Implementation of the Heated Pulsed Theory using actively heated fiber optics: measurements of soil volumetric water content and soil volumetric heat capacity

Existe una creciente necesidad de hacer el mejor uso del agua para regadio. Una alternativa eficiente consiste en la monitorizacion del contenido volumetrico de agua (? ), utilizando sensores de humedad. A pesar de existir una gran diversidad de sensores y tecnologias disponibles, actualmente ninguna de ellas permite obtener medidas distribuidas en perfiles verticales de un metro y en escalas laterales de 0.1-1,000 m. En este sentido, es necesario buscar tecnologias alternativas que sirvan de puente entre las medidas puntuales y las escalas intermedias. Esta tesis doctoral se basa en el uso de Fibra Optica (FO) con sistema de medida de temperatura distribuida (DTS), una tecnologia alternativa de reciente creacion que ha levantado gran expectacion en las ultimas dos decadas. Especificamente utilizamos el metodo de fibra calentada, en ingles Actively Heated Fiber Optic (AHFO), en la cual los cables de Fibra Optica se utilizan como sondas de calor mediante la aplicacion de corriente electrica a traves de la camisa de acero inoxidable, o de un conductor electrico simetricamente posicionado, envuelto, alrededor del haz de fibra optica. El uso de fibra calentada se basa en la utilizacion de la teoria de los pulsos de calor, en ingles Heated Pulsed Theory (HPP), por la cual el conductor se aproxima a una fuente de calor lineal e infinitesimal que introduce calor en el suelo. Mediante el analisis del tiempo de ocurrencia y magnitud de la respuesta termica ante un pulso de calor, es posible estimar algunas propiedades especificas del suelo, tales como el contenido de humedad, calor especifico (C) y conductividad termica. Estos parametros pueden ser estimados utilizando un sensor de temperatura adyacente a la sonda de calor [metodo simple, en ingles single heated pulsed probes (SHPP)], o a una distancia radial r [metodo doble, en ingles dual heated pulsed probes (DHPP)]. Esta tesis doctoral pretende probar la idoneidad de los sistemas de fibra optica calentada para la aplicacion de la teoria clasica de sondas calentadas. Para ello, se desarrollaran dos sistemas FO-DTS. El primero se situa en un campo agricola de La Nava de Arevalo (Avila, Espana), en el cual se aplica la teoria SHPP para estimar ? El segundo sistema se desarrolla en laboratorio y emplea la teoria DHPP para medir tanto ? como C. La teoria SHPP puede ser implementada con fibra optica calentada para obtener medidas distribuidas de ? , mediante la utilizacion de sistemas FO-DTS y el uso de curvas de calibracion especificas para cada suelo. Sin embargo, la mayoria de aplicaciones AHFO se han desarrollado exclusivamente en laboratorio utilizando medios porosos homogeneos. En esta tesis se utiliza el programa Hydrus 2D/3D para definir tales curvas de calibracion. El modelo propuesto es validado en un segmento de cable enterrado en una instalacion de fibra optica y es capaz de predecir la respuesta termica del suelo en puntos concretos de la instalacion una vez que las propiedades fisicas y termicas de este son definidas. La exactitud de la metodologia para predecir ? frente a medidas puntuales tomadas con sensores de humedad comerciales fue de 0.001 a 0.022 m3 m-3 La implementacion de la teoria DHPP con AHFO para medir C y ? suponen una oportunidad sin precedentes para aplicaciones medioambientales. En esta tesis se emplean diferentes combinaciones de cables y fuentes emisoras de calor, que se colocan en paralelo y utilizan un rango variado de espaciamientos, todo ello en el laboratorio. La amplitud de la senal y el tiempo de llegada se han observado como funciones del calor especifico del suelo. Medidas de C, utilizando esta metodologia y ante un rango variado de contenidos de humedad, sugirieron la idoneidad del metodo, aunque tambien se observaron importantes errores en contenidos bajos de humedad de hasta un 22%. La mejora del metodo requerira otros modelos mas precisos que tengan en cuenta el diametro del cable, asi como la posible influencia termica del mismo. ABSTRACT There is an increasing need to make the most efficient use of water for irrigation. A good approach to make irrigation as efficient as possible is to monitor soil water content (? ) using soil moisture sensors. Although, there is a broad range of different sensors and technologies, currently, none of them can practically and accurately provide vertical and lateral moisture profiles spanning 0-1 m depth and 0.1-1,000 m lateral scales. In this regard, further research to fulfill the intermediate scale and to bridge single-point measurement with the broaden scales is still needed. This dissertation is based on the use of Fiber Optics with Distributed Temperature Sensing (FO-DTS), a novel approach which has been receiving growing interest in the last two decades. Specifically, we employ the so called Actively Heated Fiber Optic (AHFO) method, in which FO cables are employed as heat probe conductors by applying electricity to the stainless steel armoring jacket or an added conductor symmetrically positioned (wrapped) about the FO cable. AHFO is based on the classic Heated Pulsed Theory (HPP) which usually employs a heat probe conductor that approximates to an infinite line heat source which injects heat into the soil. Observation of the timing and magnitude of the thermal response to the energy input provide enough information to derive certain specific soil thermal characteristics such as the soil heat capacity, soil thermal conductivity or soil water content. These parameters can be estimated by capturing the soil thermal response (using a thermal sensor) adjacent to the heat source (the heating and the thermal sources are mounted together in the so called single heated pulsed probe (SHPP)), or separated at a certain distance, r (dual heated pulsed method (DHPP) This dissertation aims to test the feasibility of heated fiber optics to implement the HPP theory. Specifically, we focus on measuring soil water content (? ) and soil heat capacity (C) by employing two types of FO-DTS systems. The first one is located in an agricultural field in La Nava de Arevalo (Avila, Spain) and employ the SHPP theory to estimate ? The second one is developed in the laboratory using the procedures described in the DHPP theory, and focuses on estimating both C and ? The SHPP theory can be implemented with actively heated fiber optics (AHFO) to obtain distributed measurements of soil water content (? ) by using reported soil thermal responses in Distributed Temperature Sensing (DTS) and with a soil-specific calibration relationship. However, most reported AHFO applications have been calibrated under laboratory homogeneous soil conditions, while inexpensive efficient calibration procedures useful in heterogeneous soils are lacking. In this PhD thesis, we employ the Hydrus 2D/3D code to define these soil-specific calibration curves. The model is then validated at a selected FO transect of the DTS installation. The model was able to predict the soil thermal response at specific locations of the fiber optic cable once the surrounding soil hydraulic and thermal properties were known. Results using electromagnetic moisture sensors at the same specific locations demonstrate the feasibility of the model to detect ? within an accuracy of 0.001 to 0.022 m3 m-3. Implementation of the Dual Heated Pulsed Probe (DPHP) theory for measurement of volumetric heat capacity (C) and water content (? ) with Distributed Temperature Sensing (DTS) heated fiber optic (FO) systems presents an unprecedented opportunity for environmental monitoring. We test the method using different combinations of FO cables and heat sources at a range of spacings in a laboratory setting. The amplitude and phase-shift in the heat signal with distance was found to be a function of the soil volumetric heat capacity (referred, here, to as Cs). Estimations of Cs at a range of ? suggest feasibility via responsiveness to the changes in ? (we observed a linear relationship in all FO combinations), though observed bias with decreasing soil water contents (up to 22%) was also reported. Optimization will require further models to account for the finite radius and thermal influence of the FO cables, employed here as “needle probes”. Also, consideration of the range of soil conditions and cable spacing and jacket configurations, suggested here to be valuable subjects of further study and development.