The National Science Foundation provided support to the American Geophysical Union (AGU) to engage its relevant community and help clarify the need for a Near-Surface Geophysics (NSG) Center and identify how it could advance key science questions, provide benefits for society, and develop the geophysical workforce of the future. This report synthesizes the broad input from the community. The listed authors represent the Steering Committee, led by Sarah Kruse and Xavier Comas, and AGU staff leads. They were responsible for most of the editing and connective writing. The major conclusions are: ● The capability and importance of NSG is expanding rapidly, and NSG is providing key science and knowledge to many specific scientific challenges in diverse disciplines–from ecology and anthropology to hydrology, oceanography, cryosphere science, soil and critical zone science, and more. ● This has been thanks to diverse new instruments and approaches, expanded monitoring, improved resolution, interoperable data sets, and new computing power and approaches, among other developments. ● As a result, advancing NSG is critical to addressing many societal challenges at local to global scales. Human society depends on and interacts with the NSG environment in deep and diverse ways at all scales. ● Despite these developments, integration of NSG approaches and awareness of these across related disciplines are not nearly robust enough for these needs. ● Major challenges include providing equipment and training around its use, developing and deploying new equipment and sensors, developing interoperable data, and developing computation techniques. ● In particular, educating both current researchers and developing an NSG-enabled workforce is a major challenge. ● Integrating education with societal and scientific challenges provides a great opportunity and means to expand inclusivity and diversity in the Earth sciences and to address climate justice and equity challenges. ● Thus there was a strong consensus for support of an NSG Center designed to address these challenges and needs and to foster convergent science, provide broad and hands-on educational training, and engage communities and the public meaningfully. ● We were not charged with envisioning the specific model for a Center—and indeed emphasized that the term “Center” was generic and did not necessarily imply that these efforts were envisioned to be in one location–but note that NSF is supporting important complementary facilities include the new EarthScope Consortium combining IRIS and UNAVCO, NCALM, and CTEMPS. ● In sum, we strongly encourage the NSF to take the next step in considering the best implementation model for a NSG Center that addresses these needs, enables these opportunities, and leverages and complements existing efforts.

A 24-hour 2D time-lapse electrical resistivity imaging (ERI) survey was conducted in an altered mangrove forest on a barrier island in southeast Florida, USA, to (1) assess the method’s utility in hypersaline conditions and (2) understand how trees respond to hypersaline conditions. ERI measurements serve as a proxy for pore water salinity and saturation. Here, resistivity changes suggest a lag between the tidal cycle and changes in ground resistivity. ERI data show that overall changes within 24 hours are very small, but there is more variability in resistivity in the root zone of mangroves than in open salt flat portions along a fixed transect. Two to three hours after sunset, root zone resistivity increased from initial, midday conditions. Overnight, the root zone was less resistive than midday. By sunrise, root zone resistivity was once again higher than initial conditions. Measurements from the salt flat where roots are absent remained generally constant throughout the survey. Thus, changes in resistivity over time are inferred to reflect mangrove tree physiological influences related to diel water use. A mechanistic explanation for the decreased resistivity two hours after sunset from the re-distribution of salts to the soil around the roots is the Cohesion-Tension Theory, which suggests that trees continue water uptake after sunset to balance the pressure after leaf stomates have closed. The corresponding overnight drop in ground resistivity just prior to sunrise may be explained by redistribution of freshwater from the tree to the soil that was delayed until the early morning hours. The limited period of data acquisition limits definitive data interpretations, but the study illustrates the monitoring potential of ERI in hypersaline environments such as a mangrove forest.

Ground Penetrating Radar (GPR) is shown to be a successful tool in detecting tunnels and voids. Lava tubes are tunnel-like features in volcanic terrains that can be potential safe places for human crews and equipment on the Moon and Mars. We utilize GPR to detect and map lava tubes (Valentines cave, Skull cave and Hercules Leg cave) in Lava Beds National Monument, CA. Our preliminary results show that the ceiling of the lava tubes are readily detectable by GPR. However, due to the strong radar velocity contrast between lava and the air-filled tubes, accurate recovery of the position of the lava tube floor is much more challenging. Careful migration of the GPR data is required to resolve the floor signature and create an image with the tube floor restored to its correct depth. We are developing an optimal workflow for recovering complete lava tube geometries. We can do this because we have collected centimeter-scale LiDAR data from the interior of tubes as well as on the surface along GPR transect lines. Thus we can test the accuracy of GPR migration methods against the LiDAR-measured tube geometry. We are testing conventional 2D migration techniques as well as topographic migration. At selected field sites we have limited 3D ‘grids’ of data. We expect to compare the results of different migration techniques to identify optimal methods for this problem. As a part of this project, we also seek to develop a library of different lava tube geometries and their corresponding GPR image from their migrated sections. The GPR image library will encompass a range lava tube geometries, including tubes of different heights, widths, shapes, and structures (e.g., pillars), plus a variety of floor textures (e.g., smooth, ropey, rubble) and overhead thickness. This library will be an asset for determining the utility of deploying GPR technology in mapping a tube-rich environment.

Lava tubes can offer protection for human crews and their equipment on other solar system bodies, in particular from radiation threats and extreme surface temperatures. Developing strategies to survey regions of other terrestrial bodies (such as the Moon or Mars) for tubes suitable for potential habitation will likely become an important part in planning future space exploration projects. A variety of surface geophysical techniques, such as ground penetrating radar (GPR) have the potential to help recognize and map tubes. GPR shows promise for providing high resolution information on tube geometries. To investigate GPR’s capacity and limitations, we use GPR, as well as comparative methods of seismic and magnetic surveys, in conjunction with LiDAR mapping of tube interiors at the Lava Beds National Monument (LBNM) in California, USA. LBNM offers a wide variety of tube geometries and textures. We have collected 2D GPR profiles and small 3D GPR grids (of parallel 2D lines) with antenna frequencies of 100 and 200 MHz on four lava tubes with different geometries, textures and at different depths. Challenges in recovering tube geometries include wave scattering in fractured rock covering tubes, irregular and “drippy” ceilings and walls, and blocky floors. Our primary results show that the top of the LBNM tubes can generally be resolved in the GPR data, while resolving the bottom is more challenging. The utility of various GPR processing techniques can be directly assessed by comparing resolved GPR images against the LiDAR-measured tube geometries.