SMARTPHONE GEOPHYSICS A&G • December 2017 • Vol. 58 • aandg.org 6.35 G eophysics is fun to teach because of the many real-world examples that students can relate to. However, it is diffcult to put good geophysics experi- ments together without expensive gadgetry and technical support, and many teachers will have thought, “Darn! I wish we could do that in class…” We remember when we were students ourselves, walking around during lab ses- sions to fnd a free “station” to do an activ- ity, because there were limited – often only one – pieces of equipment. We were also frustrated because we couldn’t do things requiring greater technology. For instance, we measured the speed of P-waves in the Earth’s crust using printed seismic sections from studies published by other people, giving little insight into the nuts and bolts of the actual data collection and record- ing process. Now, as teachers, many of us share the same thoughts: it would be great to have multiple sets of instruments and to be able to do something fancier in lab time. But until recently, getting enough GPS units, seismometers with digitizing units, gravimeters and digital thermometers for a lab required a pot of money. One solution is to use smartphones. Students are already familiar – perhaps unduly so – with their smartphones. They can be used to do homework or make basic measurements (e.g. Armstrong 2014) that improve comprehension and motivate active learning (Bromley 2012). But they also have built-in instruments that can make useful measurements for geo- sciences: measurements that have been too expensive or simply impossible in the lab. Built-in GPS units, accelerometers and ther- mometers are used for phone operations such as navigation, screen rotation and personal customization of apps; data from these sensors are not necessarily displayed, but third-party apps can access and extract them (e.g. Griscom 2006). Three simple experiments illustrate how data from smartphone sensors can be use- ful in geophysics labs. We have developed these experiments to address the topics of positioning (accuracy and precision), seismic signals and gravity in an introductory geophys- ics course called Earth’s Interior at Northwestern University, USA. The students are either from Earth sciences or engineering. Positioning – accuracy and precision For the frst experiment, exploring posi- tioning, students used their phones to make measurements, at least six hours apart, of the latitude and longitude of the corner of a planter in front of a build- ing (red arrow on fgure 1). Then they extracted the coordinates using Google Maps, entered their results in a spread- sheet and sent it to the instructor. Figure 1 shows a cumulative dataset, with Android and iPhone data plotted in red and blue, showing a slight difference between the two. The midpoints from the two groups are more than ~3 m apart, demonstrating how instruments can bias data. We asked students to assess which of the measure- ment groups is more precise and/or accurate, using his- tograms like those in fgure 2. Using the entire data set together shows that making more measurements yields a more precise position (green triangle in fgure 1). This point is close to (~2 m) the geospatial reference (yellow star in fgure 1 and green lines in fgure 2). Considering the ~8 m accuracy of the positioning system used by smartphones (Zandbergen 2009), this result is pretty good. This experiment lets students use their own data to explore concepts typically shown in texts with schematic illustrations. Seismic wave velocity Smartphones can produce and record time-series with their built-in sensors. The signals from the accelerometers in smart- phones turn a phone into a small seismom- eter (e.g. Minson et al. 2015, Kong et al. 2016, Panizzi 2016). This requires software that Turn your smartphone into a geophysics lab Amir Salaree and colleagues explain how that smartphone in a student’s pocket can be used to perform geophysics experiments. “The signals from the accelerometers in a phone turn it into a small seismometer” 1 Students measured values for latitude and longitude of the point shown by the red arrow (inset, from Google Maps). The map shows position measurements made by Android phones and iPhones (red and blue dots) with red and blue triangles showing the two groups’ midpoints. The green triangle represents the halfway point of all the measurements and the yellow star shows the geospatial reference point from Google Earth. Contours are at multiples of 1 m from the reference point. Downloaded from https://academic.oup.com/astrogeo/article/58/6/6.35/4638410 by guest on 07 November 2021