BTEM Low-Altitude Test

15 July 2019 - 2 November 2019

Lead Scientist: Robert Grimm

Observatory: SGP

Electromagnetic (EM) sounding measures the inductive response of the Earth to natural or artificial signals to determine the profile of electrical resistivity with depth, and thus inform a variety of studies for science, the environment, or resources. The objective of this project is to measure ambient electric and magnetic fields from 5 Hz to 2 kHz from an aerial platform and prove the recent theory that transverse electromagnetic (TEM) waves can be measured at arbitrary altitude and still yield information on the subsurface. The Balloon-borne, Transverse Electromagnetic Measurement (BTEM) instrument will enable remote sensing of the lithospheric structure of Venus, groundwater on Mars, or a subsurface ocean on Titan, all from balloons or other aerial vehicles. The prototype system comprises sensors to measure three components of the electric field and the horizontal components of the magnetic field, as well as inertial navigation, digitizer/computer, two-way communications to the ground via the balloon avionics, batteries, and thermal and impact management. The project is supported by NASA’s Planetary Instrument Concepts for the Advancement of Solar System Observations (PICASSO) and Flight Opportunities (FO) programs: the former funded the payload development and science analysis and the latter contracted the stratospheric balloon flights.

Two high-altitude flights were completed and clearly demonstrated interaction with the ground and ionosphere as expected. However, sensitivity is strongly weighted toward the much higher resistivity of the ionosphere (compared to the ground), with the result that the ground resistivity could not be well determined. In order to complete this proof of concept, we wish to acquire electromagnetic data at low altitude from a stable platform. The ARM aerostat is an ideal platform as measurements can be obtained at a number of fixed positions to a maximum altitude of about 1000 m. ARM personnel have already built and tested a BTEM mass simulator and we need to install their mounting hardware on our payload. We anticipate that a few hours of data acquisition will be sufficient, but we will plan for several days to prove payload accommodation and allow for weather, in either the summer or fall 2019 campaigns at SGP. No external power or communications are required.

We note that the original high-altitude approach will work for Venus, Mars, and Titan, because their crustal and ionospheric resistivities are more comparable, resulting in a more even distribution of signal contribution and thus better discrimination of the resistivity of the crust. The serendipitous ARM aerostat test will provide complementary and confirmatory data to our stratospheric flights and support further development of BTEM to higher technical readiness levels needed prior to spaceflight proposals in the 2020s.