Measuring the Distribution of Percolation Rate in an Artificial Recharge Basin Using Fiber Optic Distributed Temperature Sensing

Transcription

1 ABSTRACT & POWERPOINT PRESENTATION Measuring the Distribution of Percolation Rate in an Artificial Recharge Basin Using Fiber Optic Distributed Temperature Sensing Matthew W. Becker, Ph.D. Professor of Geology and Conrey Chair in Hydrogeology California State University, Long Beach Long Beach, California Managed Aquifer Recharge Symposium January 25-26, 2011 Irvine, California Symposium Organizers: National Water Research Institute Orange County Water District Water Research Foundation

2 Measuring the Distribution of Percolation Rate in an Artificial Recharge Basin using Fiber Optic Distributed Temperature Sensing Matthew W. Becker, Brian Bauer, Department of Geological Sciences, California State University Long Beach, Long Beach, California 90840, It is well known that recharge (spreading) basins do not percolate water homogeneously in space or time. Variation in surficial geology can cause relative dead zones in the basin that do not contribute significantly to recharge. Over time, permeability of the basin will decrease due to siltation and biofouling. Understanding the distribution and rate of percolation can reduce costs of operation by focusing workover efforts where percolation rates are potentially high and by shutting in regions that are not contributing to recharge. We describe the development of fiber optic distributed temperature sensing (DTS) as a monitoring technique for percolation rates in recharge basins. The DTS system operates by sending a laser signal along a standard fiber optic cable. Light backscatter is proportional to temperature, allowing temperature to be measured every meter along a cable at resolutions of up to 0.02 degrees centigrade. Distances of up to 5 km can be measured along cables using commercially available sensing units. In the pilot project, a 150 meter section of a basin was monitored using a cable layered at three depths (surface, 0.3 m depth, 1 m depth). Cables at depth were trenched in and buried using a backhoe and grader. The diurnal temperature fluctuations in the surface water were monitored as they propagated downward through basin sediments with the percolating water. The percolation rate could be easily measured by fitting standard heat transport equations to the time series temperature measurements. In addition, the variation in percolation rate along the cable could be determined by examining the phase lag between the temperature oscillation in the surface water and the temperature oscillation at depth. The pilot project demonstrated that percolation rate should be accurately measured in well mixed basins using a single strand of buried cable with simultaneous measurement of surface water temperatures using an inexpensive temperature datalogger. Thus, percolation rates should be measurable along up to 5 km of cable at a capital cost of approximately $1/m of cable at each installation, in addition to the costs of the sensing unit and labor.

3 Measuring the Distribution of Percolation Rate in an Artificial Recharge Basin using Fiber Optic Distributed Temperature Sensing Matthew Becker, Brian Bauer, Department of Geological Sciences California State University Long Beach NWRI Managed Aquifer Recharge Symposium, January 24-25, 2011 Atrium Hotel Orange County

4 Heat as Tracer The heat energy of water can be used to trace recharge flow paths through space and time Not as persistent as chemical/isotope tracers, but easier to measure

5 Orange County Example Orange County, California used temperature data to track flow from spreading basins vertically and horizontally. Gsi/Water, Interrelationship of spreading basins and ground water interpreted from temperature data, Anaheim Forebay Area, Orange County, Califorina, Report Prepared for the Orange County Water District, 1997

6 Orange County Example Temperature anomaly could be tracked more than a mile down gradient from Kramer Basin Gsi/Water, Interrelationship of spreading basins and ground water interpreted from temperature data, Anaheim Forebay Area, Orange County, Califorina, Report Prepared for the Orange County Water District, 1997

7 Orange County Example Temperature anomaly could be tracked more 450 feet deep below Santa Ana off-river basin Gsi/Water, Interrelationship of spreading basins and ground water interpreted from temperature data, Anaheim Forebay Area, Orange County, Califorina, Report Prepared for the Orange County Water District, 1997

8 Orange County Example Interpretation limited by temperature accuracy and vertical resolution Temporal resolution also critical to interpretation Gsi/Water, Interrelationship of spreading basins and ground water interpreted from temperature data, Anaheim Forebay Area, Orange County, Califorina, Report Prepared for the Orange County Water District, 1997

9 Fiber Optic Distributed Temperature Sensing We are introducing new technology to the heat tracer approach DTS can be used to measure temperature at depth or in basins Installation can be made where ever a cable can be placed Long history of application in petroleum industry and more recently in stream monitoring

10 Principle of Operation Conditions along the fiber optic cable are determined by the travel time of light. Ratio Anti-Stokes / Stokes scattering provides a quantity based on the fiber temperature. Measurement for Oryx: Accuracy ~0.02 C Spatial Resolution 1m Max Distance: 5 km

11 Cable Selection Sealing Armoring Turning Radius Number of Fibers Flexibility

12 Little Anaheim Experimental Design Sensing Unit Fiber Optic Cable N 120 m

13 Installation: March 24, 2010 Shallow (left) and deep (right) trenches with cables laid out

14 Installation: March 24, 2010 Backfilling over trenches and cable

15 Installation: March 25, 2010 Fusing fiber optic cable

16 Data Collection Temperature (C) Temperatures collected between March 26 and June 1 = 67 days Temperature collected every meter every 15 or 30 minutes am April 1 5 am April 2 5 am April 3 5 am April 4 Distance Along Cable (m) Data collection reinitiated October 20.

17 Average Temperatures in Basin 20 Temperature (C) Surface Water (x= m) Shallow (x= m) Deep (x= m) Days Th ti f th bl hi h t t i d i h b l

18 Modeled Percolation Rate Temp Surface Water Measured Deep Temperature Measured Deep Temperature Modeled hrs The modeled specific discharge (percolation rate) is 10 cm/hr (8.1 feet per day)

19 Composite April 2010 Temperature deep shallow surface Low permeability Time

20 Wavelet Analysis XWT: Basin-Deep Period /2 1/4 1/8 64 1/16 1/ /

21 Wavelet Analysis Lag (Hours)

22 Other Potential Deployments Fiber optic cables can be deployed anywhere, over a length of up to 5 km. Security required for sensing box but battery operated.

23 Off River Basins: Percolation Rate / Siltation Monitoring Trench cable into bed Measure lag and attenuation in diurnal signal between surface water and bed temperature loggers cable

24 Wells: Aquifer Stratigraphy / Travel Times Cable grouted into monitoring wells Temperature measured along entire borehole every hour, every meter

25 Embankments: Seepage/Piping Detection Temperature anomaly indicates seepage and potential points of erosion. Cable can be installed in monitoring wells and/or toe of dam. Used/required in Europe on earthen dams Figure compliments of Sensornet

26 Summary Heat can be a powerful tracer of flow Fiber optic distributed temperature sensing offers unprecedented opportunity measure flow using heat tracing Cable can be deployed in a variety of environments for multiple purposes We have tested this successfully for measuring percolation rates in a spreading basin

27 Acknowledgements This work is funded by a contract between Orange County Water District and California State University Long Beach Support of Roy Herndon, Adam Hutchinson, Chris McConaughy of OCWD is greatly appreciated Michael Mondanos of Sensornet provided training and setup at Mini Anaheim.