Foreword

Groundwater science has advanced largely by measurements of parameters such as water pressure, temperature and salinity, in boreholes or wells or along the bottom of lakes and rivers. Well-established geophysical methods using electrical, electromagnetic, seismic, radar or temperature signals also contribute to understanding groundwater. These signals are collected using instruments located in airplanes, on the land surface or in boreholes.

This book is an introduction to a new field of geophysical technology that can be used in boreholes, or on the bottom of rivers or lakes to gain insight into the aquifer system. This method is known as Distributed Fiber Optic sensing where fibers are embedded in a cable to acquire measurements along the cable. The fibers are continuous strands of glass, each about the diameter of a human hair. The optical fiber is the core of a cable assemblage that can range from as small as less than 1mm diameter for shallow applications to several centimeters in diameter for deep ocean or geothermal-well installations. An apparatus (commonly referred to as an “interrogator”) connected to the cable sends and records the feedback of pulses of light (photons) along the fiber which are influenced by the temperature or the strain along the cable and in the surrounding aquifer. Hence, a continuous distribution of temperature, or strain, parameters are profiled along the cable. For example, in fractured rock, the temperature profiles over time identify the fractures with the most active groundwater flow.

For temperature, this is the equivalent of having an assemblage of many direct temperature measuring devices (e.g., thermistors) spaced at small intervals. However, the thermistors give point measurements with no data between the points and in contrast, the fiber optic cable provides values averaged over short segments of the cables and short time intervals. Typically, the distribution of temperature in groundwater is influenced substantially by groundwater flow, and by using fiber optics, the precision of the temperature values can be as small as a fraction of a degree Celsius. Therefore, the cables, which are relatively low in cost, can provide important insights into the flow conditions for which no other methods are practical. This technology has become well established in the petroleum industry over the past two decades. As the cost of the method is continually decreasing and its capabilities increase, these technologies are routinely used in research applications, and rapidly becoming common for routine aquifer monitoring. How this technology can serve groundwater science continues to expand.

The authors of this book are at the forefront of fiber optic research and have used the method in many types of applications in North America and Europe: Scott Tyler and John Selker are Professors at the University of Nevada, Reno and Oregon State University, respectively, while Tom Bogaard, Nick van de Giesen and Juan Aguilar-Bopez are professors at the Delft University of Technology, The Netherlands.

John Cherry, The Groundwater Project Leader
Guelph, Ontario, Canada, June 2022