Measuring aeolian sand transport using acoustic sensors Ate Poortinga a,⇑ , Hans van Rheenen b , Jean T. Ellis c , Douglas J. Sherman d a Soil Physics and Land Management Group, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands b Eijkelkamp Agrisearch Equipment, Postbus 4, 6987 ZG Giesbeek, The Netherlands c Marine Science Program and Department of Geography, University of South Carolina, Columbia, SC 29208, United States d Department of Geography, University of Alabama, Tuscaloosa, AL 35487, United States article info Article history: Received 2 October 2014 Revised 31 December 2014 Accepted 31 December 2014 Available online 23 January 2015 Keywords: Field measurements Aeolian sand transport Saltation Saltiphone Miniphone Laboratory experiment abstract Acoustic sensors are frequently used to measure aeolian saltation. Different approaches are used to pro- cess the signals from these instruments. The goal of this paper is to describe and discuss a method to measure aeolian saltation with acoustic sensors. In a laboratory experiment, we measured the output from an advanced signal processing scheme on the circuit board of the saltiphone. We use a software implementation of this processing scheme to re-analyse data from four miniphones obtained during a field experiment. It is shown that a set of filters remove background noise outside the frequency spectrum of aeolian saltation (at 8 kHz), whereas signals within this frequency spectrum are amplified. The resulting analogue signal is a proxy of the energy. Using an AC pulse convertor, this signal can be converted into a digital and analogue count signal or an analogue energy signal, using a rectifier and integrator. Spatio-temporal correlation between field deployed miniphones increases by using longer integration times for signal processing. To quantify aeolian grain impact, it is suggested to use the ana- logue energy output, as this mode is able to detect changes in frequency and amplitude. The analogue and digital count signals are able to detect an increase in frequency, but are not able to detect an increase in signal amplitude. We propose a two-stage calibration scheme consisting of (1) a factory calibration, to set the frequency spectrum of the sensor and (2) a standardized drop-test conducted before and after the experiment to evaluate the response of the sensor. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Aeolian research aimed at understanding the processes associ- ated with saltation has advanced rapidly over the last decade, in part as a consequence of substantial improvement in the instru- ments available for field experiments. The ability to measure the wind near the sand surface, especially, has improved greatly with the adoption of ultrasonic anemometry (e.g. Van Boxel et al., 2004; Walker, 2005). Until recently, however, instruments and methods to measure characteristics of sand flux have remained disproportionately unsophisticated. A number of recent articles and discussion papers have been published on the use of acoustic sensors in aeolian research. Several (Poortinga et al., 2013; Yurk et al., 2013; Schönfeldt, 2012) demonstrate that acoustic sensors are suitable for the measurement of aeolian mass fluxes, although the work of Ellis et al. (2009) and Sherman et al. (2011) show some discrepancies between the output of the acoustic sensors and coin- cidental measurements of sand transport. Electronic saltation sensing instruments operate from one of three physical bases: acoustic detection; piezoelectric detection; or optical detection. These types of sensors have been reviewed and discussed extensively elsewhere (e.g. Davidson-Arnott et al., 2009; Van Pelt et al., 2009; Barchyn and Hugenholtz, 2010; Hugenholtz and Barchyn, 2011; Sherman et al., 2011) and we focus, therefore, mainly on acoustic sensors. The use of acoustic sensors in aeolian research dates back to Spaan and Van den Abeele (1991), who designed and tested a microphone-based device called the saltiphone. The microphone responded to the impact of saltating grains that compressed its diaphragm, thereby generating an acoustic signal that could then be digitally recorded. A large number of studies have relied on saltiphone technology. These studies were conducted in a variety of aeolian settings, including coastal environments (e.g. Arens, 1996, 1997; van der Wal, 2000; Schönfeldt and von Lowis, 2003; Poortinga et al., 2014), (semi-) arid regions (Mei et al., 2006; Visser et al., 2005; Youssef et al., 2012; Visser et al., 2004; Leenders et al., 2005), nature reserves (Riksen and Goossens, 2007), as well as wind tun- nels (e.g. Van Pelt et al., 2009; Goossens et al., 2000; Youssef et al., 2012). Similar custom built microphone systems have been http://dx.doi.org/10.1016/j.aeolia.2014.12.003 1875-9637/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: ate.poortinga@wur.nl (A. Poortinga). Aeolian Research 16 (2015) 143–151 Contents lists available at ScienceDirect Aeolian Research journal homepage: www.elsevier.com/locate/aeolia