NS43A-01 INVITED
Mapping Soil Properties for Ecohydrological Studies in Small Semi-Arid Watersheds using Electromagnetic Induction
While traditional methods of soil mapping provide a qualitative description of soil properties across a landscape they fail to provide spatially detailed quantitative data needed in ecohydrological studies. Electromagnetic induction (EMI) mapping is a useful tool for mapping soil properties where the penetration depth of a handheld sensor is just over a meter when carried 0.2 m above the soil surface, and the measurement integrates over a volume of several cubic meters, similar to the scale of a soil pedon. Our recent research efforts have focused on mapping soil properties in a semi-arid watershed at Reynolds Creek, ID, using geophysical methods with the idea of complimenting point-measured data using distributed sensors. In the Reynolds Creek environment the stream flow response is controlled largely by the subsurface and hence the identification of soil hydrological properties and the delineation of soil boundaries is important for modeling the watershed hydrology. Data collected using electromagnetic induction provide spatially informative maps tied to the subsurface bulk electrical conductivity (EC). The utility of bulk EC measurements for delineating soils stems from measured sensitivity to the physical and biogeochemical properties. Instrument output tends to increase in soils with more 2:1 clay, higher water content and higher solute concentration. Therefore, maps of bulk electrical conductivity are good for differentiating between coarse and fine textured soils and for identifying hydrological flow pathways. Preliminary data suggest EMI maps can be correlated to provide detailed spatial information of soil texture, water content, hydraulic conductivity and hydrological flow pathways in catchment hydrology, potentially improving hydrological model parameter estimation.
NS43A-02
Geophysical Characterization Of A Cover With Capillary Barrier Effect
Covers with capillary barrier effects (CCBE) can be used to limit gas flux into or out of mine waste disposal sites. The hydraulic behaviour of such type of cover is influenced by slope inclination, which can induce a local desaturation that is detrimental to its efficiency. The covers we have studied consist of three layers, from bottom to top: a 0.5 m layer of sand used as support and a capillary break layer; a fine-grain moisture-retaining layer with a thickness of 0.8 m; and a 0.3 m sand and gravel layer on top for drainage and used as a protective layer against erosion and evaporation . The fine-grain layer must remain permanently saturated in order to be an effective barrier. TDR probes are installed to monitor saturation level; however the number of probes installed is always too small over the very large area of mine wastes protected by CCBE to lead to a good spatial estimation of water content. Also, suction can vary, especially on slopes, which can lead to desaturation by drainage of the moisture- retaining layer and the loss of imperviousness to oxygen. We have investigated ways to estimate water saturation changes of the cover using geophysics. Modelling show that changes in water content within the cover will affect the GPR and resistivity responses. We have carried out a survey over the flanks and the top of a large mining waste site located in North-western Quebec (Canada). We use GPR with 200 MHz, 450 MHz and 900 MHz antennas as well as a Lund resistivity imaging system from ABEM with 0.30 m dipoles to image stratigraphy and water content from surface to 1.7 m depth. Thicknesses of each of the three layers within the cover can be mapped with detail. Correlations with TDR data show that the techniques can be used to estimate saturation conditions within the fine-grained barrier with good lateral resolution. The survey has also allowed mapping a suction break installed within one of the slope profile.
NS43A-03
Geotechnical and Petrophysical Properties of Unconsolidated Materials from Electrical Measurements
Micro-structural modifications in unconsolidated geo-materials due to changes in effective stress affect the stability and strength of such materials. These processes often result in geo-hazards (e.g., landslides, liquefaction, debris flow) which claim so many lives, destroy the environments and cause considerable amounts of property damage. The effect of confining and pore pressure (effective stress) on the petrophysical, geotechnical as well as the electrical properties of sand-clay mixtures is investigated in a laboratory environment. Spectral electrical response (SER) measurements are performed over a range of frequencies (0.001Hz-10kHz) in a load cell connected to a transmitter-receiver unit. Useful electrical parameters are extracted from the responses and related to the relevant petrophysical/geotechnical parameters, that is, the void ratio, dry density and the modulus of elasticity. These relations can be useful in predicting the physical, mechanical and geotechnical properties of unconsolidated geo-materials in transition from frame-supported to suspensions or (from high to low effective stress). Utilization of insights and understanding gained from the laboratory measurements will be useful in achieving the ultimate goal of using non-invasive geophysical methods to monitor and assess the potential instabilities in unconsolidated geo-materials supporting sensitive structures.
NS43A-04
Long term monitoring of biogenic gasses in peat soils using electromagnetic (EM) measurements
Peatlands are a critical component of the global carbon cycle and play a major role in atmospheric fluxes. Peat soils are considered one of the largest natural sources for greenhouse gas emissions (e.g. methane and carbon dioxide) to the atmosphere, but the mechanisms of formation and spatial distribution of these gasses within the soil matrix still remain uncertain. In this work we investigate the use of ground penetrating radar (GPR) as a geophysical tool to estimate the long term temporal evolution and spatial variability of biogenic gasses at the field scale. We performed high resolution EM measurements over a large section (4m x 4m x 6m) of a northern peatland (Caribou Bog, Maine) for almost an entire year. Methane emissions and surface peat deformation were concurrently monitored. All measurements were performed along a platform built to avoid disturbance of the peat surface and consistently revealed large shifts in gas content. Our results demonstrate that GPR measurements are a totally non-invasive way of investigating long term gas evolution without any disturbance to the natural gas regime (such as inserting monitoring probes), and provide unique information on the spatial distribution of biogenic gasses in peat soils. These findings also have implications for climate modeling efforts as related to biogenic gas emissions in peat soils and its impact on global warming.
NS43A-05
Resistivity-based geophysical monitoring of biogenic gas dynamics in a large peat block
Northern peatlands are an important component of the global carbon (C) cycle, estimated to account for 10 percent of annual methane flux to the atmosphere. Yet considerable uncertainty exists regarding the spatial and temporal character of free phase gas (FPG) build up and release in peat soils. We describe here an experiment on a large peat block (dimensions .28 m x 0.2 m x 0.2 m) whereby electrical resistivity measurements were made over a 66-day period of biogenic gas build up and release. Vertical profiles of resistivity were obtained using five vertical resistivity arrays (20 electrodes at 0.01 m spacing per array) distributed throughout the block. In addition to the geophysical measurements, methane release was quantified with a portable gas detector and surface deformation (due to peat expansion as a result of gas build up) recorded using 30 rods distributed equally across the block surface. Pore fluid conductivity was also recorded at three depths (0.05, 0.09 and 0.15 m below the peat surface). The geophysical measurements, after correction for increases in pore fluid conductivity apparently driven by methanogenesis, exhibit a distinct pattern of resistivity variation that we interpret as a result of repeated build up and release of a resistive gas phase. This interpretation is supported by the significant correlation between resistivity change and associated surface deformation due to expansion, as well as periods of maximum resistivity change generally occurring during maximum gas emission as estimated with the portable gas detector. Inversion of the resistivity datasets obtained from the five arrays yielded clear images of the spatial distribution of gas accumulation within the block. The inversion clearly shows that the FPG build up is non- uniform within the block and primarily concentrated in (a) a zone about 5 cm below the peat surface, and (b) a zone below 0.16 m towards the base of the peat block. The images of predicted gas build up are remarkably consistent with previously published vertical profiles of methane concentration produced/accumulated in peat soils and support the concept that production is focused within a higher porosity zone of more rapid organic matter decomposition just below the water table.
NS43A-06
Degree And Extension Of Soil Pollution Around Polish Iron Factories And Main Roads As Indicated By Magnetometric Studies
Magnetic iron minerals, whose presence in soils can easily be detected by magnetic susceptibility measurements, are components of numerous industrial dusts. There is a linear dependence between magnetic particles content and content of heavy metals like: Cu, Fe, Pb and Zn (in urban dusts). A similar dependence was observed in atmospheric dusts (for Pb, Cu, Zn, and Cd). If the origin of magnetic particles and a considerable part of heavy metals permeating into soils as a result of industrial emission or imission is the same, the application of magnetic susceptibility measurements for the detection of potential high risk-areas is possible. Several roads and ironworks are surrounded by the cereal and other plants cultivation fields. Cultivated soils are often polluted by heavy metals. Two Polish ironworks and several sectors of roads were chosen for the case study. Lateral and vertical extension of soils pollution are studied and compared with several parameters like type of soil and directions of prevailing winds. Maps of magnetic susceptibility were prepared basing on measurements collected directly in the field, using Bartington device. Extents of increased values are explicitly visible on maps. Their directions correspond with prevailing wind directions, and increased concentrations of heavy metals are expected at a distance up to 1.5 km from the ironworks and up to 80 m from the roads. Several soils profiles were taken from the polluted areas. Vertical migration of heavy metals were determined by further petromagnetic and geochemical analysis of profile samples, taken with 5cm step. In the soils from ironworks areas magnetic susceptibility decreases rapidly from the top level to 15 cm where it stabilizes at the level characteristic for natural podzols and sands. It is confirmed by geochemical analysis, showing decrease of Fe as well as Zn and Pb content at the same depth distance. SEM images shows that there are numerous Fe spherules in top samples of studied profiles. They have miscellaneous structures, usually with empty space inside. Some titanomagnetites appears at subsoils of profile from ironworks area causing abovementioned increased values of magnetic susceptibility. Those minerals are visible at SEM images as irregular grains, sometimes with Al, Ca, Si jacket.
NS43A-07
Acoustic Techniques for Studying Soil-surface Seals and Crusts
The impact of raindrops on a soil surface during a rainstorm may cause soil-surface sealing and crusting. Soil- surface sealing is a result of the clogging in interaggregate pores by smaller suspended particles in the water, which reduces the infiltration capacity of soils. Soil-surface crusting refers to the increase in soil strength or mechanical stiffness associated with near surface compaction or densification. The formation of soil-surface seals and crusts have a profound influence on the erodability of soils, with the consensus being that the reduced hydraulic conductivity due to sealing is the more important factor. However, studies note that measured values of seal hydraulic conductivity are few. The reason so few measurements may be because the thickness of the altered surface layer is on the order of millimeters. For example Lee (2006) states that a soil-surface seal consist of two parts: a 0.1mm thick upper skin seal attributed to compaction by the rain drop impact and a deeper 1.5 mm "washed in" zone with decreased porosity due to the accumulation of particles. Bulk density profiles measured using X-radiography show maximum changes in the top 5 mm of the soil. The surface of the ground (soil) has an influence on the propagation of sound outdoors. The porosity, air flow- resistivity, and tortuosity of the ground are the properties, which characterize the influence of the ground on the airborne sound. The air flow-resistivity of a dry soil is equivalent to the hydraulic conductivity of a water-saturated soil. In this presentation we discuss two acoustic techniques, one with sensitivity to changes in hydraulic properties (sealing) and the other to changes in mechanical stiffness (crusting). These non-contact techniques excite the soil using a suspended loudspeaker to impinge acoustic energy from the air (sound) onto the sample. The response of the soil is quantified using a microphone to measure the total pressure above the soil surface and a laser Doppler vibrometer (LDV) to measure the surface solid particle velocity. Changes in soil-surface hydraulic conductivity are examined by observing the relative change in total pressure at the soil surface. The soil-surface stiffening is quantified by the ratio of LDV response to the measured total pressure and is referred to as the acoustic-to-seismic admittance. Measurements on two different soils having different erodability characteristics and subject to a simulated rainstorm are presented.