using mechanical gauge technology. Surface strain distributions were measured on both ends of 12 ties, and their associated transfer lengths were subsequently extracted. The measurements of the transfer length using the laser-speckle strain sensor were unprecedented because it was the first time that the laser-speckle technique had been applied to pretensioned concrete inspection, and particularly for use in transfer length measurements of concrete railroad ties. It was also demonstrated that the technique was able to withstand the harsh manufacturing environment, making transfer length measurements possible on a production basis for the first time.

Micromechanical testing with microstrain resolution

Simple test equipment has been developed for studying the elastic limit and plastic deformation of thin metal wires and thin foils (down to 10 μm) under torsion, tension, and bending. Using load-unload methods and gauge lengths up to 1 m, plastic strain as low as 10(-6) can be measured accurately.

High resolution Bragg edge transmission spectroscopy at pulsed neutron sources: Proof of principle experiments with a neutron counting MCP detector

The high spatial and temporal resolution of a neutron counting detector using microchannel plates (MCPs) combined with Medipix2/Timepix readout can substantially improve the spatial resolution of neutron transmission spectroscopy, as shown in our proof-of-principle experiments. Provided that the neutron fluence and data acquisition time are sufficient, transmission spectra can be acquired in each 55×55 μm 2 pixel of the detector, allowing high spatial resolution mapping of Bragg edge positions. Our first experiment demonstrates that energy resolution as high as Δ E/ E<1% or Δ E<4 mÅ can be achieved. Variation of the residual strain in a well-characterized VAMAS round robin shrink-fitted Al ring-and-plug sample was measured with ∼200 microstrain resolution through an accurate mapping of the first (1 1 1) Bragg edge. The measured stress profile agrees well with the expected values for that particular sample. More developments on the detector processing electronics are required in order to reduce the data acquisition times by enabling simultaneous measurements of spectra in a wide energy range covering multiple Bragg edges.

Rockslide deformation monitoring with fiber optic strain sensors

Abstract. With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion.

Quasi-distributed long-gauge fiber optic sensor system

This paper presents a quasi-distributed, long-gauge, sensor system for measurement optical path length variation. This system can be directly applied to long gauge strain and/or temperature sensing. The proposed sensor system is comprised of sensing fiber, which is divided into the sensor's segments separated by semi reflective mirrors made out of standard optical connectors. Short duration radio-frequency modulated optical bursts are launched into the sensing fiber and phase differences among individual reflected bursts are measured to determine the optical path-length variations among neighboring mirrors. Twenty sensing fiber segments were successfully addressed by a single-signal processor, while relying on standard telecommunication PIN diode, and a Fabry Perot laser diode. The resolution of a fiber-length variation better than 5 microm was demonstrated in practice. Since the long sections of fiber can be employed for constructing individual sensors within the sensor's array, a microstrain resolution can be achieved in practice. The drift of the sensor's system can be predominantly attributed to the temperature sensitivity of the electronic components, which proved to be below 20 microm/ degrees C. The entire system relies on simple and widely-used components that are low-cost.

Fiber Bragg sensors interrogation based on carrier generation by modulating the coupling length of a wavelength-division multiplexer

A compact all-fiber interrogation technique for Bragg sensors is presented. It is based on the generation of an electric carrier by modulating the coupling length of a wavelength-division multiplexer (WDM). Theoretical and experimental results are presented when both serrodyne and sinusoidal modulations are considered. It is shown that this technique provides static microstrain resolution in measurement intervals larger than some millistrain.

Interrogation of 60 fibre Bragg grating sensors with microstrain resolution capability

The authors report the demonstration of an instrumentation system capable of monitoring a large number of Bragg gratings using a common source and scanning narrowband filter. The system described monitors five arrays of 12 Bragg gratings sensors for a total of 60 sensor elements with µstrain resolution.

Electronic tracking system for multiplexed fibre grating sensors

The authors demonstrate for the first time an electronic lock-in system using an acousto-optic tunable filter (AOTF) to track the wavelength of a fibre Bragg grating. The change in the system output is proportional to the change in grating wavelength. Initial results show microstrain resolution at data acquisition rates of 10 Hz.

Extrinsic Fabry-Perot sensor for strain and crack opening displacement measurements from -200 to 900 degrees C

The authors present the design of a fiber optic extrinsic Fabry-Perot interferometric sensor element and its operation in several applications. The sensor has been demonstrated in a coal-fired combustor for the measurement of both thermally induced strains and crack opening displacements in ceramic materials. As a strain sensor, the device is demonstrated at temperatures ranging from -200 to 900 degrees C. It is shown that a Fabry-Perot strain gage with a one centimetre gage length can be operated in differential mode with 0.01 microstrain resolution in real-time, and in an absolute mode with 0.5 microstrain resolution with a three-second scan time.