Application of the relative uranium-series disequilibrium in soil to locate and/or confirm precisely active fault traces: A new technique

Fault traces have been previously located from measurements of ²²²Rn in soils taken a constant soil-depth across the fault trace. In this paper, we have studied the uranium-series disequilibria of the ²²⁶Ra, ²²²Rn (gas) and ²¹⁴Bi radionuclides, not only for their horizontal spatial patterns across the fault trace, but also for their vertical spatial patterns near and at the fault trace itself. Radon-222 activities in the soil-gas were measured on-site with a radiation monitor and a Lucas cell. Radium-226 and ²¹⁴Bi were determined in soil samples in the laboratory by gamma-ray spectroscopy. A new technique employing the measurement of ²²²Rn versus soil-depth shows a decrease in ²²²Rn activity at the fault trace due to the much higher soil-gas permeability as a result of the fractured soil, as well as relative larger uranium-series disequilibria, in respect to an increase in ²²²Rn activity at normal sites, where the soil is not fractured. Finally, it is suggested that fault trace detection could possibly also be performed by measuring ²¹⁴Bi in surface soils (0-100 cm) along a transect.     

The spatial distribution of cesium-137 in surface soils in the Gran Sabana and Sierra de Pacarima (Venezuela)

More than fifty surface (2-10 cm) soil samples were collected in the Gran Sabana and Sierra de Pacarima, State of Bolivar (Venezuela) and analyzed for their ¹³⁷Cs activity. The measurements were performed by high resolution gamma-ray spectrometry employing a hyperpure germanium detector with an energy resolution of about 1.8 keV for the 1.33 MeV line of ⁶⁰Co and a counting efficiency greater than 25%. About 500 ml portions of dried soil samples with less than 85 mm particle size were counted for 20,000 seconds and the data were collected, stored and analyzed with an IBM-compatible computer. The concentrations of ¹³⁷Cs in most samples were below the detection limit of 1.2 Bq/kg, but many of these soil samples could have been disturbed not only by man but the nature itself, for example, by the large amount of rainfall in the wet season. Only 15% of the soil samples were determined to have values equal or greater than 2 Bq/kg and only two of these were greater than 10 Bq/kg. The study area was divided into five zones, which had different environmental characteristics: climate, annual average rainfall, temperature ranges, elevations (m.a.s.l.), soil types and vegetation. Finally, it was concluded that the high ¹³⁷Cs activities could be associated with the areas of higher annual rainfall and with very humid pre-mountainous climate rather than with humid tropical climate. This revised version was published online in July 2006 with corrections to the Cover Date.     

A simple and rapid method to locate fault traces by measurements of 214Bi in soils

A simple and rapid technique is presented to determine the relative counts of ²¹⁴Bi in surface soils to locate active fault traces of the El Pilar Fault in the state of Sucre, Venezuela. The method employes a portable differential gamma-ray spectrometer on site with 300 seconds of measuring time. Three transects across the El Pilar fault of very different geological aspects were studied. One of the advantages of this technique in respect to determining ²²²Rn in soil-gas is that no soil-gas probes are required to be inserted in the soil and it is not necessary to know the appropriate depth. Finally, it has been suggested that measurements of 1000 seconds would be preferred rather than 300 seconds for future studies even though this would limit the number of measurements to about 20 per day.This revised version was published online in November 2005 with corrections to the Cover Date.     

Anomalously high values of cesium-137 in soils on the Península de Paraguana (Venezuela)

The activity of ¹³⁷ Cs in surface soils (2-5 cm) was determined at twenty-one sampling sites along the northwestern and eastern coast of the Paraguana peninsula (Venezuela), as well as, at nine locations, between 95 and 535 m.a.s.l. on Cerro Santa Ana. The measurements were performed by high-resolution gamma-ray spectroscopy employing a compatible IBM computer. Most of the values were much higher than those found along the coastline of the mainland; four sites were found to be anomalously high, with ¹³⁷ Cs values greater than 10 Bq/kg. It is difficult to explain these anomalous ¹³⁷ Cs values by geographical or climatological factors since there is little rainfall here and the clouds and fog are rarely if never present along the coast of the peninsula. Possibly, some mechanism of the mist that is blown ashore could explain these anomalies. The values of the ¹³⁷ Cs versus altitude on the Cerro Santa Ana show an increase of two or three times at 500 m.a.s.l. level, thus we have concluded that the base of the clouds was at this height when the fallout was directly deposited by condensation in this cloud forest. These results in the Cerro Santa Ana cloud forest are similar to those of other cloud forests along the Venezuelan coast, but the altitude (m.a.s.l.) of the base of the clouds here are much lower.