Catálogo de publicaciones - revistas

<jats:p> Organic shales, defined as mudrocks with total organic carbon (TOC) exceeding 1.5%, are prone to overpressure generation when buried well into the oil window temperature/depth range because the original solid organic matter is characterized by density greater than the liquid hydrocarbons produced from it during their thermal maturation and oil generation. This process of overpressure generation results in a significant reduction of the mean effective stress the rock has been subject to prior to its maturation, commonly referred to as unloading. We combine the Vernik shale normal compaction model with the Vernik-Kachanov rock-physics model adapted for organic shales to invert for the vertical effective stress. When combined with the total vertical principal stress, this yields a reasonable pore pressure estimate wherever the accurate information about the sonic (seismic) velocity, mineralogy, and TOC content is available. Although similar in nature to the empirical Bowers unloading-related pore pressure prediction model, our approach is based on the solid rock-physics understanding of the organic shales and their velocity controls. The model presented was tested against measured pore pressure data available in the Marcellus Shale. </jats:p>

<jats:p> The Editorial Calendar details upcoming (approximately one year in advance) publication plans for The Leading Edge. This includes all special sections, guest editors, and information about submitting articles to TLE. </jats:p>

<jats:p> Imagine for a moment you have just attended another terrific event hosted jointly by SEG and partner societies. This joint-society approach has paid off in terms of broadening the viewpoints, attendees, and subjects available to SEG members. Geoscientists, engineers, environmentalists, city planners, and policy makers all joined in the conversations. Global participation was high as everyone has become comfortable with digital event components. Online interactions went far beyond just playing recordings of presentations. Web attendees benefited from real-time analysis and commentary on the work shown in the technical presentations. The digital exhibition with hosted tours was also a huge success. </jats:p>

<jats:p> While geophysical exploration of Earth is well established as a critical method for understanding planetary processes, many current and planned missions offer great opportunities for geophysicists to apply their skills and expertise to space exploration. Programs such as NASA's Artemis aiming to bring humans back to the moon and the James Webb Space Telescope for deep space imaging, the 10-year extension plan for the Chinese Lunar Exploration Program's Chang'e missions, the Japanese Aerospace Exploration Agency's Martian Moons eXploration program, and the European Space Agency's European Large Logistics Lander targeting the moon are just a few examples. NASA's Commercial Lunar Payload Services program proposes to send two commercial landers to the moon every year for the remainder of the decade. Several already-manifested payloads in the program involve geophysical instrumentation including heat flow probes, magnetotelluric sounding systems, and seismometers. Seismic data continue to arrive from NASA's InSight mission to Mars as the dusty solar panels still deliver a little energy, emphasizing that geophysics is and will remain an important tool for planetary exploration moving forward. </jats:p>

<jats:p> Geophysical techniques were first tested beyond Earth during the Apollo program. Of those examined, radio-wave propagation methods appeared to be the most suitable for the moon and other solar system bodies. This was due to the electromagnetic characteristics of planetary subsurfaces and the possibility to remotely perform measurements on board spacecrafts and rovers. After the first successful experiment on the moon, more than 20 years passed before ground-penetrating radar (GPR) was included in the payload of a planetary mission. Technological advancements in GPR design and successful results of radio echo sounding measurements for the detection of basal water below terrestrial ice sheets paved the way for the application of similar techniques to search for liquid water in the Martian subsurface. Since deployment of the first two subsurface radar sounders above Mars, the number of proposed planetary missions relying on GPR for surveying the subsurface of planets, moons, and other objects has grown progressively. Six orbiting radar sounders and five GPRs mounted on rovers/landers have been employed so far to explore the moon, Mars, and comet 67P/GC. Some of these are in full operation and some are just starting to operate. Planned missions to the icy moons of Jupiter will also depend heavily on radar sounders to detect evidence of an internal ocean on Europa and to understand the habitability conditions on Europa, Ganymede, and Callisto. Finally, planetary missions to Earth's twin, the planet Venus, could take advantage of GPR to understand the cause of its drastic change in climatic conditions and the geologic phenomena that contributed to changing a watery and hospitable surface into a hot and asphyxiating inhabitable planet. </jats:p>

<jats:p> Prior to using in-situ planetary resources, efficient mapping of geochemical and physical characteristics of the near surface will be required. As part of an integrated geophysical instrument suite on exploration and prospecting vehicles, we investigated the suitability of seismic piezo-sensors rigidly mounted on the interior of a generic rover wheel. Factors that can compromise proper data acquisition for this system include the natural mechanical resonance of the wheel and wheel-to-ground coupling. We characterized the natural resonance frequency bands of a generic wheel with an electromagnetic shaker. We also collected seismic shot gathers for subsequent seismic surface-wave analysis using a wheel in both a dry, laboratory sand tank (1.8 × 1.8 × 0.6 m) and in frozen loess soils within the United States Army Corps of Engineers Permafrost Tunnel in Alaska. For our wheel, self-weighted coupling to the ground was found to be adequate, although suitable wheelbase dimensions can constrain field acquisition geometries. In unconsolidated sediments, represented by medium sand (0.25–0.5 mm), wheel resonance of 1 kHz does not affect the fundamental mode used in shear-wave-velocity-to-depth inversion. When analyzed, shot-gather data collected from both wheel-mounted sensors and sand-planted sensors, in loose dry sand, effectively captured similar fundamental surface-wave modes. This is evident in frequency-versus-phase velocity dispersion images. Because narrow frequency bands of wheel resonance exhibit a high signal-to-noise ratio, they also readily detect lateral attenuation changes. Thus, wheel resonance can also be used to capture soil attenuation changes, including those produced when pore H<jats:sub>2</jats:sub>O ice acts to cement the regolith or loose-grained soils. </jats:p>

<jats:p> Lunar surface activities during Apollo and terrestrial analogue lunar mission simulations have commonly focused on traverses that prioritize surface observations and sample collection activities. Along the way, geophysical measurements are often made. However, they are not necessarily made in a way that optimizes information about the physical subsurface properties, which is something that geophysics can provide. In 2010, NASA simulated a high-quality multiweek human lunar rover traverse analogue mission in the San Francisco volcanic field in Arizona. The traverse route and associated science station locations were selected based on addressing surface observation and sampling tasks. Geophysical studies were not included in the simulation. We returned to the same field area and obtained data on 19 active seismic refraction geophone lines from the science station locations accessed during the simulation. We analyzed the data to calculate 1D seismic velocity profiles for each of the lines. Results revealed up to seven distinct seismically defined material types, including a nearly ubiquitous veneer of regolith of variable thickness at the surface. Results also provided depth and thickness of the seven material types in the first 60 m of the subsurface at each of the science station locations. These cannot be obtained by geologic observations of the outcrops. Systematic interpretation of the area's overall subsurface stratigraphy was not feasible due to the geophysically nonsystematic nature of the original traverse's prioritization of the science station locations. The added geophysical understanding of a region could drive additional geologic investigations to locate samples of otherwise unknown material through the location of surface exposures or coring. This emphasizes the importance of synchronizing geologic and geophysical research requirements during lunar traverse planning and execution to optimize addressing scientific and utilization questions. </jats:p>

<jats:p> Teleseismic receiver function (RF) analysis traditionally has targeted deep earth structures in the context of observational seismology, allowing the detection of converted seismic phases originating from major boundaries of impedance. Recent efforts to adapt this passive-source seismic imaging technique to basin-scale subsurface characterization have retrieved primary basin geometries and key stratigraphic boundaries by relying on densely deployed broadband seismometer networks. While accurate interpretation of RF time series on Earth benefitted from complementary subsurface constraints (e.g., core and well logs), lack of such resources and instrumental limitations must be considered in the context of planetary geophysics, where passive-source imaging may emerge as a key frontier exploration technique. Thus, it is important to establish an analysis framework in which observed RF signals are interpreted by relying on first-order suppositions about the physical properties of the underlying subsurface. In this work, we seek to simulate this approach in single-station scenarios and qualitatively examine the baseline information inferable from the RF time series. Our results suggest that signals observed in the sedimentary interval can be reasonably attributed to major impedance changes expected from the generalized lithostratigraphy of the local subsurface, including the transition to the underlying crust. We also find patterns of anisotropy-related directional variations in high-frequency signals as well as unique frequency-dependent responses likely associated with the depth and vertical dimension of converting boundaries. Together, these seismic observables enable inference of various geologic attributes within the underlying sedimentary unit, proving it to be a critical tool in future efforts of planetary exploration. </jats:p>

<jats:p> Studies of Martian fault-related landform morphology have noted an interesting phenomenon associated with large thrusts — isolated channels that run from the crests of these landforms down their slopes. One such example is Ogygis Rupes, an east-verging, 180 km long thrust-fault-related landform located near 55°W, 33°S. Geometric modeling efforts have indicated the fault underlying this landform has experienced approximately 2850 m of slip, and a large anticlinal fold has grown above the fault. We mapped 72 channels on back and front slopes of this massive landform at 1:24,000 scale on 10 mosaiced Context Camera images (6 m/pixel) and one High-Resolution Imaging Experiment image (2.5 m/pixel). The morphology of these channels indicates that they formed through flash flood events that originated near the hinge of the fold. We propose that these flooding events were due to the melting of subsurface ice during fault slip. We apply a thermodynamic model for heat generation along the fault surface during slip events of various magnitudes to estimate the volumes of water produced during each hypothetical event. By comparing these estimates to channel discharge values estimated from channel morphologies, we resolve fault slip magnitudes that could produce the channels. We find that slip events of approximately 28 m produce enough heat to melt the ice associated with channel flow (4.867 × 10<jats:sup>9</jats:sup> m<jats:sup>3</jats:sup>). This estimate is based on slip within a 1 m basaltic fault zone, with a friction coefficient of 0.6 where all pore space is occupied by water ice. Results indicate that only mature faults with well-developed, fractured hinges are capable of both (1) melting enough subsurface ice and (2) transporting the water to the surface to result in flooding, explaining why this impressive phenomenon remains a rare but important hazard of which to be aware during planning of the exploration and settlement of Mars. </jats:p>