GEOPHYSICAL MAPPING OF A LAVA TUBE CAVE ON MAUNA LOA VOLCANO, HAWAI‘I

Lava tube caves are important science targets on planets due to their access to an environment relatively pristine from surface weathering and conditions that might be favorable for the preservation of life. They are also important exploration targets because the subsurface environment can provide astronauts protection from radiation, dust, and large ambient temperature variations. Noninvasive characterization of the size and extent of a cave passage is advisable before a rover or human can safely enter one of these voids on another world. To evaluate which techniques and operational best practices might be most appropriate for such characterization, we have carried out a series of geophysical field campaigns at a lava tube cave near the Hawai‘i Space Exploration Analog and Simulation (HI-SEAS) planetary analog site on the north flank of Mauna Loa Volcano, Hawai‘i. Ground Penetrating Radar (GPR) surveys were performed using a GSSI SIR 3000 system with a 270 MHz antenna capable of penetrating up to 6 meters in depth. Due to the field site’s rough terrain, readings were taken in sounding mode at discrete stations spaced on 5 and 2 m grids. We found that GPR data quality collected in this manner did not allow for efficient survey or identification of the subsurface target. Magnetometer surveys were performed with total field GEM GSM-19 Overhauser and Geometrics G-859 cesium vapor magnetometers. Readings were recorded at a 1 Hz sampling rate, along a repeated grid survey with a spacing of 10-20 meters. Magnetometer readings display drops of approximately 1500 nT in remnant background magnetic fields associated with subsurface cavities and thus enabled mapping of the subsurface lava tube cave passage extent. These surveys complement interior studies of the lava tube system carried out at the site using tripod-mounted Light Detection and Ranging (LiDAR) scanning and sample collection to study mineralogy, chemistry and biology.