HEM Data Research Articles

Development of a novel approach for recovering SIP effects from 1-D inversion of HEM data: Case study from the Alut area, northwest of Iran

We have developed a novel approach for recovering the induced polarization (IP) effects from one-dimensional (1-D) inversion of helicopter-borne electromagnetic data (HEM). To achieve this aim, we incorporate both electromagnetic and spectral induced polarization effects within the HEM data by taking the frequency dependence behaviour of resistivity/conductivity of polarizable formation into account. Furthermore, we have included the EM sensor altitude, magnetic permeability and dielectric permittivity of subsurface layers in order to conduct the full solution of forward operator. In this regard, we have calculated the magnetic permeability from the inversion of aeromagnetic data and incorporated the sensor altitude and dielectric permittivity as additional model parameters to be recovered using the inversion of the HEM data. We have carried out both forward and inverse modelling of the HEM data by implementing fast Hankel transform and hybrid algorithm including very fast simulated annealing (VFSA) and Marquardt-Levenberg (ML) strategies, respectively. This approach enables us to recover model parameters comprising of relaxation parameters of Cole-Cole model, dielectric permittivity and thickness of layered half space model and sensor altitude above the earth surface using 1-D inverse modelling of HEM data.We have tested our approach on both synthetic and field DIGHEM data. Implementing the forward solution indicates that incorporation of the IP effects may shift the frequency domain electromagnetic (FDEM) response so that it yields the sign reversal signature in the in-phase component at low frequencies. Moreover, the results of the inverse modelling of the HEM data show that the IP effect could be recovered using our approach effectively. It provides fast and cost-effective quantitative tool for recovering the IP effect from HEM data for the sake of more precise characterization of buried polarizable formations in the wide area. In addition, incorporation the IP effect along with magnetic permeability and sensor altitude provides a significant degree of improvement in the inversion and interpretation of negative responses occurred in the in-phase component at low frequencies.

Comparing resistivity models from 2D and 1D inversion of frequency domain HEM data over rough terrains: cases study from Iran and Norway

ABSTRACTFrequency domain helicopter-borne electromagnetic (FHEM) surveys have been used as an effective tool for the exploration of underground resources for as long as airborne electromagnetics (AEM) has existed. Large FHEM data sets are commonly interpreted with very fast 1D inversion algorithms that often numerically adequately fit the data sets, even though they yield incorrect results near 2D/3D geological structures. The present study aims to compare 1D and 2D inversion algorithms when applied to the reconstruction of geologically complex regions. We have developed a 2D inversion algorithm incorporating the Levenberg–Marquardt least-squares approach regularised through spatial constraints to retrieve 2D electrical resistivity models associated with arbitrary surface topography. The approach uses a 2D finite element frequency domain solution and a tailored triangular meshing algorithm based on the Ruppert’s Delaunay refinement for the forward modelling. We illustrate how rough topographic effects obscure the FHEM response and affect recovered resistivity models through numerical experiments. We also demonstrate the influence of acquisition frequency and resistivity structure on the topographic effect. In the Appendix, we discuss the FHEM footprint concept from a 2D perspective to assess how 2D effects affect and bias 1D inversion results. Complex 2D synthetic scenarios are presented to compare 1D and 2D inversion in various settings. Two field cases from Norway and Iran are presented to show the model improvements with 2D inversion. For the Norwegian case, the 2D FHEM inversion aligns well with a model retrieved from ground-based electrical resistivity tomography. We show the bias imposed on the 2D inversion of the data set from Iran by improper system calibration.

Base level (drift) estimation for frequency domain HEM data using causality, an update on dispersion relations and the Hilbert transform for EM system response

As a check on frequency domain systems for calibration and drift, we aim to predict in-phase from quadrature data and vice versa. It is known that, for causal systems, the real and imaginary components of the response of the system under excitation are related through dispersion relations. The formal relationships between the components of any complex response function are the Kramers-Kronig relations, which are satisfied by any complex function which is analytic in the upper half plane, representing causality. Geophysical signals are inherently causal and as such it is in principle always possible to relate in-phase to quadrature measurements. In the frequency domain these dispersion relations take the form of the Hilbert transformation relationship between scaled in-phase and quadrature measurements which for specific ideal Earth structures are able to be solved analytically. However real Earth responses are rarely captured by these idealised models and numerical approaches are required. The major limitation on the successful implementation of the numerical Hilbert transform is limited bandwidth measurements which introduce significant noise constraints. Frequency domain measurements rely on the existence of frequency dependent responses of materials for the detection of anomalies. The response function is nominally the resistivity (conductivity) and experimental determination of this response function involves two measurements per frequency, the in-phase and quadrature components. There are several possible applications of the prediction of in-phase response from quadrature data including 1) base level, calibration and phase checks on electromagnetic and induced polarisation systems 2) prediction and validation of noise levels in in-phase from quadrature measurements and vice versa and 3) interpolation and extrapolation of sparsely sampled data enforcing causality and better frequency-domain - time-domain transformations. This could be used for example to speed up forward and inverse modeling without loss of accuracy. In practice, using recalibrated Resolve HFEM data, in-phase data points can only be predicted using a scaled Hilbert transform with a standard deviation between 20 ppm (central frequencies) and 100 ppm (outer frequencies. This error is too large to be used reliably in any of the potential applications listed above.

Using HEM surveys to evaluate disposal of by-product water from CBNG development in the Powder River Basin, Wyoming

Production of methane from thick, extensive coal beds in the Powder River Basin of Wyoming has created water management issues. Since development began in 1997, more than 650 billion liters of water have been produced from approximately 22,000 wells. Infiltration impoundments are used widely to dispose of by-product water from coal bed natural gas (CBNG) production, but their hydrogeologic effects are poorly understood. Helicopter electromagnetic surveys (HEM) were completed in July 2003 and July 2004 to characterize the hydrogeology of an alluvial aquifer along the Powder River. The aquifer is receiving CBNG produced water discharge from infiltration impoundments. HEM data were subjected to Occam’s inversion algorithms to determine the aquifer bulk conductivity, which was then correlated towater salinity using site-specific sampling results. The HEM data provided high-resolution images of salinity levels in the aquifer, a result not attainable using traditional sampling methods. Interpretation of these images reveals clearly the produced water influence on aquifer water quality. Potential shortfalls to this method occur where there is no significant contrast in aquifer salinity and infiltrating produced water salinity and where there might be significant changes in aquifer lithology. Despite these limitations, airborne geophysical methods can provide a broadscale (watershed-scale) tool to evaluate CBNG water disposal, especially in areas where field-based investigations are logistically prohibitive. This research has implications for design and location strategies of future CBNG water surface disposal facilities within the Powder River Basin.

Respiratory function in rats restrained for extended periods: Assessment of the effects of bethanecol

Introduction The ICH guideline S7A recommends that the effects of drugs on the respiratory system are evaluated in laboratory mammals prior to administration in man. Previously, animals have been placed in plethysmography chambers for short durations. This study investigates the possibility of restraining animals in chambers for a longer duration to assess respiratory function over extended periods. Methods Respiratory function in conscious rats was assessed using plethysmography chambers where the rat body was enclosed in a sealed chamber while the head was free. Thoracic movements were measured by pressure transducers linked to a Buxco amplifier system and respiratory parameters were captured and analyzed by the Notocord HEM data acquisition system. Each animal was subjected to 5 acclimatization sessions of escalating duration (1, 2, 4, 5, and 6 hours (h)) over 5 days prior to testing, with a baseline recording session conducted the day prior to dosing. Animals (8 males/group) were dosed subcutaneously with saline or bethanecol (3, 10, or 30 mg/kg) and placed in the chambers for 6 h of continuous recording. Additionally, a recording session was conducted at 24 h post-dose. Results Subcutaneous administration of 30 mg/kg bethanecol decreased respiration rate by up to 33% during the first 1.5 h post-dose and increased tidal volume by up to 46% from 0.25 to 1.25 h post-dose when compared to vehicle group data. A decrease in minute volume of up to 33% was observed 0.25 h following administration of the 10 and 30 mg/kg doses. Discussion These data show a respiratory depression caused by the cholinergic agonist bethanecol, an effect partially compensated for by an increase in tidal volume. This also demonstrates the ability to continuously restrain and record respiratory parameters in conscious rats for up to 6 h without any negative impact on the quality of the data.

Theoretical treatment of amplitude calibration problems in HEM data

A recent increase in the environmental usage of Airborne EM has shown the need to accurately provide values of depth and conductivity. Calibration problems in helicopter EM data produce imprecise conductivity depth images (CDIs), maps and sections, which are essential requirements in order to target smaller, near?surface objectives, such as salinity outbreaks. To ensure agreement between ground?truth, such as conductivity logs and CDIs, data recalibration is often applied before processing. Ground?based methods, if available, have spatial limitations. An alternative statistical average method has been developed to provide theoretical consistency. In conductivity?independent asdomain, the median response of a variety of expected synthetic models based on expected geology is calculated and compared with the median of the larger amplitude field data. The data are then rescaled in the data domain so that the recalibrated median response lies exactly on the theoretical curve. The amplitude rescaling was applied to an HEM dataset, collected in the Riverland area in South Australia. The results were compared using maps and CDI images of the raw and recalibrated data. The original delivered data produced CDI images that were generally inconsistent with borehole conductivity data. However, rescaling to ensure `thin?sheet' consistency has produced remarkable agreement between ground truth and the CDI sections.

The Validation of RESOLVE AEM Data in the Riverland, South Australia, to Determine their Value in Mapping a Regionally Significant, Near Surface Hydrogeological Unit

In the Riverland region, the South Australian Salinity Mapping and Management Support Project used the helicopter-borne RESOLVE EM system to map conductive fine textured materials (the Blanchetown Clay) in the unsaturated zone as a basis for groundwater recharge modelling. Ground validation of the AEM data included 14 boreholes, which target various observed conductivity responses that were present in the 106 kHz and 25 kHz apparent conductivity images. Evidence from down-hole induction logs, measurements of water and chloride content and texture from drill core, indicates that conductivity can be used to map near surface clay. However, ambiguity arises due to the presence of shallow conductive saline groundwater, which can be confused with the presence of clay. A ``Clay thickness map', produced from the constrained inversion of the HEM data effectively depicts the spatial distribution and thickness of clay, and eliminates problems of separating areas of clay from areas of conductive groundwater.

Some observations on the sedimentary framework of the Loxton-Bookpurnong region, South Australia as defined by borehole, ground and airborne geophysical data - implications for informing the development of groundwater interception schemes

The construction of salt interception schemes (SIS) in the Riverland region of South Australia forms an integral part of a broader strategy to manage saline groundwater intrusion into the River Murray. Results from the inversion of airborne electromagnetic data provided some insight into the distribution and variability of the Loxton sands aquifer. These data indicated regional facies variations associated with the main barrier systems of this prograded strandline sedimentary sequence. The relevance of this information is being followed up in an examination of options for the Bookpurnong highland borefield. A program involving the acquisition and interpretation of neutron, gamma and inductive conductivity borehole logs, NanoTEM ground TEM traverses, and the analysis of HEM data was undertaken to better define the sedimentary and hydrogeological model of the area. This approach has been critical in explaining local scale changes in the sedimentary environment and elucidating reasons for the variable aquifer yield in the Loxton sands aquifer.

Correcting Drift Errors in HEM Data

Much HEM survey data suffers from low (spatial) frequency along-line variability in amplitude caused by slow changes in the system geometry and electronic drift. Even though the system is periodically taken up to a sufficient altitude to re-zero the primary field correction, the errors are not totally removed. This paper describes a procedure to reduce the effect of these errors. Each flight line is corrected by subtracting a slowly varying function of time that has been chosen so that the between-line differences over the whole survey area are minimized. The parameters of the correction functions are estimated using weighted, damped least squares. The procedure produces a marked improvement in the quality of the images of low frequency, in-phase channels that have been corrupted by drift noise.