Seismometer Detection of Dust Devil Vortices by Ground Tilt

Abstract

We report seismic signals on a desert playa caused by convective vortices and dust devils. The long-period (10-100s) signatures, with tilts of ~10$^{-7}$ radians, are correlated with the presence of vortices, detected with nearby sensors as sharp temporary pressure drops (0.2-1 mbar) and solar obscuration by dust. We show that the shape and amplitude of the signals, manifesting primarily as horizontal accelerations, can be modeled approximately with a simple quasi-static point-load model of the negative pressure field associated with the vortices acting on the ground as an elastic half space. We suggest the load imposed by a dust devil of diameter D and core pressure {\Delta}Po is ~({\pi}/2){\Delta}PoD$^2$, or for a typical terrestrial devil of 5 m diameter and 2 mbar, about the weight of a small car. The tilt depends on the inverse square of distance, and on the elastic properties of the ground, and the large signals we observe are in part due to the relatively soft playa sediment and the shallow installation of the instrument. Ground tilt may be a particularly sensitive means of detecting dust devils. The simple point-load model fails for large dust devils at short ranges, but more elaborate models incorporating the work of Sorrells (1971) may explain some of the more complex features in such cases, taking the vortex winds and ground velocity into account. We discuss some implications for the InSight mission to Mars.Comment: Contributed Article for Bulletin of the Seismological Society of America, Accepted 29th August 201

The Noise Model of the SEIS Seismometer of the InSight Mission to Mars

“…This simpler model gives very similar results, as described in Murdoch et al (2016a). The advantage of this simpler model is that it can be considered as validated on Earth: Lorenz et al (2015) has shown that the size and amplitude of seismic signal generated on a seismometer by a dust devil (which can be assimilated to a pressure point source) can be measured, and is consistent with the Greens' function approximation used. In purple (dashed), the estimated self-noise of the InSight flight model micro-barometer.…”

Field Measurements of Terrestrial and Martian Dust Devils

“…These systems have allowed large numbers (hundreds to thousands) of unperturbed vortex encounters to be obtained without chase biases, finally yielding robust statistics on vortex populations from pressure drops (Lorenz and Lanagan 2014; and on dust loading . Furthermore, the small measurement stations can be deployed in spatial arrays to make simultaneous measurements that resolve the two-dimensional horizontal structure of dust devils (Lorenz et al 2015a(Lorenz et al , 2015b(Lorenz et al , 2015c.…”

“…Lorenz et al (2015b) employed another field site, La Jornada Experimental Range in New Mexico (operated by the US Department of Agriculture) where access is restricted and so larger installations can be left safely unattended.…”

“…This facility was used by Lorenz et al (2015b) to deploy a line array of pressure/solar loggers in summer 2013. In addition to providing population statistics at this site, the survey noted that the number of vortices encountered varied quite substantially over a distance of a few tens of meters, due presumably to the influence of topography on dust devil migration and/or the effects of different scrub bushes on dust availability and surface roughness.…”

“…The experiment included a seismometer buried at very shallow depth, together with a suite of meteorological instruments. During this field campaign, ground tilt was measured by the seismometer at the same time as vortex encounters were documented by an array of pressure loggers (Lorenz et al 2015a). The negative load of a dust devil vortex pulls up the ground as it passes, causing the ground-and seismometer-to tilt away from the dust devil center.…”