Results in Geophysical Sciences最新文献

A mineralisation potential assessment in part of Ilesha Schist belt of southwestern Nigeria was carried out using multi criteria decision analysis (MCDA) in the context of analytical hierarchy process (AHP). Three parameters sourced from aeromagnetic, electrical resistivity and geological data were identified to favour gold mineralisation in the study area. These parameters include lineament density (Ld), lithology (Lt), and electrically derived coefficient of anisotropy (An). The application of data enhancement techniques such as total horizontal derivative, 3-D Euler deconvolution and source edge detection were employed in the production of lineament map of the study area. Two hundred and sixty-five (265) vertical electrical sounding (VES) points were investigated using Schlumberger configuration. Coefficient of anisotropy derived from the Dar-Zarouk parameters was obtained from the synthesized VES data. The lithologic information was obtained from the existing geological map of the area. The orientations of lineaments displayed on the aeromagnetic results suggest predominantly NE-SW trending structures, which are characteristic of the Pan African orogenic events in the area. However, few of the E-W trending lineaments are imprints of secondary structural alteration. Furthermore, the Euler deconvolution solutions revealed that the area is characterised by geological contacts with estimated depth of 94–600 m. The coefficient of anisotropy results obtained from the geo-electric parameters vary from 1.00–2.87. AHP weighting and ranking technique was used to develop a reliable mineralisation potential model (MPM). In line with the recommended lower limit of 10 %, the consistency ratio of the developed model is 9.61 %. The produced MPM was classified into low, moderate and high mineralisation potential zones. Preliminary validation of the MPM using the existing pits in the area showed 79 % success rate. The validation of the model using the existing geochemical information from the previous studies indicated 85 % agreement with the conceptual model; thus, confirming the reliability of the produced MPM. The developed model is therefore believed to serve as a future reference in the resource management of the study area.

The focus of the present study is to identify potential groundwater zones using resistivity Schlumberger array and magnetic method for mapping hidden lineaments in and around the Valliyar river basin situated in the tail of Southern India. Estimation by 2-D radial power spectrum indicates that the maximum depth at which the magnetic source is present is about 700 m to 800 m. In case of shallow depth magnetic source rock, it is about 40 m to 45 m. The results of magnetic data have revealed the presence of a major magnetic lineament trending in NW to SE direction. The electrical resistivity survey in our study area gives information on sub-surface layers like the weathered zone, aquifer resistivity and aquifer thickness. The locations corresponding to ves-35, ves-40, and ves-47 have weathered and fracture layers having a thickness greater than 20 m, situated in the magnetic major lineament trend. Regarding master curves for layer analysis, H and HA type curves are generally observed in the hard rock region. The interpretation of results from the present study endorses that the coastal region of the study area has good groundwater potential.

Waterlogging and raised soil salinity are leading to land degradation and desertification problems worldwide. It can significantly reduce the agricultural practices and crop yields of affected lands. The present study is attempted to map waterlogged and saline affected areas using LISS-III satellite imagery of pre-monsoon and post-monsoon season of the year 2012. It was observed that the total size of seasonal waterlogged (only Rabi or only Kharif) is 454.96 ha (2.37%) of the total area. The land covered with perennial waterlogged area was found to be 535.54 ha (2.79 %) of the entire command area. The waterlogged area concerning the depth of groundwater was further categorized into most critical, critical, less critical and non-critical. The identification and mapping of salt affected (saline) area was performed during Rabi and Kharif season. Only 0.46% of the command area identified as saline land either in both or single season patches, remaining 99.54% area comes under non saline category. Out of total command area 80.58 and 19.42% areas comes under non-saline and saline zones respectively. The saline area was further classified into medium, high and very high zones. The saline affected area covered with medium, high and very high salt categories is 2.25 ha (0.012%), 26.380 ha (0.137%), and 0.624 ha (0.003%), respectively. In-situ measurements such as pH, electrical conductivity (EC) and exchangeable sodium percentage (ESP) were also used to validate the results. The methodology proposed in this study can potentially be used for cost-effective mapping of soil salinity in other geographical regions.

The broader area of Pournari dam (Western Greece) is investigated regarding the potential seismic hazard change after its impoundment in the early 1980’s. During this impoundment, an increase in local earthquake activity with certain features of induced seismicity was observed. Within the third and fourth month after the first filling, two moderate seismic events occurred, on March 10, 1981 (M_L=5.6) and on April 10, 1981 (M_L=4.7), with focal depths 13 km and 10 km, respectively, indicating migration of seismicity to shallower depths. The latter is considered as mechanical response change of background from undrained to the drained response (flysch formation).

We investigate the potential local seismic hazard and the induced seismicity changes due to the establishment of the reservoir using three time-windows. The first one covers the period up to 1980, before the impoundment, the second starts immediately after the impoundment (1981-2009), while the third is focused on the whole instrumental period 1900-2009. For these time-windows, seismic hazard was initially assessed using the extreme values method. The traditional Cornell-McGuire approach was following applied for the whole instrumental period as a validation test, regarding the reliability of the results obtained by the extreme values method. The results reveal a small decrease of the estimated values of Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) and maximum expected magnitude (M_max). The results are compared to the seismic hazard study case of Polyphyto dam (NW Greece), located on a different seismotectonic regime.

The magnetic fabric of the meta-ultramafic rocks and the associated gneisses in the Wayanad district, in the Moyar Shear Zone (MOSZ) of the Southern Granulite Terrane (SGT) of the state of Kerala, in India has been investigated using Anisotropy of Magnetic Susceptibility (AMS) and its low-field variation, as well as temperature variation of magnetic susceptibility vs temperature (Tc). The dominant magnetic minerals that control and contribute to the magnitude of the AMS were quantified and the regional deformation in the area and its effects on the rock types deciphered. The mean bulk susceptibility of the two rock types indicates the ferromagnetic character in the area. Ferromagnetic and ferrimagnetic minerals – magnetite and maghemite – are the major magnetic mineral contributors of AMS in the meta-ultramafic rocks of Wayanad whereas the mafic minerals – garnet, hornblende, biotite, and pyroxene – influence the magnetic bulk susceptibility in the gneisses. The development of fabric in the meta-ultramafics of Wayanad has an intimate evolutionary relationship with the gneisses and the shear zone itself. This has been demonstrated by magnetic and field-based studies. Both the magnetic foliation of the meta-ultramafics and the gneisses have orientations that fit well with the occurrence of the meta-ultramafic body in the field within the orientation of the Moyar Shear Zone.

Until recently, one obstacle to forecasting paleoclimatic data with time series models was gaps in the record. In 2020, highly resolved data sets became available. Time series models make it possible to determine how accurately climate can be predicted using techniques such as regressions and artificial intelligence. This paper runs forecasting tests for δ¹⁸O and δ¹³C using data spanning 34 Ma. The data exhibit several features: long-memory, irregular trending, and nonlinear variability. The probability distribution has heavy tails, and there are intermittent outliers. Because of the repeated changes in state, a simple train-validate-forecast method is inappropriate for these data sets. Instead, the testing methodology is iterative forecasting over moving windows: only recent observations are used to predict the future. There are several findings. First, at horizons of 2–4 kyr, all the methods perform well. Regressions and neural networks including the orbital parameters achieve the most accurate predictions for δ¹⁸O. In the tests for δ¹³C, the contest between the models is much closer. Second, as the horizon extends, accuracy deteriorates. At 10 kyr, the models track the central tendency of the data but miss the fluctuations. Third, forecast accuracy is found to vary substantially over time. There is a marked deterioration in accuracy starting around 2.58 Ma, coinciding with a period of steeper trending and greater amplitude in the cyclical fluctuations. The main limitation of time series models is that they do not incorporate the underlying physics. A more effective approach may be to combine statistical and physics-based models.

The paper presents generalized parameters of digital seismograms of the Borovoye Geophysical Observatory (BRVK). The seismograms were obtained after reformatting the archive of the Institute of Geosphere Dynamics (IDG RAS). For this research, we consider looking at the most representative entries – underground nuclear explosions (UNE). The particular interest is the analysis of the P-wave travel times at the regional distances Δ≈5–7° and Δ≈89–91°. These deep paths cross the boundaries of the Earth crust, mantle and outer core. There are UNE digital records at the Semipalatinsk Test Site (Δ_STS ≈5.56–6.27°) and at the Nevada Test Site (Δ_NTS≈89.77–90.15°). For this reason, we use UNEs from STS and NTS from 1967 to 1992. UNEs locations and magnitudes are taken from published open sources. The results for P-wave travel time curves on the BRVK – NTS and BRVK – STS paths and the isochron maps are obtained. The linear functions for travel time curves are defined by the linear regression. The resulting seismic travel time curves for P waves are compared to AK135. Because global one-dimensional seismic reference models are non-unique solution to regional seismic data, in terms of physical structure. The results help identify seismic wave velocities at the source-receiver travel paths and character of geologic media. Such physical models could significantly facilitate physical interpretation of seismic structures in the area of Central Asia (e.g. Tien Shan) and global.

This study investigates the distribution of potential kimberlitic pipes in the Kafur-Masari-Malumfashi region of northwestern Nigeria from satellite gravity and regional aeromagnetic data beyond the Kafur kimberlite pipe discovered previously. The interpretation indicates the presence of some roughly circular to oval shaped analytical signal anomalies which coincide with locations of concealed kimberlitic pipes. These results confirm the Kafur kimberlite and delineate six other suspected kimberlitic pipes. The integrated 2D model images and the signatures of the anomalies obtained allow us to interpret and confirm the field responses as possible multiple kimberlitic pipe intrusions within the basement. Thus suggesting that the region is a potential kimberlite field and represents targets for follow-up surveys.