Ambient Seismic Noise Analysis Associated with the 2010 Eruption of Merapi Volcano Using Temporal Variations of Randomness and Background NoiseRead the full article
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Typology of Sounding Curves and Lithological 1D Models of Mineral Prospecting and Groundwater Survey within Crystalline Basement Rocks in the East of Cameroon (Central Africa) Using Electrical Resistivity Method and Koefoed Computation Method
Resistivity method using seventy-sixth (976) Schlumberger vertical electrical soundings along forty-one (41) profiles are conducted in the Batouri and Ngoura subdivisions, East region of Cameroon, to investigate the subsurface layering, mineral potential, and groundwater resource characteristics. Results of quantitative and qualitative interpretation of data using Koefoed computation method reveal two to five layers having geometrical and electrical characteristics of geological layer models: topsoil (; ), lateritic soils (; ); conductive layer (; ); fractured/weathered granite (; ); clayey layer (; (dry) Ω.m); and fresh granites (). From the qualitative interpretation of VES curves, the subsurface layering is depicted by nine (09) types of sounding curves (G, H, A, QH, KH, HK, HA, HKH, and KHK) characterizing the vertical changes and the typology of sounding curves in the East Cameroon crystalline basements. The lithology of the subsurface is dominated (more than 80%) by geoelectrical and lithological 1D models derived by the H, QH, KH, HK, HA, HKH, and KHK curve types. These models are characterized by the presence of conductive layers and fractured/weathered granites derived from tectonic activities of the region. Also, the resistivity method (VES) applied in this study bring information about variation of the resistivity with depth, geological structures, fractures, and rupture zones in the underground until 120 m depth. These abovementioned information reveal proper hydrogeological and mining conditions for an efficient evaluation of the mineral potential and groundwater resources.
A Feasibility Study on Monitoring Crustal Structure Variations by Direct Comparison of Surface Wave Dispersion Curves from Ambient Seismic Noise
This work assesses the feasibility of the direct use of surface-wave dispersion curves from seismic ambient noise to gain insight into the crustal structure of Bransfield Strait and detect seasonal seismic velocity changes. We cross-correlated four years of vertical component ambient noise data recorded by a seismic array in West Antarctica. To estimate fundamental mode Rayleigh wave Green’s functions, the correlations are computed in 4-hr segments, stacked over 1-year time windows and moving windows of 3 months. Rayleigh wave group dispersion curves are then measured on two spectral bands—primary (10–30 s) and secondary (5–10 s) microseisms—using frequency-time analysis. We analyze the temporal evolution of seismic velocity by comparing dispersion curves for the successive annual and 3-month correlation stacks. Our main assumption was that the Green’s functions from the cross-correlations, and thus the dispersion curves, remain invariant if the crustal structure remains unchanged. Maximum amplitudes of secondary microseisms were observed during local winter when the Southern Ocean experiences winter storms. The Rayleigh wave group velocity ranges between 2.1 and 3.7 km/s, considering our period range studied. Interannual velocity variations are not much evident. We observe a slight velocity decrease in summer and increase in winter, which could be attributed to the pressure melting of ice and an increase in ice mass, respectively. The velocity anomalies observed within the crust and upper mantle structure correlate with the major crustal and upper mantle features known from previous studies in the area. Our results demonstrate that the direct comparison of surface wave dispersion curves extracted from ambient noise might be a useful tool in monitoring crustal structure variations.
Twenty-First Century Projected Changes in Extreme Temperature over Côte d’Ivoire (West Africa)
The projection of the future climate changes is of paramount importance inasmuch as it contributes to provide useful information for adaptation planning worldwide to local scales. This study investigated the future changes using four temperature related indices based on an ensemble of 14 CORDEX-Africa simulations at 0.44° × 0.44° of resolution under the RCP4.5 and RCP8.5 scenarios. These indices indicate moderate extremes over Côte d’Ivoire. The results show an increase in the warm extreme indices such as the warm spell days index (HWFI), very warm days frequency index (TX90P), and the warm nights frequency index (TN90P) over the entire country under both emission scenarios. The increase in these indices was higher under RCP8.5 and reached 85, 72, and 90% for HWFI, TX90P, and TN90P respectively. In addition, the magnitude of the changes is relevant along the coastal areas in the 2031–2060 and 2071–2100 periods. Moreover, the intra period extreme temperature range (ETR) shows future decrease following a south-north gradient with values in the range [−0.5; 1.5°C] over the country during January–March (JFM) and October–December (OND) seasons whereas an increase (~0.5°C) is projected for April–June (AMJ) and July–September (JAS) seasons, particularly in the central and northern parts. The minimum temperature increases faster than the maximum, except in AMJ and JAS in the central and northern regions. On the other hand, the changes in the indices based on the mean values of the reference period (1976–2005) are in concordance to the expected warming at the end of the twenty-first century with important trends. The projected changes are, however, subject to uncertainties, which are higher under RCP8.5 than under RCP4.5 scenarios. Overall, these changes are meaningful as all the 14 CORDEX-Africa simulations agree to an increase of warm extreme temperature.
Contaminant Delineation of a Landfill Site Using Electrical Resistivity and Induced Polarization Methods in Alice, Eastern Cape, South Africa
A combination of electrical resistivity and induced polarization methods were applied to a solid waste landfill in Alice, Eastern Cape, South Africa to delineate the lithologic layers and locate possible leachate plumes. Resistivity and IP data were collected along six profiles; VES on two and the dipole-dipole configuration was used in the rest four. The result shows a 4-layered earth system with a shallow depth to the top of the bedrock (<10 m). Contaminants ranging from unsaturated waste with high ion content to dense aqueous phase liquid contaminants, characterized by low resistivity (34–80 Ohm-m) and low chargeability values (0.05–5.75 ms). The contamination was interpreted based on resistivity/IP anomalies considering the background geology. The shallow bedrock indicated a low risk to groundwater contamination because of its competent nature from its geology, and characteristic high resistivity values (≥1000 Ohm-m). However, the steep nature of the landfill terrain due to its location at the foot of a vertical slope favours the rapid migration of the contaminants into the immediate vicinity of the landfill. The construction of containment structures such as waste cells will help in enhancing effective waste management practices in the landfill.
Critical Frequency foF2 Variations at Korhogo Station from 1992 to 2001 Prediction with IRI-2012
In this paper we report the foF2 data measured at Korhogo station (Lat. 9.3° N; Long. 354.6° E; dip. 0.6° S) compared to predictions with IRI-2012 subroutine URSI and CCIR for different solar cycle phases (minimum, ascending, maximum, descending) and different geomagnetic activity classes (quiet, fluctuating, recurrent, shock). According to our investigations, predictions with IRI are in agreement with the measured data during daytime and show significant differences between them at night-time and especially before sunrise. Except at solar minimum, the gap between predictions and measured data are more appreciable during recurrent and shock conditions compared to quiet and fluctuating conditions. Our results also show that only URSI model expresses the signature of EXB drift phenomenon at solar maximum phase during the recurrent days and at ascending phase for fluctuating activity.
Self-Potential Studies in Volcanic Environments: A Cheap and Efficient Method for Multiscale Fluid-Flow Investigations
We demonstrate the value of using the self-potential method to study volcanic environments, and particularly fluid flow in those environments. We showcase the fact that self-potential measurements are a highly efficient way to map large areas of volcanic systems under challenging terrain conditions, where other geophysical techniques may be challenging or expensive to deploy. Using case studies of a variety of volcano types, including tuff cones, shield volcanoes, stratovolcanoes, and monogenetic fields, we emphasize the fact that self-potential signals enable us to study fluid flow in volcanic settings on multiple spatial and temporal scales. We categorize the examples into the following three multiscale fluid-flow processes: (1) deep hydrothermal systems, (2) shallow hydrothermal systems, and (3) groundwater. These examples highlight the different hydrological, hydrothermal, and structural inferences that can be made from self-potential signals, such as insight into shallow and deep hydrothermal systems, cooling behavior of lava flows, different hydrogeological domains, upwelling, infiltration, and lateral groundwater and hydrothermal fluid flow paths and velocities, elevation of the groundwater level, crater limits, regional faults, rift zones, incipient collapse limits, structural domains, and buried calderas. The case studies presented in this paper clearly demonstrate that the measured SP signals are a result of the coplay between microscale processes (e.g., electrokinetic, thermoelectric) and macroscale structural and environmental features. We discuss potential challenges and their causes when trying to uniquely interpret self-potential signals. Through integration with different geophysical and geochemical data types such as subsurface electrical resistivity distributions obtained from, e.g., electrical resistivity tomography or magnetotellurics, soil CO2 flux, and soil temperature, it is demonstrated that the hydrogeological interpretations obtained from SP measurements can be better constrained and/or validated.