Mapping pollution and coastal hydrogeology with helicopterborne transient electromagnetic measurements

Christensen, N. E.; Halkjær, Max

doi:10.1071/eg13071

Cited by 12 publications

(9 citation statements)

References 33 publications

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“…Since the publication of Christensen and Tølbøll ()’s work, the LPC method has been used in the inversion of airborne data from several surveys for both hydrogeophysical and geo‐technical purposes (Christensen et al . ; Christensen and Halkjær ; Christensen and Reid ; Reid J., Christensen, and Godber; Lawrie et al a, b).…”

Section: A Strictly Horizontal Lateral Parameter Correlationmentioning

confidence: 99%

Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets

Christensen

2016

Near Surface Geophysics

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This paper presents new developments to the lateral parameter correlation method, a method that can be used to invoke lateral smoothness in model sections of one‐dimensional inversion models. The lateral parameter correlation method has three steps. First, all datasets are inverted individually. Next, a laterally smooth version of each model parameter is found by solving a simple constrained inversion problem by postulating identity between the uncorrelated and correlated parameters and solving the equations including a model covariance matrix that ensures lateral smoothness. As a final step, all sounding data are reinverted with the correlated model parameter values as a priori values to produce models that better fit the data. Because the method separates inversion and correlation, it is much faster than methods where the inversion and correlation are solved simultaneously. The new development to the lateral parameter correlation method presented in this paper is an option to perform a strictly horizontal correlation, thereby avoiding the model artefacts sometimes seen when correlating along layers, namely that formations tend to follow the topography. Furthermore, a solution to the intractable computation times arising with large datasets is formulated, employing a tessellation of the plane and an averaging scheme within the subareas that reduces the size of the numerical lateral parameter correlation inversion problem while maintaining correct correlation within a very large area. In a field example, correlation along layers and the new strictly horizontal correlation are compared, and it is demonstrated that the horizontal correlation removes the above mentioned artefacts that can appear when correlating along layers. The lateral parameter correlation method is very flexible and is capable of correlating models from inversion of different data types, including information from boreholes, and it lends itself easily to “embarrassingly parallel” computation.

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Section: A Strictly Horizontal Lateral Parameter Correlationmentioning

confidence: 99%

Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets

Christensen

2016

Near Surface Geophysics

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“…The AEM method is particularly advantageous for watershed to regional-scale salinity studies as it provides high-resolution resistivity information with greater spatial coverage than is practically feasible with well data or ground-based geophysical surveys. AEM surveys have been used to evaluate groundwater salinity for various hydrogeologic scenarios that include studies of coastal aquifers and saltwater intrusion (Christensen & Halkjaer, 2014;Delsman et al, 2018;Faneca Sànchez et al, 2012;Fitterman & Deszcz-Pan, 1998;Goebel et al, 2019;Haider et al, 2015;Jørgensen et al, 2012;Kirkegaard et al, 2011;Schamper et al, 2013;Siemon et al, 2015;Teatini et al, 2011;Viezzoli et al, 2010), salinity mapping in irrigated and arid regions (Cresswell et al, 2004;Cresswell et al, 2007;, the location of brines Dugan et al, 2015), and the shallow occurrence of produced water from oil and gas production (Paine, 2003;Paine et al, 1997;Paine & Collins, 2003;Smith et al, 1992;Thamke & Smith, 2014). equations in light of other hydrogeologic properties can be assessed (e.g., Gillespie, Davis, Stephens, et al, 2019;Stephens et al, 2019).…”

Section: Salinity Mapping Using Aem Datamentioning

confidence: 99%

Probabilistic Categorical Groundwater Salinity Mapping From Airborne Electromagnetic Data Adjacent to California's Lost Hills and Belridge Oil Fields

Ball

Davis

Minsley

et al. 2020

Water Resources Research

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Growing water stress has led to emerging interest in protecting fresh and brackish groundwater as a potential supplement to water supplies and raised questions about factors that could affect the future quality of fresh and brackish aquifers. Limited well infrastructure, particularly in regions where elevated salinity has led to limited historical groundwater development, hinders traditional mapping of salinity distributions through groundwater sampling. This paper presents a quantitative salinity mapping approach of the upper 300 m using high-resolution, regionally comprehensive resistivity models derived from Bayesian inversion of an airborne electromagnetic survey adjacent to the Lost Hills and Belridge oil fields in the southwestern San Joaquin Valley of California. Using local water quality observations as an interpretational foundation, a probabilistic approach yields maps of fresh, saline, and brackish groundwater while quantifying joint uncertainty inherited from the geophysical data and interpretational relations. Saline and fresh regions are mapped with relatively high confidence in many locations, while areas of lower confidence, particularly at depth, can be mapped as their most probable salinity category while reflecting the relative uncertainty in the interpretation. These maps identify a stratified salinity structure, where saline water commonly occurs in the surficial aquifer overlying fresher groundwater in the Tulare aquifer, separated by regional confining clay layers. Downgradient of unlined surface water diversions, recharge of imported surface water results in relatively fresh groundwater throughout the depth of investigation. Plain Language Summary Salinity, a measure of salt concentration, can impact the potential use of groundwater. Salinity can be difficult to map using wells alone, especially where groundwater development has been limited in the past. Dissolved salts are excellent conductors of electricity, and resistivity measurements are commonly used to estimate salinity where wells are not available. Uncertainty in estimating salinity from resistivity data can be introduced into the interpretation by two sources: (1) how well we can estimate salinity from resistivity and (2) how accurately we can measure resistivity. We present a method for mapping salinity that accounts for both sources of uncertainty using regional-scale, high-resolution airborne geophysical data, leading to spatially comprehensive 3-D maps of likely fresh (low salt) and saline (high salt) groundwater. The method is applied in the southwestern San Joaquin Valley of California adjacent to the Lost Hills and Belridge oil fields. Shallow groundwater is shown to have relatively high salinity in much of the study area, and a clay layer protects lower salinity water in the underlying Tulare aquifer. Leaking surface water canals have imported relatively fresh water into the area, resulting in fresher groundwater downgradient of the canals.

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“…Comparatively few studies have used EMI methods in coastal environments (Paine et al, 2004;Seijmonsbergen et al, 2004;Vrbancich, 2009;Christensen and Halkjaer, 2010;Nenna et al, 2013;Delefortrie et al, 2014b), with most of these focusing on mapping saltwater intrusion. Most of these studies use Geonics EM31, 34, 38, and similar frequencydomain sensors; Geonics EM47, 63, and similar time-domain electromagnetic (TDEM) sensors in addition to various airborne electromagnetic (AEM) systems.…”

Section: Application Of Emi Methods In Coastal Studiesmentioning

confidence: 99%

“…AEM surveys are important for coastal studies but are beyond the scope of this paper. Previous coastal EMI studies have explored subsurface σ because it is related to framework geology (Seijmonsbergen et al, 2004;Vrbancich, 2009), classification of coastal wetlands (Paine et al, 2004), and investigation of coastal groundwater dynamics and pollution (Goldman et al, 1991;Fitterman and Deszcz-Pan, 1998;Christensen and Halkjaer, 2010;Nenna et al, 2013). Seijmonsbergen et al (2004) use the EM34 (albeit not a portable multifrequency EMI profiler) at 20 m station spacing and 20 m coil separation to acquire a 14.5 km transect along a segment of the Dutch coast, Netherlands.…”

Section: Application Of Emi Methods In Coastal Studiesmentioning

confidence: 99%

“…The results suggest that TDEM methods can be used to characterize saltwater intrusion in coastal aquifer systems and infer the continuity of confining layers between saturated layers with different water qualities. A different approach using transient AEM was used by Christensen and Halkjaer (2010) to map North Sea coastal hydrology at a heavily polluted site in western Jutland, Denmark, which suggests that transient AEM systems can be used to delineate the extent of the pollution plume, the fresh/saltwater boundary, and the complex pattern of subsurface preferential flow channels along the coast. Although the above studies demonstrate the value of EMI sensors for coastal research, most have not examined in detail the effects of changing hydrology as it relates to framework geology over different spatialtemporal scales.…”

Section: Application Of Emi Methods In Coastal Studiesmentioning

confidence: 99%

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Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

et al. 2016

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Electromagnetic induction (EMI) techniques are becoming increasingly popular for near-surface coastal geophysical applications. However, few studies have explored the capabilities and limitations of portable multifrequency EMI profilers for mapping large-scale (10 1 -10 2 km) barrier island hydrogeology. The purpose of this study is to investigate the influence of groundwater dynamics on apparent conductivity σ a to separate the effects of hydrology and geology from the σ a signal. Shorenormal and alongshore surveys were performed within a highly conductive barrier island/wind-tidal flat system at Padre Island National Seashore, Texas, USA. Assessments of instrument calibration and signal drift suggest that σ a measurements are stable, but vary with height and location across the beach. Repeatability tests confirm σ a values using different boom orientations collected during the same day are reproducible. Measurements over a 12 h tidal cycle suggest that there is a tide-dependent step response in σ a , complicating data processing and interpretation. Shore-normal surveys across the barrier/wind-tidal flats show that σ a is roughly negatively correlated with topography and these relationships can be used for characterizing different coastal habitats. For all surveys, σ a increases with decreasing frequency. Alongshore surveys performed during different seasons and beach states reveal a high degree of variability in σ a . Here, it is argued that surveys collected during dry conditions characterize the underlying framework geology, whereas these features are somewhat masked during wet conditions. Differences in EMI signals should be viewed in a relative sense rather than as absolute magnitudes. Small-scale heterogeneities are related to changing hydrology, whereas low-frequency signals at the broadest scales reveal variations in framework geology. Multiple surveys should be done at different times of the year and tidal states before geologic interpretations can confidently be made from EMI surveys in coastal environments. This strategy enables the geophysicist to separate the effects of hydrology and geology from the σ a signal.

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Mapping pollution and coastal hydrogeology with helicopterborne transient electromagnetic measurements

Cited by 12 publications

References 33 publications

Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets

Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets

Probabilistic Categorical Groundwater Salinity Mapping From Airborne Electromagnetic Data Adjacent to California's Lost Hills and Belridge Oil Fields

Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

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