Laboratory Calibration of a Magnetic Bed Load Movement Detector Jason Rempel 1 , Marwan A. Hassan 1 , and Randy Enkin 2 1 Department of Geography, The University of British Columbia, Vancouver, British Columbia, Canada, 2 Geological Survey of Canada, Sidney, British Columbia, Canada. Abstract A series of laboratory experiments was conducted to test and calibrate the Bedload Movement Detector (BMD), a magnetic system for measuring coarse bedload movement in gravel bed rivers. Empirical relations were derived between the amplitude, width and integral of the sensor response, and particle size, magnetic content and velocity. Because of high variability in magnetic field strength across the sensor face, the system is highly sensitive to particle trajectory; therefore the present design of the BMD system cannot be used to reliably predict the particle size from an individual signal. Introduction Magnetic detection systems are designed to track the movement of either artificially tagged, or naturally magnetic coarse particles (for review see Bunte and Ergenzinger, 1989; Hassan and Ergenzinger, 2003). The underlying principle is that when a magnet passes over an iron-cored coil of wire (an inductor), a measurable electronic pulse is generated. The first known system of this kind was built by Ergenzinger and Conrady (1982). They inserted magnets into pebbles, and a magnetic detector was used to monitor their passage. The second, a similar but more advanced system developed by Ergenzinger and Conrady, was used to detect the passage of naturally magnetic cobbles and pebbles past a fixed point during flow events in Squaw Creek, Montana (Ergenzinger and Custer 1983; Custer et al., 1987, Bunte, 1996). It was estimated that the system was sensitive enough to detect 40% of the coarse material (>32 mm) in Squaw Creek. The passage of particles was recorded on a strip chart recorder. This made data analysis time consuming, and limited the resolution of the system to approximately 200 particles per hour (Spieker and Ergenzinger, 1990; Bunte, 1996). The third magnetic system was developed by Reid et al. (1984). Their system consisted of two elongated unscreened coils; the sensors were fully balanced over the entire width of the channel. The passage of the particles over the sensor distorted the magnetic field and produced a change in the inductance of the coils. To avoid double registration of tracers and the influence of particles settling on or very close to the system, a self balancing system was built into the circuit that tuned out the influence of such particles after a predetermined time interval. The system operated automatically and was Published online in 2010 as part of U.S. Geological Survey Scientific Investigations Report 2010-5091. 400