Geophysical Tracer Research Articles

Monitoring Atmospheric Atomic Mercury by Optical Techniques

Mercury is a serious neurotoxic agent, and the control and monitoring of emissions are important. Optical spectroscopy is a powerful technique for measurement of mercury, which in the atmosphere predominantly appears in atomic form. The mercury resonance line close to 254 nm can be utilized in long-path absorption measurements of average concentrations or in light detection and ranging (lidar) studies, where range-resolved concentration values can be obtained. In addition, point monitors often use optical detection, frequently after pre-concentration, as well as for mercury compounds in conjunction with denuders, which transfer the compounds into atomic mercury. The present review discusses mercury measurement methods with respect to merits and sensitivity to interference. The main focus is on remote-sensing techniques, and many examples from industrial and mining monitoring are given. Further, mercury emissions related to the extraction of geothermal energy are discussed. Finally, an example from archaeology—the Qin tomb in Xi’an—is presented. Advanced measurement techniques can help in shaping an environment largely free from mercury contamination. Further, the aspect of mercury being an important geophysical tracer gas can also be exploited.

Devendra Lal. 14 February 1929—1 December 2012

Devendra Lal was an Indian nuclear physicist who began his career studying particle physics while a student at the Tata Institute of Fundamental Research (TIFR) in Bombay, using tracks in nuclear emulsions to study cosmic ray particles and their interactions. He soon moved on to the search for radionuclides produced in the atmosphere by cosmic ray bombardment, independently (with colleagues) discovering radioisotopes of Be, P and Si and using them as geophysical tracers for atmospheric, meteorological and oceanographic processes. His career revolved principally around multiple aspects of cosmic rays, employing theory and experiment to examine their flux, chemical composition and energy spectrum, both at present and in the past through (for example) studies of particle tracks in the minerals of meteorites and lunar samples. He played a major role in developing approaches for the use of terrestrial cosmic-ray-produced isotopes as dating tools and tracers for a wide range of Earth processes, from biological cycles in the ocean to landscape evolution and ice ablation in the Antarctic. At various stages of his career Lal was professor at TIFR and led the geophysics group there, was professor and director of the Physical Research Laboratory in Ahmedabad, India, and was professor at the Scripps Institution of Oceanography, University of California San Diego. He was elected fellow of numerous scientific organizations and academies internationally, and was the recipient of many scientific awards and prizes.

Mercury as a Geophysical Tracer Gas - Emissions from the Emperor Qin Tomb in Xi\xb4an Studied by Laser Radar

Mercury is, because of its high vapor pressure and its prevalence in the atmosphere as atoms, an interesting geophysical tracer gas, also with potential archaeological applications. According to historical records dating back 2200 years, the mausoleum chamber of the “Terracotta Army Emperor” Qin in Xi´an, China, contains large amounts of liquid mercury, considered as an elixir of life at the time. We here report on measurements of the atmospheric contents of atomic mercury above the tomb mound performed with a mobile differential absorption lidar (light detection and ranging) system. Our measurements, which were performed from three different locations around the mound, indeed indicate elevated atmospheric mercury levels, with localizations, which correlate with previous in situ soil sampling results. Concentrations up to 27 ng/m3 were observed, significantly higher than the typical general pollutant level in the area which was found to be around 5–10 ng/m3. An out-flux of about 5×10−8 kg/s was estimated. Highly volatile mercury may be escaping through cracks, which developed in the structure over time, and our investigation supports ancient chronicle records on the tomb, which is believed never to have been opened/looted. Our findings also have bearings on the proposed use of mercury as a tracer gas for valuable ores and geothermal resource exploration, and also bring problematics around reliable nuclear waste long-term underground storage to mind.

Heat as a Proxy to Image Dynamic Processes with 4D Electrical Resistivity Tomography

Since salt cannot always be used as a geophysical tracer (because it may pollute the aquifer with the mass that is necessary to induce a geophysical contrast), and since in many contaminated aquifer salts (e.g., chloride) already constitute the main contaminants, another geophysical tracer is needed to force a contrast in the subsurface that can be detected from surface geophysical measurements. In this context, we used heat as a proxy to image and monitor groundwater flow and solute transport in a shallow alluvial aquifer (<10 m deep) with the help of electrical resistivity tomography (ERT). The goal of our study is to demonstrate the feasibility of such methodology in the context of the validation of the efficiency of a hydraulic barrier that confines a chloride contamination to its source. To do so, we combined a heat tracer push/pull test with time-lapse 3D ERT and classical hydrogeological measurements in wells and piezometers. Our results show that heat can be an excellent salt substitution tracer for geophysical monitoring studies, both qualitatively and semi-quantitatively. Our methodology, based on 3D surface ERT, allows to visually prove that a hydraulic barrier works efficiently and could be used as an assessment of such installations.

Radon exhalation and radiometric prospecting on rocks associated with Cu–U mineralizations in the Singhbhum shear zone, Bihar

The Singhbhum thrust belt is a 200 km long arcuate orogenic belt in Bihar, eastern India. The huge mineral resources, viz. copper, uranium, magnetite, apatite and molybdenite, etc., make it significant from an economic as well as a geological point of view. The belt hosts three types of mineralization: sulphides of copper and other metals, uranium oxides and apatite-magnetite. Several distinct geological episodes are responsible for the evolution of mineralization and the thrust zone itself. Extensive and reliable radiometric prospecting and assaying have been carried out by us for the past 5 years from Dhobani in the east to Turamdih in the west of the Singhbhum shear zone. The present work indicates uranium mineralization in the Pathargora–Rakha area presently being mined for copper and also within areas in the vicinity of Bhatin. Studies on radon emanation have also been undertaken in some parts of the shear zone which indicate reasonably high radon emanation of the soils and rocks studied. This suggests the need for regular monitoring and suitable controls on the mine environment (air quality) and its vicinity. Radon emanation studies coupled with gamma-ray spectrometry and the subsequent modelling of the radiometric and radon measurements will help in the application of radon as a geophysical tracer in exploration of radioactive ore bodies and in radon risk assessment as well as in delineating active and passive faults and even in petroleum exploration.

Modelling the dynamics and thermodynamics of icebergs

Icebergs are a significant hazard for polar shipping, and, geophysically, are significant components of the mass balance of continental ice sheets while providing major freshwater inputs to the polar oceans. Some modelling of iceberg trajectories has been undertaken in the past, principally in the Labrador Sea, but here we present a hemispheric-wide attempt to model iceberg motion in the Arctic and North Atlantic Oceans. We show that the basic force balance in iceberg motion is between water drag and water advection, but with the pure geostrophic balance being only a minor component of the latter. Iceberg density maps essentially demonstrate the effect of the major boundary currents but we show that the time and size of calving from individual tidewater glaciers are important variables in determining the ultimate fate of bergs. The biggest bergs never leave the Arctic Ocean. All modelled icebergs have melted after about 5 years from their release date, although most melt over the first year. During their lifetime most, but not all bergs, overturn several times. Our model shows good agreement with the limited observational data. We therefore suggest that icebergs, both modelled and observed, may be exploited as previously little-used geophysical tracers.

Resolving the intermediate and deep advective flows in the Indian Ocean by using temperature, salinity, oxygen and phosphate data: the interplay of biogeochemical and geophysical tracers

For computing large-scale advective flow in the Indian ocean (including the Indian-Antarctic sector), we use a box-model approach and perturbed inverse method. The top 400 meters is not considered in this study, in view of the dominant seasonal dynamics. We use 1244 hydrographic stations, to estimate mean values for temperature, salinity, oxygen and phosphate concentratons. Fifty perturbed inversions of steady-state tracers conservations and thermal-wind equations are done using box-averages standard deviations and a 25% perturbation on the thermal-wind coefficients. The mean solutions represent the large-scale advective flow and carbon-decomposition rates in which we are interested. Solutions with only advective processes are first considered. The broad features of the circulation in the Indian Ocean are resolved in the intermediate levels, but in deeper layers, an input from North Atlantic Deep Water (NADW) is not apparent. Inspection of oxygen and phosphate residuals reveals a biochemical signal. Therefore, we introduce in the oxygen and phosphate budgets a linear parameterization (Redfield ratios) for the organic-decomposition processes. The structure of the residuals for oxygen and phosphate is changed in that the biochemical signal vanishes. The advective solutions are nearly the same in intermediate waters; however, in deep layers the new solution shows an inflow of 11 (±8) Sv of NADW south of Africa. The calculated total organic decomposition of 0.93 (±0.25) 10 15g C year −1 is about one fifth of the estimated world ocean amount, but total residuals of oxygen and phosphate lead to an unexplained 0.5 10 15g C year −1 missing carbon sink. The new solution does contain unrealistic elements (e.g. large deep flow between Indonesia and Australia). Finally, to investigate this last result, we add one advective constraint at the Indonesia-Australia boundary. This addition changes the circulation in the northeastern part of the Indian Ocean. The circumpolar flow between 400 m and 27.65 (σo) remains unchanged at 20°E and at 80°E; however, at 130°E the constraint increases the advective flow by 20%. Total organic-carbon-decomposition rates are not affected by the additional inflow from Indonesia.

Resolving the intermediate and deep advective flows in the Indian Ocean by using temperature, salinity, oxygen and phosphate data: the interplay of biogeochemical and geophysical tracers

For computing large-scale advective flow in the Indian ocean (including the Indian-Antarctic sector), we use a box-model approach and perturbed inverse method. The top 400 meters is not considered in this study, in view of the dominant seasonal dynamics. We use 1244 hydrographic stations, to estimate mean values for temperature, salinity, oxygen and phosphate concentratons. Fifty perturbed inversions of steady-state tracers conservations and thermal-wind equations are done using box-averages standard deviations and a 25% perturbation on the thermal-wind coefficients. The mean solutions represent the large-scale advective flow and carbon-decomposition rates in which we are interested. Solutions with only advective processes are first considered. The broad features of the circulation in the Indian Ocean are resolved in the intermediate levels, but in deeper layers, an input from North Atlantic Deep Water (NADW) is not apparent. Inspection of oxygen and phosphate residuals reveals a biochemical signal. Therefore, we introduce in the oxygen and phosphate budgets a linear parameterization (Redfield ratios) for the organic-decomposition processes. The structure of the residuals for oxygen and phosphate is changed in that the biochemical signal vanishes. The advective solutions are nearly the same in intermediate waters; however, in deep layers the new solution shows an inflow of 11 (±8) Sv of NADW south of Africa. The calculated total organic decomposition of 0.93 (±0.25) 10 15g C year −1 is about one fifth of the estimated world ocean amount, but total residuals of oxygen and phosphate lead to an unexplained 0.5 10 15g C year −1 missing carbon sink. The new solution does contain unrealistic elements (e.g. large deep flow between Indonesia and Australia). Finally, to investigate this last result, we add one advective constraint at the Indonesia-Australia boundary. This addition changes the circulation in the northeastern part of the Indian Ocean. The circumpolar flow between 400 m and 27.65 (σo) remains unchanged at 20°E and at 80°E; however, at 130°E the constraint increases the advective flow by 20%. Total organic-carbon-decomposition rates are not affected by the additional inflow from Indonesia.

Steam and water pressure gauges

Icebergs are a significant hazard for polar shipping, and, geophysically, are significant components of the mass balance of continental ice sheets while providing major freshwater inputs to the polar oceans. Some modelling of iceberg trajectories has been undertaken in the past, principally in the Labrador Sea, but here we present a hemispheric-wide attempt to model iceberg motion in the Arctic and North Atlantic Oceans. We show that the basic force balance in iceberg motion is between water drag and water advection, but with the pure geostrophic balance being only a minor component of the latter. Iceberg density maps essentially demonstrate the effect of the major boundary currents but we show that the time and size of calving from individual tidewater glaciers are important variables in determining the ultimate fate of bergs. The biggest bergs never leave the Arctic Ocean. All modelled icebergs have melted after about 5 years from their release date, although most melt over the first year. During their lifetime most, but not all bergs, overturn several times. Our model shows good agreement with the limited observational data. We therefore suggest that icebergs, both modelled and observed, may be exploited as previously little-used geophysical tracers.