ADVANCES IN DEVELOPMENT OF
GEOPHYSICAL SEISMIC AND NON-SEISMIC
TECHNIQUES FOR CTBT ON-SITE INSPECTION

On-site inspection (OSI) is the final verification measure of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). According to paragraphs 69(e), 69(f) and 69(g) of Part II of the Protocol to the CTBT, an OSI may involve the following seismic and non-seismic geophysical techniques to search for, locate and characterize underground anomalies associated with a nuclear explosion: passive seismological monitoring for aftershocks; resonance seismometry and active seismic surveys; magnetic and gravitational field mapping; ground penetrating radar; and electrical conductivity measurements. In preparation of the CTBT's entry into force, the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is actively developing OSI capabilities. Most recent advances in passive seismological monitoring include upgrade of the telemetry system for data transmission and development of the data processing software to accommodate topographically challenging environments. To assess current OSI geophysical imaging capabilities for the other geophysical techniques and for deep site characterization applications in an integrated manner, an extensive field test was conducted in September 2022 in the Austrian Ybbstaler Alps in a topographically challenging environment. Resonance seismometry and active seismic surveys, magnetic and gravitational field mapping, as well as electrical conductivity measurements were performed along three profiles over a cave system at 40-350 m depth mimicking underground cavities produced by an underground nuclear explosion. This was the first field test of a newly acquired active seismic data recording system, with the aim of developing OSI methods for active seismic surveys. Out of all geophysical techniques, active seismic surveys have the potential of providing the highest resolution for deeper site characterization. Other recent advances include the development of forward models to characterize the magnetic anomalies created by complex geometric bodies simulating different OSI-relevant observables. As a follow-up of this project, a series of multi-level magnetic surveys (ground, near surface and airborne) were conducted in central Italy to further develop magnetic field mapping workflow in an OSI. We present these recent advances in development of the CTBT OSI regime for geophysical techniques, with specific focus on aspects that are also applicable in the wider context, for example for site characterization of potential nuclear waste repository sites.