International Journal of Geophysics

This paper aims to study the foF2 seasonal asymmetry diurnal variation at Dakar station from 1976 to 1995. We show that equinoctial asymmetry is less pronounced and somewhere is absent throughout 21 and 22 solar cycles. The absence of equinoctial asymmetry may be due to Russell-McPherron mechanism and the vertical drift . The solstice anomaly or annual anomaly is always observed throughout both 21 and 22 solar cycles as measured at Dakar ionosonde. The maximum negative value of σfoF2, fairly equal to -65%, is observed during the decreasing phase at solstice time; this value appeared usually at 0200 LT except during the maximum phase during which it is observed at 2300 LT. The maximum positive value, fairly equal to +94%, is observed at 0600 LT during solar minimum at solstice time. This annual asymmetry may be due to neutral composition asymmetric variation and solar radiation annual asymmetry with the solstice time. The semiannual asymmetry is also observed during all solar cycle phases. The maximum positive value (+73%) is observed at 2300 LT during solar maximum, and its maximum negative (-12%) is observed during the increasing phase. We established, as the case of annual asymmetry, that this asymmetry could not be explained by the asymmetry in vertical velocity phenomenon but by the axial mechanism, the “thermospheric spoon” mechanism, and the seasonally varying eddy mixing phenomenon.

We present the combination of permutation entropy (PE) and power spectral density (PSD) analysis on continuous seismic data recorded by short-period seismic stations during the 2010 Merapi volcano eruption. The calculation of PE aims at characterizing the randomness level in seismic noise, while the PSD parameters use to detect the background noise level in various frequency bands. It was previously observed that a significant reduction of randomness before the volcano eruption could be indicated as one of the short-term precursors due to the lack of high frequencies (>1 Hz) in the noise wave-field caused by high absorption losses as the hot magma uprises to the upper crust. The results show no significant reduction in signal randomness before the eruption series. The characteristic of events during the preeruptive period and the crisis tends to be chaotic (PE in the range 0.9 to 1). Further calculations show that the standard deviation in PE decreased in four days before the first eruption onset on 26 October. PE was stable at the highest values (very close to 1) and gradually returned to the previous fluctuation after the eruption onset. The level of background noise in the low- and high-frequency bands appeared to have the same tendency. The two main eruptions correspond to the two highest peaks of noise levels.

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.

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.

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.

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.