ESA’s next-generation gravity mission concepts

The paper addresses the preparatory studies of future ESA mission concepts devoted to improve our understanding of the Earth’s mass change phenomena causing temporal variations in the gravity field, at different temporal and spatial scales, due to ice mass changes of ice sheets and glaciers, continental water cycles, ocean masses dynamics and solid Earth deformations. The ESA initiatives started in 2003 with a study on observation techniques for solid Earth missions and continued through several studies focusing on the satellite system, technology development for propulsion and distance metrology, preferred mission concepts, the attitude and orbit control system, as well as the optimization of the satellite constellation. These activities received precious inputs from the GOCE, GRACE and GRACE-FO missions. More recently, several studies related to new sensor concepts based on cold atom interferometry (CAI) were conducted, mainly focusing on technology development for different instrument configurations (GOCE-like and GRACE-like) and including validation activities, e.g. a first successful airborne survey with a CAI gravimeter. The latest results concerning the preferred satellite architectures and constellations, payload design and estimated science performance will be presented as well as remaining open issues for future concepts.

Fracture Seismic: Mapping Subsurface Connectivity

Fracture seismic is the method for recording and analyzing passive seismic data for mapping the fractures in the subsurface. Fracture seismic is able to map the fractures because of two types of mechanical actions in the fractures. First, in cohesive rock, fractures can emit short duration energy pulses when growing at their tips through opening and shearing. The industrial practice of recording and analyzing these short duration events is commonly called micro-seismic. Second, coupled rock–fracture–fluid interactions take place during earth deformations and this generates signals unique to the fracture’s physical characteristics. This signal appears as harmonic resonance of the entire, fluid-filled fracture. These signals can be initiated by both external and internal changes in local pressure, e.g., a passing seismic wave, tectonic deformations, and injection during a hydraulic well treatment. Fracture seismic is used to map the location, spatial extent, and physical characteristics of fractures. The strongest fracture seismic signals come from connected fluid-pathways. Fracture seismic observations recorded before, during, and after hydraulic stimulations show that such treatments primarily open pre-existing fractures and weak zones in the rocks. Time-lapse fracture seismic methods map the flow of fluids in the rocks and reveal how the reservoir connectivity changes over time. We present examples that support these findings and suggest that the fracture seismic method should become an important exploration, reservoir management, production, and civil safety tool for the subsurface energy industry.

Strain and tilt records of the CANNIKIN nuclear test

abstract During the CANNIKIN underground nuclear test of November 6, 1971, nearsurface earth deformations were observed by a total of 26 strainmeters and eight tiltmeters at nine sites in the Aleutian Islands at distances of 10 to 1125 km from ground zero on Amchitka Island. Permanent strain steps were observed at all stations. The size of strain steps observed at about 10 km (> 10−4) implies that the zone of nonelastic behavior extended to more than 10 km from ground zero. The size of strain steps observed at 295 and 1125 km (< 10−8) indicates that the residual strains at these distances were not as large as the amplitude of the daily earth tide. The permanent steps exhibit a pronounced directional assymmetry but, at five stations, are in reasonable agreement with predictions based on the work of Wideman and Major (1967). Redundant observations of areal strains at two stations yield values of −29.5 × 10−9 versus −24.2 × 10−9 and −2.1 × 10−7 versus −3.2 × 10−7.