Characterization of lava tubes using ground penetrating radar at Craters of the Moon National Monument, Idaho, USA
Colin Rowell, Adam Pidlisecky, James D. Irving, Robert James Ferguson
Craters of the Moon National Monument is a Pleistocene to Holocene volcanic field in southern Idaho. It consists of numerous lava flows erupted between 15,000 and 2000 years ago. One of the largest lava flows, called the `Blue Dragon' flow, is approximately 2100 years old and covers an area of 280 km2. In many of these larger flows, lava tubes provide an important means of transporting lava underneath the flow over large distances. The goal of this project is to use near-surface geophysical techniques to build images of lava tubes in the sub-surface, and use this information to better understand the mechanisms of lava transport that were active during the eruption. Ground-penetrating radar (GPR) is a geophysical technique that transmits radio waves into the ground, which are then reflected off of boundaries in the sub-surface. These reflected waves return to the surface to be recorded by the radar device. From these reflections, a 2-dimensional image of the sub-surface can be assembled. This technique was used to image lava tubes at Craters of the Moon National Monument. GPR data were acquired over known lava tubes in order to assess the usefulness of the method for detecting lava tubes and other lava flow features. A standard GPR processing flow consisting of dewow filter, gain correction, and elevation correction were applied to these data. Additional processing, in the form of phase-shift migration and Gabor deconvolution was also applied to the data set. The migration algorithm corrected for, among other things, the non-vertical incidence of the GPR raypaths at shallow depths, while the deconvolution accounted for the non-stationary nature of GPR signals throughout the time record. These processing steps greatly improved the quality of the GPR images. Lava tubes were successfully imaged to depths of at least 10 meters using elevated antennas, however, voids smaller than about 2 meters were very difficult to distinguish at any depth.