Frederick Boudin

Inferring field-scale properties of a fractured aquifer from ground surface deformation during a well test

Fractured aquifers which bear valuable water resources are often difficult to characterize with classical hydrogeological tools due to their intrinsic heterogeneities. Here, we implement ground surface deformation tools (tiltmetry and optical leveling) to monitor groundwater pressure changes induced by a classical hydraulic test at the Ploemeur observatory. By jointly analyzing complementary time constraining data (tilt) and spatially constraining data (vertical displacement), our results strongly suggest that the use of these surface deformation observations allows for estimating storativity and structural properties (dip, root depth, lateral extension) of a large hydraulically active fracture, in good agreement with previous studies. Hence, we demonstrate that ground surface deformation is a useful addition to traditional hydrogeological techniques and opens possibilities for characterizing important large-scale properties of fractured aquifers with short-term well tests as a controlled forcing.

Dual-modulation fiber Fabry-Perot interferometer with double reflection for slowly-varying displacements.

This Letter describes a dual-amplitude modulation technique incorporated into a double reflection extrinsic-type fiber Fabry-Perot interferometer to measure periodic, nonperiodic as well as quasi-static displacements. The modulation scheme simultaneously maintains the interference signal pair in quadrature and provides a reference signal for displacements inferior to a quarter of the source wavelength. The control and phase demodulation of the interferometer carried out via software enable quasi-real-time measurement and facilitates sensor alignment. The sensor system can be exploited in the low frequency range from 10(-3) to ∼500 Hz and has a resolution better than 2.2 nm, targeting applications in geophysics.

Amplitude and Phase Drift Correction of EFPI Sensor Systems Using Both Adaptive Kalman Filter and Temperature Compensation for Nanometric Displacement Estimation

Nanometric displacement measurements by Extrinsic Fiber Fabry-Perot interferometers (EFPI) is extremely susceptible to external environmental changes. Temperature, in particular, has a remarkable influence on the optical power and wavelength of the laser diode in use, in addition to the thermal expansion of the mechanical structure. In this paper we propose an optimization of the EFPI sensor in order to use it for very long-term (more than one year) and for high-precision displacement measurements. For this purpose, a real time and adaptive estimation procedure based on a homodyne technique and a Kalman filter is established. During a sinusoidal laser diode current modulation, the Kalman filter provides a correction of the amplitude drift caused by the resultant optical power modulation and external perturbations. Besides, stationary temperature transfer operators are estimated via experimental measurements to reduce the additive thermal noise induced in the optical phase and mechanical components.

Physical Modelling To Remove Hydrological Effects At Local And Regional Scale: Application To The 100-M Hydrostatic Inclinometer In Sainte-Croix-Aux-Mines (France)

New inclinometers devoted to hydrological studies were set up in the Vosges Mountains (France). Two orthogonal 100-m base hydrostatic inclinometers were installed in December 2004 as well as a hydrometeorological monitoring system for the 100-km2 hydrological unit around the inclinometer. As inclinometers are very sensitive to environmental influences, this observatory is a test site to confront hydrological modelling and geodetic observations. Physical modelling to remove hydrological effects without calibrating on geodetic data is tested on these instruments. Specifically, two deformation processes are most important: fluid pressure variations in nearby hydraulically active fractures and surface loading at regional scale

A phase-tracking fiber interferometer for seismologic applications

A fiber Fabry-Pe acuterot interferometer is proposed for high precision seismic wave detection. Simple polarization optics are employed in the sensor head design to generate two inter-dependent quadrature phase-shifted interference fringes, with the added advantage of reducing/suppressing signal attenuation due to polarization scrambling in the fiber system. Interference signal demodulation exploits an algorithm which optimizes the phase calculation on the alternatively evolving quadrature pair as well as enables phase compensation for out-of-quadrature signals. Vibration measurements carried out concurrently with a seismometer under free oscillating conditions show excellent agreement. Differences obtained from both instruments in displacement amplitude and velocity amounted to ~50 nm and 5 nm/ms, respectively.

Understanding the Hydromechanical Behavior of a Fault Zone From Transient Surface Tilt and Fluid Pressure Observations at Hourly Time Scales

Flow through reservoirs such as fractured media is powered by head gradients which also generate measurable poroelastic deformation of the rock body. The combined analysis of surface deformation and subsurface pressure provides valuable insights of a reservoirs structure and hydromechan-ical properties, which are of interest for deep-seated CO 2 or nuclear waste storage for instance. Among all surveying tools, surface tiltmeters offer the possibility to grasp hydraulically induced deformations over a broad range of time scales with a remarkable precision. Here we investigate the information content of transient surface tilt generated by the pressurization a kilometer scale subvertical fault zone. Our approach involves the combination of field data and results of a fully coupled poromechanical model. The signature of pressure changes in the fault zone due to pumping cycles is clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in (1) tilt time series alone from a set of four instruments and 2) the ratio of tilt over pressure. We evidence that the shape of tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. The ratio of tilt over pressure predominantly encompasses information about the systems dynamic behavior and extent of the fault zone and allows separating contributions of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes associated with fault zone hydrogeology at short time scales, where spaceborne surveying methods fail to recognize any deformation signal.

A fiber Fabry-Perot interferometer for geophysics applications

A fiber interferometer interfaced to 3 geo-mechanical elements is presented for applications in geophysics. The fiber sensor is based on an extrinsic fiber Fabry-Perot interferometer (EFFPI) which incorporates a modulation scheme to lock the interferometer at quadrature and to enable displacement measurements below a quarter of the interrogating wavelength. It operates over a relatively large frequency dynamic of ~500000 with a precision better than 2 nm. The fiber interferometer is next interfaced to a differential hydrostatic long baseline inclinometer, a 3-axis borehole tiltmeter and a single-axis seismometer, respectively. Results obtained demonstrate that the fiber interferometrically-interrogated instruments exhibit performances equivalent to or even surpassing those of the reference instruments employed for comparison during their deployment to an underground test site since March 2012.