1 Introduction

For the past several decades, earth scientists have been part of a sensing revolution. Driven by advances in miniaturization, computing power, solid-state physics and efficiencies in power requirements, almost every tool available to hydrogeologists has become smaller, faster, and more efficient. We can measure and retrieve data with higher resolution in space and time at a lower cost than ever before, allowing us to image and monitor the subsurface at ever finer and faster scales.

Still, many sensors including hydrogeophysical sensors, are essentially point sensors, which measure a quantity at a single point (or support volume). Only by distributing hundreds of sensors or making creative use of a limited number of sensors, such as in electric resistance tomography (ERT), can higher spatial resolution be obtained. This limitation is rapidly being erased by distributed sensing on optical fibers (e.g., Selker et al., 2006a; Tyler et al., 2009; Bense et al., 2016; Schenato, 2017). In hydrology, and most sciences, when it is possible to measure at higher spatial and temporal scales, the underlying processes, such as groundwater inflow into a stream, become much clearer.

Distributed sensing measures at sub-meter resolution on optical fibers and makes use of the continuously changing (in both space and time) properties of light transmission and scattering along tens of kilometers of optical fiber. While optical fibers have been used for decades for transmitting data and sensing at discrete points (where either the light within the fiber is directed outward, or where the light is directed through a sensor), continuous-in-space monitoring has only recently become possible and practical. In the next sections, we describe the theory, operation and example applications of distributed temperature, strain and strain rate applications in hydrogeology and hydrogeophysics. The book concludes with a few thoughts on the future and where it may take us.