Effect of tropical weathering on electrical and electromagnetic measurements

Geophysics ◽  
1979 ◽  
Vol 44 (1) ◽  
pp. 69-88 ◽  
Author(s):  
G. J. Palacky ◽  
Kiyoshi Kadekaru
Keyword(s):  
Electrical Properties ◽  
Mineral Exploration ◽  
Annual Rainfall ◽  
Tropical Regions ◽  
Rock Types ◽  
Electromagnetic Measurements ◽  
Weathered Layer ◽  
Efficient Exploration ◽  
Effective Use ◽  
Airborne Em

Electrical properties of the weathered layer in tropical regions of Brazil were investigated by means of resistivity soundings, airborne, and ground electromagnetic measurements. Five case histories illustrate how changes of climate, lithology, and geomorphology affect geophysical measurements. In humid and subhumid tropical regions (annual rainfall over 650 mm) the weathered layer is between 10 and 80 m thick and moderately conductive. Results from one region (Minas Gerais) indicate that excessive depth of weathering and leaching of massive sulfides, rather than the conductivity of overburden, present the greatest obstacle to effective use of airborne EM methods in mineral exploration. Seasonal variations of precipitation cause changes in soil resistivity, but such changes are not apparent in the underlying weathered layer. In semiarid and temperate regions of Brazil, the weathered layer is 10 to 20 m thick and regional airborne EM surveys are an efficient exploration tool. In all regions, the degree of weathering depends upon lithology and, in several areas, anomaly patterns obtained from airborne EM surveys correlate well with the surface geologic map. However, when comapring electrical properties of similar rock types among regions of the same climatic type, a considerable variation is observed. It seems that also geomorphology plays an important role in weathering. A careful interpretation of airborne EM data is necessary to distinguish anomalies caused by the weathered layer from those due to underlying conductors. Highly conductive, saline alluvia, which cause strong EM anomalies in Australia, were encountered (sporadically) in only one region of Brazil, the semiarid Valley of Curaçá, Bahia.

Investigations of electrical properties of weathered layers in the Yala area, western Kenya, using resistivity soundings

Geophysics ◽  
1991 ◽  
Vol 56 (1) ◽  
pp. 133-138 ◽  
Author(s):  
J. O. Barongo ◽  
G. J. Palacky
Keyword(s):  
Electrical Properties ◽  
Volcanic Rocks ◽  
Silica Content ◽  
Data Sets ◽  
Geologic Mapping ◽  
Weathering Profile ◽  
Western Kenya ◽  
Tropical Regions ◽  
Weathered Layer ◽  
First Time

Investigation in many parts of the world have indicated that resistivity of saprolite (a chloritization zone at the base of the weathering profile) depends uniquely on the underlying rock type. A study of electrical properties of the weathered layer was undertaken near Yala in the Nyanzian‐Kavirondian greenstone belt in western Kenya. Resistivity soundings were systematically carried out in a 24 × 40 km area. Saprolite formed over basalt had the lowest resistivity (14 Ω•m), followed by andesite (27 Ω•m) and rhyolite (67 Ω•m). Statistically meaningful data sets conclusively show, for the first time, that saprolite resistivity increases with the silica content of parent volcanic rocks. Plutonic and sedimentary rocks had higher average resistivities (granite 135 Ω•m, mudstone 79 Ω•m, grit 213 Ω•m). Thickness of the weathered layer varied between 10 and 40 m, with the largest values observed over grit (sandstone). Compared with other tropical regions, the saprolite layer in western Kenya appears relatively thin, possibly because of the relatively rapid tectonic uplift of the area which leads to fast erosion. As observed in earlier studies, resistivity and electromagnetic surveys can increase the speed and reliability of geologic mapping in tropical terrains.

The effective use of exploration techniques for ore search

1971 ◽  
Vol 2 (1) ◽  
pp. 70
Author(s):  
D.R. Cook
Keyword(s):  
Mineral Exploration ◽  
Business Approach ◽  
Effective Use

It has been said that mineral exploration is the "Triumph of Hope over Reality". I suggest that mineral explorationists can improve their confidence in discovery through a sound scientific and good business approach to exploration.

scholarly journals Hyperspectral analysis of lithologies in the Arctic in areas with abundant lichen cover

2018 ◽  
pp. 51-55
Author(s):  
Sara Salehi
Keyword(s):  
Remote Sensing ◽  
Chemical Composition ◽  
Mineral Exploration ◽  
Spectral Characteristics ◽  
Image Data ◽  
Geological Mapping ◽  
The Arctic ◽  
Electromagnetic Spectrum ◽  
Rock Types ◽  
The Impact

Lithological mapping using remote sensing depends, in part, on the identification of rock types by their spectral characteristics. Chemical and physical properties of minerals and rocks determine their diagnostic spectral features throughout the electromagnetic spectrum. Shifts in the position and changes in the shape and depth of these features can be explained by variations in chemical composition of minerals. Detection of such variations is vital for discriminating minerals with similar chemical composition. Compared with multispectral image data, airborne or spaceborne hyperspectral imagery offers higher spectral resolution, which makes it possible to estimate the mineral composition of the rocks under study without direct contact. Arctic environments provide challenging ground for geological mapping and mineral exploration. Inaccessibility commonly complicates ground surveys, and the presence of ice, vegetation and rock-encrusting lichens hinders remote sensing surveys. This study addresses the following objectives: 1. Modelling the impact of lichen on the spectra of the rock substrate; 2. Identification of a robust lichen index for the deconvolution of lichen and rock mixtures and 3. Multiscale hyperspectral analysis of lithologies in areas with abundant lichens.

scholarly journals Precipitation Patterns and Rainfall Erosivity Return Period Under Savanna Conditions in Formosa, Goiás, Brazil

2020 ◽  
Vol 8 (4) ◽  
pp. 554
Author(s):  
Aline Franciel de Andrade ◽  
Roriz Luciano Machado ◽  
Cássia Cristina Rezende ◽  
Elizabete Alves Ferreira ◽  
Daniel Fonseca de Carvalho ◽  
...  
Keyword(s):  
Annual Rainfall ◽  
Rainfall Erosivity ◽  
Return Periods ◽  
Tropical Regions ◽  
Average Annual Rainfall ◽  
Physical Attributes ◽  
Erosivity Index ◽  
Rainfall Patterns ◽  
Agricultural Areas ◽  
Intermediate Pattern

Losses of soil and nutrients affect a large part of agricultural areas in tropical regions, regardless of the level of technology adopted. This study evaluated the physical attributes and erosivity indices associated with rainfall patterns and return periods in the region of Formosa, State of Goiás, Brazil. Using series of pluviographic (2002-2008) and pluviometric (1975-1998) data from a station located at municipality of Formosa, the erosive potential (EI30 and KE>25), rainfall patterns (advanced, intermediate and delayed) and the erosivity associated with the rainfall return periods were determined. The average annual rainfall of the region was 1,391.6 mm with 87.4% of the rains concentrated in October to March. The average annual values of EI30 and KE>25 corresponded to 8,041.6 MJ mm ha-1 h-1 year-1 and 125.7 MJ ha-1 year-1, respectively. The months of the year did not differ based on rainfall pattern. The advanced hydrological pattern had the highest frequency of occurrence, followed by the delayed and intermediate patterns. The highest EI30 and KE>25 indices for individual rainfall seasons occurred under the intermediate and the advanced patterns in February and under the intermediate pattern in October for the index KE>25. The average annual erosivity index (R factor of USLE) (8041.6 MJ mm ha-1 h-1 year-1) is expected to occur at least once every 1.89 years, corresponding to a probability of occurrence of 52.84%. The average annual values of EI30 estimated for the return periods of 2, 5, 10, 25, 50 and 100 years were 8,230, 10,225, 10,889, 11,222, 11,421 and 11,488 MJ mm ha-1 h-1 year-1, respectively.

Herbaceous standing crop in relation to surface and subsurface rockiness

Bothalia ◽  
1983 ◽  
Vol 14 (2) ◽  
pp. 259-264 ◽  
Author(s):  
M. C. Rutherford
Keyword(s):  
Standing Crop ◽  
Soil Surface ◽  
Plant Production ◽  
Annual Rainfall ◽  
Plant Responses ◽  
Desert Grassland ◽  
Surface Rock ◽  
Rock Types ◽  
Rock Cover ◽  
Geological Substrates

A study, carried out in non-wooded parts of savanna and in desert grassland, demonstrated that herbaceous standing crop can decrease with increasing surface rock cover. However, such decreases corresponded more directly to a soil volumetric decrease than to a decrease in soil surface area per se. The relationship was also found to depend strongly on subsurface rock weathering patterns which may reverse the indicated relationship between standing crop and surface rock cover. The application of surface rock cover data without regard to profile stoniness, is, therefore, likely to be of limited use for comparing plant production across different geological substrates. The role of different rock types in giving rise to soils of different fertility with corresponding plant responses, was also indicated by the data. Sensitivity of plant standing crop to surface rock cover was possibly dependent on mean annual rainfall, but further data are required. The interpretation of aboveground standing crop data in terms of aboveground annual production is briefly discussed.

scholarly journals Drone-based electromagnetic survey system for geophysical applications

2022 ◽  
Vol 2 ◽  
pp. 3
Author(s):  
Markku Pirttijärvi ◽  
Ari Saartenoja ◽  
Pekka Korkeakangas
Keyword(s):  
Mineral Exploration ◽  
Theoretical Background ◽  
Geological Mapping ◽  
Current Version ◽  
Horizon 2020 ◽  
Large Loop ◽  
Maximum Payload ◽  
Geotechnical Investigations ◽  
Survey System ◽  
Airborne Em

Geophysical electromagnetic (EM) methods are used in geological mapping, mineral exploration, groundwater studies and geotechnical investigations. Airborne EM methods have the benefit of avoiding terrain obstacles such as lakes, rivers, swamps, and ravines. Compared to manned aircrafts, drones or unmanned aerial vehicles (UAVs) have benefits of their own. Drone-based surveys are versatile, fast to deploy, economical and ecologically more friendly. Presently, magnetic surveying is the only geophysical method that is routinely conducted with drones. The modest maximum payload limit of drones imposes severe restrictions on the applicability of other methods including EM and radiometric methods, for example. Finnish company, Radai Ltd has been developing Louhi, a novel drone-based frequency-domain EM survey system, in an EU funded Horizon 2020 project NEXT – New Exploration Technologies. The EM system has two operation options – the first uses a large loop on the ground as an EM source and the other uses a small portable EM transmitter loop. Both systems utilize a stand-alone and light-weight three-component EM receiver that can be towed by a drone. This article presents the theoretical background of the EM methods, the solution developed by Radai Ltd, the current version of the EM device, and results from field and flight tests that demonstrate the applicability of the drone-based EM system under development.

Cross-equatorial dry intrusions and their impact on Indian summer monsoon-associated water cycle

2020 ◽  
Author(s):  
Deepika Rai ◽  
Shira Raveh-Rubin
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Water Cycle ◽  
Sensible Heat Flux ◽  
Atmospheric Stability ◽  
Lower Troposphere ◽  
Surface Fluxes ◽  
Annual Rainfall ◽  
Tropical Region ◽  
Tropical Regions

<p>Dry intrusion (DI) is the slantwise descent of dry air from the extratropical upper troposphere to the mid/lower troposphere of the lower latitudes. When reaching the tropical regions, DIs substantially change the overall amount of available moisture, ocean surface fluxes into the atmosphere, as well as the atmospheric stability to vertical motion and the 3-dimensional flow and associated dynamics. However, the occurrence of such events has not been quantified systematically. Here, we quantify the climatological occurrence of DIs that extend from the extratropics to tropical regions. Specifically, we focus on events that host subsequent cross-equatorial flow. Using 6-hourly ERA-Interim reanalysis data with a Lagrangian approach, we show that during the summer monsoon season (June to September) DIs enter the tropical region from the southern hemisphere with peaks that exceed 10 % frequency in time. DI arrival into the tropics is associated with dry and cold lower-tropospheric anomalies, and consequently induced ocean evaporation and sensible heat flux into the atmosphere. Although cross-equatorial DIs are rare, a hotspot of such DIs is evident in the Indian Ocean, having a potential role for Indian summer monsoon (ISM) water cycle. The dominance of the ISM for the annual rainfall over India implies that small changes in the evaporation and moisture pathways may influence the ISM precipitation downstream significantly. Indeed, we demonstrate the connection between ISM rainfall and the preceding water-cycle interaction under DI conditions, and further show that DIs entering the Indian subcontinent modify the low-level jets. </p>

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