Arthur H. Hartog

A distributed temperature sensor based on liquid-core optical fibers

The principles of operation, the design, and performance of a fiber-optic temperature-distribution sensor are discussed. The sensor uses optical time-domain reflectometry (OTDR) to detect temperature-induced changes of backscatter power at many separate locations in the fiber. In liquid-core fibers, a sensitivity of2.3 \times 10^{-2}dB/°C (0.54 percent° C-1) was observed. A measurement accuracy of 1°C with a spatial resoltuion of 1 m is attainable over a fiber length of 100 m.

On the theory of backscattering in single-mode optical fibers

A new theory of backscattering in single-mode fibers is presented. It allows backscatter waveforms to be predicted for fibers of any refractive-index profile or scattering-loss distribution. The results agree with experimental data and provide confirmation of an earlier, more restricted theory.

Vertical seismic optical profiling on wireline logging cable

Vertical seismic profiles are usually acquired by deploying downhole seismic sensors below a wireline logging cable. A seismic source is triggered at surface while recording the downhole vibration via the wireline cable. In this paper, an alternative approach based on distributed vibration measurement is tested using wireline deployment for the first time. Local axial strain of a multi-kilometre fibre optic line is measured at intervals of approximately 1 m and processed to a spatial resolution of 10 m with sub nanometer strain resolution by an optical interrogation device (distributed vibration/acoustic sensing). When deployed in a well, the optical fibre line should be mechanically coupled to the borehole wall to generate valid seismic records. A conventional vertical seismic profile was acquired with three-component sensors in a vertical well near Bottesford, UK. The impulsive seismic source was a novel portable airgun tank. Clear seismic reflections are observed within and below the borehole, in agreement with surface seismic data. A single shot generated equivalent data with an experimental optical wireline logging cable and an adequate optical interrogator at the surface. The main difference between the two records is the presence of a strong tube wave in the optical profile, which can be easily removed with conventional velocity filter processing. Corridor stacks from both conventional and optical profiles match each other and provide a reasonable tie to a nearby surface seismic line.

The Optics of Distributed Vibration Sensing

In distributed vibration sensing (DVS) - also known as distributed acoustic sensing (DAS) technology, an optical fibre is deployed in the borehole to be surveyed and is used to detect seismic waves originating from a source outside the well. Although the EAGE and SEG literature describes the results of using DVS for borehole seismic applications, essentially nothing has been written about the underlying optical technology in the geophysics technical literature. This paper therefore outlines the main methods that are probably used by the main participants in the DVS activity, insofar as this can be deduced from publications in the field of fibre optics and the patent literature.

Fundamentals of Optical Fiber Sensing Schemes Based on Coherent Optical Time Domain Reflectometry: Signal Model Under Static Fiber Conditions

The paper develops a statistical model for the signals received in phase-sensitive optical time domain reflectometry (OTDR) probed by highly coherent sources. The backscattering process is modelled by a set of discrete scatterers with properly chosen parameters. Explicit equations for calculating the amplitude and the phase of the backscattered signal are obtained. The developed model predicts spectral and autocorrelation characteristics of the amplitude signals that are validated by experimental results. Characteristics of the phase signals, practicable for studying the sensing applications of the OTDR system, are presented and studied as well, demonstrating good correspondence with experiment. A more detailed modelling of distributed vibration sensing systems and their response to disturbances along an optical fiber will be possible as an extension of the developed formalism.

Rayleigh backscattering from the fundamental mode in multimode optical fibers

Rayleigh backscattering produced by an incident fundamental mode in a multimode optical fiber is analyzed using a diffraction technique, with a full set of backward-propagating modes being taken into consideration. Explicit formulas are derived for mode excitation efficiency via radial distributions of the mode fields, and it is proved that only half-azimuthal modes are backscattered by the incident wave of a fixed polarization. Advanced analytical expressions are developed for fibers with a quadratic refractive-index profile, and mode groups of even numbers, composed of modes with equal propagation constants, are stated to be excited with equal efficiencies.

Distributed vibration sensing on optical fibre: field testing in borehole seismic applications

We describe the measurement of seismic waves in a borehole using distributed vibration sensing conveyed on wireline cable. The optical measurement is compared directly with the results of a multi-level borehole seismic survey with conventional electrical accelerometers.

A study of the geophysical response of distributed fibre optic acoustic sensors through laboratory‐scale experiments

ABSTRACT In the past few years, distributed acoustic sensing has gained great interest in geophysics. This acquisition technology offers immense improvement in terms of efficiency when compared with current geophysical acquisition methods. However, the fundamentals of the measurement are still not fully understood because direct comparisons of fibre data with conventional geophysical sensors are difficult during field tests. We present downscaled laboratory experiments that enabled us to characterise the relationship between the signals recorded by conventional seismic point receivers and by distributed fibre optic sensors. Interrogation of the distributed optical fibre sensor was performed with a Michelson interferometer because this system is suited to compact test configurations, and it requires only a very simple data processing workflow for extracting the signal outputs. We show acoustic data that were recorded simultaneously by both the fibre optical interferometer and conventional three‐component accelerometers, thus enabling the comparison of sensor performance. We present results focused on the directionality of fibre measurements, on the amplitude variation with angle of incidence, and on the transfer function that allows accelerometer signals to be transformed into optical fibre signals. We conclude that the optical fibre response matches with the array of the displacement differences of the inline accelerometers deployed along the fibre length. Moreover, we also analysed the influence of various types of coupling and fibre cable coating on the signal responses, emphasising the importance of these parameters for field seismic acquisitions when using the distributed fibre optic technology.

An Integrated System for Pipeline Condition Monitoring

Pipelines are at risk from external threats including third-party intrusion, societal development and ground movement in addition to the ongoing potential for leaks due to corrosion or other causes. Although the appropriate use of internal inspection practices and regular survey of coating and cathodic protection systems will give timely information on any deterioration in pipeline condition, external threats can be unpredictable.

Transformation of modal power distribution in multimode fiber with abrupt inhomogeneities

A transformation operator is constructed for the calculation of the modal power distribution changes caused by abrupt inhomogeneity in multimode fibers. Simplification and practical applicability of the analysis are achieved by using the mode-continuum approximation. Thus, the model enables taking into account the fiber inhomogeneities of the following types: fiber lateral offset, core diameter mismatch, numerical aperture difference, and joints of fibers with different index profiles. The admission of an arbitrary refractive index profile, input modal power distribution, and parameters of fiber mismatch is an advantageous feature of the model. The operator is applied to calculations of power loss at joints between different multimode fibers, and the results of the calculations are shown to be in a good agreement with experimental data.