Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study

Airborne time-domain electromagnetic methods (AEM) are useful for hydrogeological mapping due to their rapid and extensive spatial coverage and high correlation between measured magnetic fields, electrical conductivity, and relevant hydrogeological parameters. However, AEM data, preprocessing and modelling procedures can suffer from inaccuracies that may dramatically affect the final interpretation. We demonstrate the importance and the benefits of advanced data processing for two AEM datasets (AeroTEM III and VTEM) collected over the Spiritwood buried valley aquifer in southern Manitoba, Canada. Early-time data gates are identified as having significant flightdependent signal bias that reflects survey flights and flight lines. These data are removed from inversions along with late time data gates contaminated by apparently random noise. In conjunction with supporting information, the less-extensive, but broader-band VTEM data are used to construct an electrical reference model. The reference model is subsequently used to calibrate the AeroTEM dataset via forward modelling for coincident soundings. The procedure produces calibration factors that we apply to AeroTEM data over the entire survey domain. Inversion of the calibrated data results in improved data fits, particularly at early times, but some flight-line artefacts remain. Residual striping between adjacent flights is corrected by including a mean empirical amplitude correction factor within the spatially constrained inversion scheme. Finally, the AeroTEM and VTEM data are combined in a joint inversion. Results confirm consistency between the two different AEM datasets and the recovered models. On the contrary, joint inversion of unprocessed or uncalibrated AEM datasets results in erroneous resistivity models which, in turn, can result in an inappropriate hydrogeological interpretation of the study area

Incorporating a-priori information into AEM inversion for geological and hydrogeological mapping of the Spiritwood Valley Aquifer, Manitoba, Canada

Buried valleys are important hydrogeological structures in Canada and other glaciated terrains, providing sources of groundwater for drinking, agriculture and industrial applications. Hydrgeological exploration methods such as pumping tests, boreholes coring or ground-based geophysical methods (seismic and electrical resistivity tomography) provide limited spatial information and are inadequate to efficiently predict the sustainability of these aquifers at the regional scale. Airborne geophysics can be used to significantly improve geological and hydrogeological knowledge on a regional scale. There has been demonstrated success at using airborne electromagnetics for mapping and characterization of buried valleys in different geological contexts (Auken et al., 2008; Jørgensen et al., 2003; Jørgensen et al., 2009; Steuer et al., 2009). Despite the fact that both electromagnetic surveys and reflection seismic profiling are used extensively in hydrogeological mapping, integration of the methods is a relatively unexplored discipline (Høyer et al., 2011). The Spiritwood Valley is a Canada-USA trans-border buried valley aquifer that runs approximately NW – SE and extends 500 km from Manitoba, across North Dakota and into South Dakota (Winter et al., 1984). The Spiritwood aquifer system consists of glacially deposited silt and clay with sand and gravel bodies, infilling a broad north-south trending valley that has been identified primarily based on water wells information (Wiecek, 2009). The valley is incised into bedrock consisting of fractured siliceous shale. As part of its Groundwater Geoscience Program, the Geological Survey of Canada (GSC) has been investigating buried valley aquifers in Canada using airborne and ground-based geophysical techniques. To obtain a regional three-dimensional assessment of complex aquifer geometries for the Spiritwood, both geophysical and geological investigations were performed with the aim to develop an integrated conceptual model for a quantitative description of the aquifer system. In 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AeroTEM III) survey over a 1062 km2 area along the Spiritwood Valley, north of the US border (Oldenborger 2010a, 2010b). AEM inversion results show multiple resistive valley features inside a wider, more conductive valley structure within the conductive bedrock (Fig. 1). Furthermore, the complexity of the geometries, spatial distribution and size of the channels is evident. Other ground based data collected in the survey area make it possible to provide some constraints on the AEM resistivity model. Downhole resistivity logs were collected that provide information on the electrical model relative to the geological layers (Crow et al., 2012). In addition, over 10 line-km of electrical resistivity data and 42 km of high resolution landstreamer seismic reflection data (Figs. 2a, 2b) were collected at selected sites (Oldenborger et al., 2012). In this short paper we present results obtained from the data inversion and an example of integration of ancillary seismic data into the AEM inversion. In particular, the elevation to a layer (shale bedrock elevation) as interpreted from seismic is added to the inversion to constrain the resistivity model

Peatland Volume Mapping Over Resistive Substrates With Airborne Electromagnetic Technology

open6siDespite the importance of peatlands as carbon reservoirs, a reliable methodology for the detection of peat volumes at regional scale is still missing. In this study we explore for the first time the use of airborne electromagnetic (AEM) to detect and quantify peat thickness and extension of two bogs located in Norway, where peat lays over resistive bedrock. Our results show that when calibrated using a small amount of field measurements, AEM can successfully detect peat volume even in less ideal conditions, that is, relatively resistive peat over resistive substrata. We expect the performance of AEM to increase significantly in presence of a conductive substratum without need of calibration with field data. The organic carbon content retrieved from field surveys and laboratory analyses combined with the 3-D model of the peat extracted from AEM allowed us to quantify the total organic carbon of the selected bogs, hence assessing the carbon pool.openSilvestri S.; Christensen C.W.; Lysdahl A.O.K.; Anschutz H.; Pfaffhuber A.A.; Viezzoli A.Silvestri S.; Christensen C.W.; Lysdahl A.O.K.; Anschutz H.; Pfaffhuber A.A.; Viezzoli A

Recent AEM Case Study Examples of a Full Waveform Time-Domain System for Near-Surface and Groundwater Applications

Early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas but this poses difficulties due to a variety of issues, such as system bandwidth, system calibration and parasitic loop capacitance. In an effort to address this issue, a continued system design strategy, aimed at improving its early-channel VTEM data, has achieved fully calibrated, quantitative measurements closer to the transmitter current turn-off, while maintaining reasonably optimal deep penetration characteristics. The new design implementation, known as “Full Waveform” VTEM was previously described by Legault et al. (2012). This paper presents some case-study examples of a Full Waveform helicopter time-domain EM system for near-surface application

Circular dichroism and 1H NMR studies of Co2+- and Ni2+-substituted concanavalin A and the lentil and pea lectins.

Visible absorption, circular dichroism (CD) and magnetic circular dichroism spectra have been recorded for the Ca2+-Co2+ derivatives of the lentil (CCoLcH) and pea (CCoPSA) lectins (Co2+ at the S1 sites and Ca2+ at the S2 sites) and shown to be very similar for both proteins. The visible absorption and magnetic circular dichroism spectra indicate similar octahedral geometries for high spin Co2+ at S1 in both proteins, as found in the Ca2+-Co2+ complex of concanavalin A (CCoPL) (Richardson, C. E., and Behnke, W. D. (1976) J. Mol. Biol. 102, 441-451). The visible CD data, however, indicate differences in the environment around S1 of CCoLcH and CCoPSA compared to CCoPL. 1H NMR spectra at 90 MHz of the Co2+ and Ni2+ derivatives of the lectins show a number of isotropically shifted signals which arise from protons in the immediate vicinity of the S1 sites. Analysis of the spectra of the Co2+ derivatives in H2O and D2O has permitted resonance assignments of the side chain ring protons of the coordinated histidine at S1 in the lectins. Differences are observed in the H-D exchange rate of the histidine NH proton at S1 in concanavalin A compared to the lentil and pea lectins. NMR data of the Ni2+-substituted proteins, together with spectra of the Co2+ derivatives, also indicate that the side chains of a carboxylate ligand and of the histidine residue at S1 are positioned differently in concanavalin A than in the other two lectins. These results appear to account, in part, for the differences observed in the visible CD spectra of the Co2+-substituted proteins. In addition, binding of monosaccharides does not significantly perturb the spectra of the lectins. An unusual feature in the 1H NMR spectra of all three Co2+-substituted lectins is the presence of two exchangeable downfield shifted resonances which appear to be associated with the two protons of a slowly exchanging water molecule coordinated to the Ca2+ ion at S2. T1 measurements of CCoLcH have provided an estimation of the distances from the Co2+ ion to these two protons of 3.7 and 4.0 A

Integrated interpretation of IP and TEM data for salinity monitoring of aquifers and soil in the coastal area of Muravera (Sardinia, Italy)

The problem of aquifers and soil salinity is addressed in the coastal area of Muravera (SE Sardinia, Italy) with the combined use of electrical resistivity and induced polarization measurements along three lines, and with 41 Transient Electromagnetic (TEM) soundings distributed in the study area. The resistivity and chargeability sections provide a detailed information on the hydrogeological conditions in the first 20 - 60 m depth, clarifying the role of both clay and salt water and eventually confirming the local near-surface hydrogeological model. The resistivity maps obtained for different depth ranges by TEM data inversion, give a clear and effective representation of the saline contamination both laterally and down to a depth of about 70 m, over the whole investigated area. The combination of the geophysical techniques (resistivity tomography with Induced Polarization measurements, integrated with TEM soundings) proves very effective as an approach for soil and water salination monitorin

Modeling induced polarization effects in helicopter time domain electromagnetic data: Synthetic case studies

We have developed a synthetic multiparametric modeling and inversion exercise undertaken to study the robustness of inverting airborne time-domain electromagnetic (TDEM) data to extract Cole-Cole parameters. The following issues were addressed: nonuniqueness, ill posedness, dependency on manual processing and the effect of constraints, and a priori information. We have used a 1D layered earth model approximation and lateral constraints. Synthetic simulations were performed for several models and the corresponding Cole-Cole parameters. The possibility to recover these models by means of laterally constrained multiparametric inversion was evaluated, including recovery of chargeability distributions from shallow and deep targets based on analysis of induced polarization (IP) effects, simulated in airborne TDEMdata. Different scenarios were studied, including chargeable targets associated with the conductive and resistive environments. In particular, four generic models were considered for the exercise: a sulfide model, a kimberlite model, and two generic models focusing on the depth of investigation. Our study indicated that, in cases when relaxation time (t) values are in the range to which the airborne electromagnetic is most sensitive (e. g., approximately 1 ms), it is possible to recover deep chargeable targets (to depths more than 130 m) in association with high electrical conductivity and in resistive environments. Furthermore, it was found that the recovery of a deep conductor, masked by a shallower chargeable target, became possible only when full Cole-Cole modeling was used in the inversion. Lateral constraints improved the recoverability of model parameters. Finally, modeling IP effects increased the accuracy of recovered electrical resistivity models