Mark B. Hausner

Calibrating single-ended fiber-optic Raman spectra distributed temperature sensing data.

Hydrologic research is a very demanding application of fiber-optic distributed temperature sensing (DTS) in terms of precision, accuracy and calibration. The physics behind the most frequently used DTS instruments are considered as they apply to four calibration methods for single-ended DTS installations. The new methods presented are more accurate than the instrument-calibrated data, achieving accuracies on the order of tenths of a degree root mean square error (RMSE) and mean bias. Effects of localized non-uniformities that violate the assumptions of single-ended calibration data are explored and quantified. Experimental design considerations such as selection of integration times or selection of the length of the reference sections are discussed, and the impacts of these considerations on calibrated temperatures are explored in two case studies.

Feasibility of soil moisture estimation using passive distributed temperature sensing

Through its role in the energy and water balances at the land surface, soil moisture is a key state variable in surface hydrology and land?atmosphere interactions. Point observations of soil moisture are easy to make using established methods such as time domain reflectometry and gravimetric sampling. However, monitoring large?scale variability with these techniques is logistically and economically infeasible. Here passive soil distributed temperature sensing (DTS) will be introduced as an experimental method of measuring soil moisture on the basis of DTS. Several fiber?optic cables in a vertical profile are used as thermal sensors, measuring propagation of temperature changes due to the diurnal cycle. Current technology allows these cables to be in excess of 10 km in length, and DTS equipment allows measurement of temperatures every 1 m. The passive soil DTS concept is based on the fact that soil moisture influences soil thermal properties. Therefore, observing temperature dynamics can yield information on changes in soil moisture content. Results from this preliminary study demonstrate that passive soil DTS can detect changes in thermal properties. Deriving soil moisture is complicated by the uncertainty and nonuniqueness in the relationship between thermal conductivity and soil moisture. A numerical simulation indicates that the accuracy could be improved if the depth of the cables was known with greater certainty.

Double-Ended Calibration of Fiber-Optic Raman Spectra Distributed Temperature Sensing Data

Over the past five years, Distributed Temperature Sensing (DTS) along fiber optic cables using Raman backscattering has become an important tool in the environmental sciences. Many environmental applications of DTS demand very accurate temperature measurements, with typical RMSE < 0.1 K. The aim of this paper is to describe and clarify the advantages and disadvantages of double-ended calibration to achieve such accuracy under field conditions. By measuring backscatter from both ends of the fiber optic cable, one can redress the effects of differential attenuation, as caused by bends, splices, and connectors. The methodological principles behind the double-ended calibration are presented, together with a set of practical considerations for field deployment. The results from a field experiment are presented, which show that with double-ended calibration good accuracies can be attained in the field.

Life in a fishbowl: Prospects for the endangered Devils Hole pupfish (Cyprinodon diabolis) in a changing climate

The Devils Hole pupfish (Cyprinodon diabolis) is a federally listed endangered species living solely within the confines of Devils Hole, a geothermal pool ecosystem in the Mojave Desert of the American Southwest. This unique species has suffered a significant, yet unexplained, population decline in the past two decades, with a record low survey of 35 individuals in early 2013. The species survives on a highly variable seasonal input of nutrients and has evolved in a thermal regime lethal to other pupfish species. The short lifespan of the species (approximately 1 year) makes annual recruitment in Devils Hole critical to the persistence of the species, and elevated temperatures on the shallow shelf that comprises the optimal spawning habitat in the ecosystem can significantly reduce egg viability and increase larval mortality. Here we combine computational fluid dynamic modeling and ecological analysis to investigate the timing of thresholds in the seasonal cycles of food supply and temperature. Numerical results indicate a warming climate most impacts the heat loss from the water column, resulting in warming temperatures and reduced buoyancy-driven circulation. Observed climate change is shown to have already warmed the shallow shelf, and climate change by 2050 is shown to shorten the window of optimum conditions for recruitment by as much as 2 weeks. While there are many possible reasons for the precipitous decline of this species, the changing climate of the Mojave region is shown to produce thermal and nutrient conditions likely to reduce the success of annual recruitment of young C. diabolis in the future, leading to continued threats to the survival of this unique and enigmatic species.

The shallow thermal regime of Devils Hole, Death Valley National Park

Devils Hole, a fracture in the carbonate aquifer underlying the Death Valley Regional Groundwater Flow system, is home to the only extant population of Devils Hole pupfish (Cyprinodon diabolis). Since 1995, the population of C. diabolis has shown an unexplained decline, and a number of hypotheses have been advanced to explain this. Here, we examine the thermal regime of Devils Hole and its influence on the pupfish population. We present a computational fluid dynamic (CFD) model of thermal convection on the shallow shelf of Devils Hole, which provides critical habitat for C. diabolis to spawn and forage for food. Driven by meteorological data collected at Devils Hole, the model is calibrated with temperature data recorded in the summer of 2010 and validated against temperatures observed on the shallow shelf between 1999 and 2001.The shallow shelf experiences both seasonal and diel variations in water temperature, and the model results reflect these changes. A sensitivity analysis shows that the water temperatures respond to relatively small changes in the ambient air temperature (on the order of 1 °C), and a review of local climate data shows that average annual air temperatures in the Mojave Desert have increased by up to 2 °C over the past 30 years. The CFD simulations and local climate data show that climate change may be partially responsible for the observed decline in the population of C. diabolis that began in 1995.

Heat Transfer in the Environment: Development and Use of Fiber-Optic Distributed Temperature Sensing

In the environment, heat transfer mechanisms are combined in a variety of complex ways. Solar radiation warms the atmosphere, the oceans, and the earth’s surface, driving weather and climate (Lean & Rind, 1998). Clouds and aerosols reflect a fraction of the incoming solar radiation and partially absorb the infrared radiation that comes from the earth’s surface, allowing the existence of acceptable temperatures for the biota and human survival (Norand, 1920; Moya-Larano, 2010). In water bodies, absorption and scattering of solar radiation results in stratification of the water column (Branco & Torgersen, 2009). Cooling conditions, e.g., convective night-time cooling, at the water surface can destroy the stratification and thus, mix the water column (Henderson-Sellers, 1984). In open water bodies solar radiation also induces evaporation: as water changes its phase, heat is transferred from the water body into the atmosphere by the release of latent heat (Brutsaert, 1982). Within the earth, temperature increases with depth. The temperature at the earth’s center is estimated to be on the order of 6000 °C (Alfe et al., 2002). An average geothermal gradient of 25-30 °C km-1 (Fridleifsson et al., 2008) indicates that approximately 40 TW (4 × 1013 W) flow from the earth’s interior to its surface (Sclater et al., 1981). Much of this heat is the result of radioactive decay of potassium, uranium, and thorium (Lee et al., 2009). In the shallow subsurface, this geothermal gradient can be disturbed by groundwater flow and atmospheric conditions (Uchida et al., 2003; Bense & Kooi, 2004). By measuring the temperature in the environment, it is possible to elucidate the main heat transfer mechanisms controlling different environmental, ecological, geological or engineering processes. Many of these processes span spatial scales from millimeters to kilometers. This extreme range of spatial scaling has been a barrier limiting observation, description, and modeling of these processes. In the past, temperature measurements have been performed at small scales (spanning millimeters, centimeters, or a few meters) or at large scales (spanning tens of meters or kilometers (Alpers et al., 2004)). However, for spatial scales between these two disparate scales and in a variety of media, there is a lack of

Interpreting Variations in Groundwater Flows from Repeated Distributed Thermal Perturbation Tests

To better understand the groundwater resources of southern Nye County, Nevada, a multipart distributed thermal perturbation sensing (DTPS) test was performed on a complex of three wells. These wells penetrate an alluvial aquifer that drains the Nevada National Security Site, and characterizing the hydraulic properties and flow paths of the regional groundwater flow system has proven very difficult. The well complex comprised one pumping well and two observation wells, both located 18 m from the pumping well. Using fiber-optic cables and line heaters, DTPS tests were performed under both stressed and unstressed conditions. Each test injects heat into the water column over a period of one to two days, and observes the rising temperature during heat injection and falling temperatures after heating ceases. Aquifer thermal properties are inferred from temperature patterns in the cased section of the wells, and fluxes through the 30-m screened section are estimated based on a model that incorporates conductive and advective heat fluxes. Vertical variations in flux are examined on a scale of tens of cm. The actively flowing zones of the aquifer change between the stressed and unstressed test, and anisotropy in the aquifer permeability is apparent from the changing fluxes between tests. The fluxes inferred from the DTPS tests are compared to solute tracer tests previously performed on the same site. The DTPS-based fluxes are consistent with the fastest solute transport observed in the tracer test, but appear to overestimate the mean flux through the system.

A TDR-waveform approach to estimate soil water content in electrically conductive soils

A new methodology to determine moisture content in electrically conductive soils is developed.New aspects of TDR waveform are used to estimate soil moisture.TDR measurements in electrically conductive soils can be done without modifying the sensors. Time domain reflectometry (TDR) has been widely used by the scientific community as a reliable method to indirectly measure the volumetric water content (?) of soils, and in most soils TDR can provide observations of ? at high temporal resolution with acceptable accuracy. This technique induces an electrical wave in waveguides inserted into the soil, estimates the soil bulk dielectric permittivity (e) based on an interpretation of the reflected electromagnetic signal, and then relates e with ?. In electrically conductive soils, the reflected signal can be highly attenuated by the effect of the soils bulk electrical conductivity, resulting in very large errors in the estimation of ?; the traditional TDR methodology is thus subject to large errors and uncertainties. This work presents a simple and empirical waveform interpretation methodology based on variables less sensitive to the soils electrical conductivity than those used in the traditional TDR methodology. This approach extends the applicability of TDR sensors with reliable and accurate measures of ?, making it possible to more accurately measure soil water contents in settings that have traditionally been difficult to observe, and without modifying the TDR sensors.

Identifying and Correcting Step Losses in Single-Ended Fiber-Optic Distributed Temperature Sensing Data

Fiber-optic distributed temperature sensing (DTS) makes it possible to observe temperatures on spatial scales as fine as centimeters and at frequencies up to 1 Hz. Over the past decade, fiber-optic DTS instruments have increasingly been employed to monitor environmental temperatures, from oceans to atmospheric monitoring. Because of the nature of environmental deployments, optical fibers deployed for research purposes often encounter step losses in the Raman spectra signal. Whether these phenomena occur due to cable damage or impingements, sharp bends in the deployed cable, or connections and splices, the step losses are usually not adequately addressed by the calibration routines provided by instrument manufacturers and can be overlooked in postprocessing calibration routines as well. Here we provide a method to identify and correct for the effects of step losses in raw Raman spectra data. The utility of the correction is demonstrated with case studies, including synthetic and laboratory data sets.