Agilent Measurement Journal - Issue 3 - 2007 - (Page 69)

Basics of DTS operation C Climate change is a story that begins with temperature and often ends with water — melting glaciers, rising sea levels, storms and regional stresses on freshwater sources. Remote sensing from satellites provides the big picture, but a regional understanding of the impact of environmental change requires detailed measurements on the ground. One key measurement of a regional environment is “streamﬂow,” which hydrologists deﬁne as the movement of water in a natural channel. Lakes and rainwater runoff are obvious places to start when looking for sources of streamflows. However, most of the available freshwater exists not on the surface but in the ground, and underwater springs, which are a signiﬁcant contributor to streamﬂow, are currently not well measured or modeled. Locating and gauging these groundwater inputs requires measurements able to cover several kilometers with resolution as ﬁne as one meter. Traditional “point measurement” instruments used in hydrology cannot handle this challenge; however, new distributed measurement technologies that use ﬁber optic temperature sensors can provide the required reach and resolution. These sensors quickly measure temperature over a few kilometers with resolution down to a meter, and this temperature data can be used to uncover the groundwater interactions with stream water. However, instruments developed for in situ environmental measurements must also be ﬁeld deployable, energy efﬁcient (typically operating from batteries or solar cells) and pest-proof. The Agilent N4386A distributed temperature system (DTS) uses ﬁber optics to meet these measurement challenges. The DTS is used in a wide range of applications: downhole oil and gas reservoir performance monitoring; power cable monitoring; pipeline and water dam leakage detection; and in security applications such as ﬁre detection in tunnels, reﬁneries or other special-hazard applications. Geophysical scientists are also using this instrument to address a variety of hydrological applications. This article highlights one such project, done in collaboration with Oregon State University, Corvallis, Oregon to measure the connection between surface streamﬂows and subsurface water sources. The DTS uses light to measure temperature. It starts each measurement by launching a pulse of light from a semiconductor laser into a standard communications optical ﬁber and then measuring the backscattered light to ﬁrst determine time-of-ﬂight and then position. Three important scattering mechanisms are present in an optical ﬁber: Rayleigh, Brillouin and Raman. The Agilent DTS uses the spontaneous Raman scattering signal and measures changes in the intensity at the Stokes line, which is temperature-dependent, and the Anti-Stokes line, which is mostly temperature independent. The temperature is then computed from the ratio of these two lines after performing a ﬁber-dependent calibration procedure. The basic relation is written as I AS IS exp () h kT R where h is the Planck constant, k is the Boltzmann constant, T is the absolute temperature and DnR is the separation between Raman Anti-Stokes/Stokes and probe-light frequencies. In the DTS, temperature resolution varies with distance, spatial resolution and temporal averaging. A total of 8000 measurement points can be acquired during every averaging period. This allows spatial resolution down to 1.5 m for measurement spans of up to 12 km and down to 1 m for distances up to 8 km. For example, a temperature resolution of 0.11º C is possible at a distance of 2 km (1.5 m spatial resolution) with 10 minutes of temporal averaging. Reducing the temporal averaging to thirty seconds decreases the temperature precision to 0.35º C. During set up and calibration in the ﬁeld, temperature accuracy (along with precision) is typically checked with a few independent single-point temperature measurements. A two- or four-channel DTS offers additional capability for dual-ended or loop-back measurements. The ongoing auto-calibration present in a dual-ended measurement further simpliﬁes the calibration and improves measurement accuracy. Agilent Measurement Journal 69

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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