David Sollberger

The shallow elastic structure of the lunar crust: New insights from seismic wavefield gradient analysis

Enigmatic lunar seismograms recorded during the Apollo 17 mission in 1972 have so far precluded the identification of shear-wave arrivals and hence the construction of a comprehensive elastic model of the shallow lunar subsurface. Here, for the first time, we extract shear-wave information from the Apollo active-seismic data using a novel waveform analysis technique based on spatial seismic wavefield gradients. The star-like recording geometry of the active seismic experiment lends itself surprisingly well to compute spatial wavefield gradients and rotational ground motion as a function of time. These observables, which are new to seismic exploration in general, allowed us to identify shear waves in the complex lunar seismograms, and to derive a new model of seismic compressional and shear-wave velocities in the shallow lunar crust, critical to understand its lithology and constitution, and its impact on other geophysical investigations of the Moons deep interior.

Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study

Karstified areas are known to be difficult ground for seismic exploration. We conducted a combined numerical-modeling and field-experiment study with the objectives to study the impact of karst on seismic wave propagation and to advance geophysical characterization of karst with seismic as well as non-seismic methods (electric and electromagnetic techniques). Finite-difference simulations using models with realistic topography illustrate the pronounced impact of topographic variations in high-velocity carbonate-bedrock environments on the scattered surface/guided waves. Wavefield complexities such as strong lateral changes in the strength of surface/guided waves, which were observed in a Vibroseis gather from a karst terrain in the Middle East, were also evident in our data recorded in Switzerland. In the latter case, amplifications of surface/guided waves could be correlated with low-velocity zones, which are probably due to more intensively karstified zones. Our study demonstrates that because of the strong heterogeneity of karst terrains, dense sampling is required to properly comprehend and disentangle the observed wavefield. Furthermore, we observed in our field study that the electrical-resistivity models correlate more closely with the mapped lithology, whereas karstification seems to more strongly affect the P-wave velocity models.

Spatial Wavefield Gradient Data in Seismic Exploration

Multicomponent seismic array data provide not only recordings of the 3D particle motion and direction of arrival but also important information on the spatial gradients of the seismic wavefield. Such gradient data enable numerous new possibilities to analyse land-seismic data. Benefits of seismic spatial wavefield gradient data include enhanced wavefield separation, local slowness estimation, local elastic-parameter determination, and coherent-noise attenuation. Therefore, long-standing problems in seismic exploration such as suppression of scattered surface waves, isolation of particular wave modes, and wavefield reconstruction from spatially under-sampled data can be addressed with gradient data. Interesting applications of spatial gradients arise for shear (S) wave imaging. Considering that rotational motion (curl of wavefield) is directly linked to the S-wave presence, rotational motion measurements facilitate the identification, isolation, and processing of S-waves. If placed at the Earth’s free surface, rotational rates can be estimated from three-component (3C, vector) sensor arrays. Such data enable capture of all six degrees of freedom (three components of translation and three component of rotation). It is straightforward to apply the same principles of array-derived gradients and rotations to arrays of closely-spaced vector (3C, directed forces) seismic sources, which enables the simulation of pure rotational sources and, thus, ‘pure’ S-wave sources.