The effect of local distortions on time‐domain electromagnetic measurements

Geophysics ◽  
2004 ◽  
Vol 69 (1) ◽  
pp. 87-96 ◽  
Author(s):  
Andreas Hördt ◽  
Carsten Scholl
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Time Domain ◽  
Step Response ◽  
Model Parameters ◽  
Difficult Case ◽  
Storage Site ◽  
Transient Electromagnetic ◽  
Electromagnetic Measurements ◽  
Distortion Tensor

Based on the time‐domain integral equation, we derive expressions for the effect of an anomalous body close to the receiver or close to the transmitter on transient electromagnetic measurements. Similar to magnetotellurics, the distortion of electric fields at late times can be described by a constant distortion tensor relating the secondary electric field to the primary field components that would be obtained in the absence of the body. The distortion of a single electric field transient is a static shift only for particular configurations over a layered half‐space. In the general case, the perturbation is time dependent because the direction of the total electric field vector varies with time. The theory nicely explains spatial variations in electric field transients measured during a high‐redundancy long‐offset transient electromagnetics (LOTEM) survey over an underground gas storage site. An inversion example with synthetic data illustrates how distortion can be corrected. The elements of the distortion tensor are determined simultaneously with the model parameters. Ambiguity is reduced by a regularization of the distortion parameters. In the example, the background model is recovered well, even for the difficult case where only one transmitter is used. The distortion of the magnetic field time derivatives caused by bodies close to the receiver is proportional to the time derivative of the primary electric step response. The distortion is generally not limited to early times and cannot be neglected in general. Transmitter overprint effects resulting in static shifts of vertical magnetic field time derivatives may also be understood from the theory.

scholarly journals A parallel finite‐difference approach for 3D transient electromagnetic modeling with galvanic sources

Geophysics ◽  
2004 ◽  
Vol 69 (5) ◽  
pp. 1192-1202 ◽  
Author(s):  
Michael Commer ◽  
Gregory Newman
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Electric Fields ◽  
Time Derivative ◽  
Stable Solution ◽  
Parallel Computer ◽  
Staggered Grid ◽  
Electromagnetic Modeling ◽  
Transient Electromagnetic

A parallel finite‐difference algorithm for the solution of diffusive, three‐dimensional (3D) transient electromagnetic field simulations is presented. The purpose of the scheme is the simulation of both electric fields and the time derivative of magnetic fields generated by galvanic sources (grounded wires) over arbitrarily complicated distributions of conductivity and magnetic permeability. Using a staggered grid and a modified DuFort‐Frankel method, the scheme steps Maxwell's equations in time. Electric field initialization is done by a conjugate‐gradient solution of a 3D Poisson problem, as is common in 3D resistivity modeling. Instead of calculating the initial magnetic field directly, its time derivative and curl are employed in order to advance the electric field in time. A divergence‐free condition is enforced for both the magnetic‐field time derivative and the total conduction‐current density, providing accurate results at late times. In order to simulate large realistic earth models, the algorithm has been designed to run on parallel computer platforms. The upward continuation boundary condition for a stable solution in the infinitely resistive air layer involves a two‐dimensional parallel fast Fourier transform. Example simulations are compared with analytical, integral‐equation and spectral Lanczos decomposition solutions and demonstrate the accuracy of the scheme.

Transient electromagnetic fields of a buried horizontal magnetic dipole

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. E481-E491 ◽  
Author(s):  
Andrei Swidinsky ◽  
Misac Nabighian
Keyword(s):  
Electric Field ◽  
Magnetic Dipole ◽  
Time Domain ◽  
Major Axis ◽  
Secondary Response ◽  
Burial Depth ◽  
Transient Electromagnetic ◽  
The Earth ◽  
Smoke Ring ◽  
Electromagnetic Surveys

Electromagnetic surveys using a vertical transmitter loop are common in land, marine, and airborne geophysical exploration. Most of these horizontal magnetic dipole (HMD) systems operate in the frequency domain, measuring the time derivative of the induced magnetic fields, and therefore a majority of studies have focused on this subset of field measurements. We examine the time-domain electromagnetic response of a HMD including the electric fields and corresponding smoke rings produced in a conductive half-space. Cases of a dipole at the surface and buried within the earth are considered. Results indicate that when the current in the transmitter is rapidly switched off, a single smoke ring is produced within the plane of the vertical transmitter loop, which is then distorted by the air-earth interface. In this situation, the circular smoke ring, which would normally diffuse symmetrically away from the source in a whole space, is approximately transformed into an ellipse, with a vertical major axis at an early time and a horizontal major axis at a late time. As measured from the location of the transmitter, the depth of investigation and lateral footprint of such a system increases with burial depth. It is also observed that the electric field measured in the direction of the magnetic dipole only contains a secondary response related to the charge accumulation on any horizontal conductivity boundaries because the primary field is always absent. This field component can be expressed analytically in terms of a static and time-varying field, the latter term adding spatial complexity to the total horizontal electric field at the earth surface at early times. Applications of this theoretical study include the design of time-domain induction-logging tools, crossborehole electromagnetic surveys, underground mine expansion work, mine rescue procedures, and novel marine electromagnetic experiments.

Time-domain-induced polarization: Full-decay forward modeling and 1D laterally constrained inversion of Cole-Cole parameters

Geophysics ◽  
2012 ◽  
Vol 77 (3) ◽  
pp. E213-E225 ◽  
Author(s):  
Gianluca Fiandaca ◽  
Esben Auken ◽  
Anders Vest Christiansen ◽  
Aurélie Gazoty
Keyword(s):  
Time Domain ◽  
Mineral Exploration ◽  
Induced Polarization ◽  
Significant Loss ◽  
Step Response ◽  
Model Parameters ◽  
Full Time ◽  
Inversion Algorithm ◽  
Modeling Tools ◽  
Polarization Response

Time-domain-induced polarization has significantly broadened its field of reference during the last decade, from mineral exploration to environmental geophysics, e.g., for clay and peat identification and landfill characterization. Though, insufficient modeling tools have hitherto limited the use of time-domain-induced polarization for wider purposes. For these reasons, a new forward code and inversion algorithm have been developed using the full-time decay of the induced polarization response, together with an accurate description of the transmitter waveform and of the receiver transfer function, to reconstruct the distribution of the Cole-Cole parameters of the earth. The accurate modeling of the transmitter waveform had a strong influence on the forward response, and we showed that the difference between a solution using a step response and a solution using the accurate modeling often is above 100%. Furthermore, the presence of low-pass filters in time-domain-induced polarization instruments affects the early times of the acquired decays (typically up to 100 ms) and has to be modeled in the forward response to avoid significant loss of resolution. The developed forward code has been implemented in a 1D laterally constrained inversion algorithm that extracts the spectral content of the induced polarization phenomenon in terms of the Cole-Cole parameters. Synthetic examples and field examples from Denmark showed a significant improvement in the resolution of the parameters that control the induced polarization response when compared to traditional integral chargeability inversion. The quality of the inversion results has been assessed by a complete uncertainty analysis of the model parameters; furthermore, borehole information confirm the outcomes of the field interpretations. With this new accurate code in situ time-domain-induced polarization measurements give access to new applications in environmental and hydrogeophysical investigations, e.g., accurate landfill delineation or on the relation between Cole-Cole and hydraulic parameters.

scholarly journals Eigenvalue extraction from time domain computations

2013 ◽  
Vol 11 ◽  
pp. 23-29 ◽  
Author(s):  
T. Banova ◽  
W. Ackermann ◽  
T. Weiland
Keyword(s):  
Electric Field ◽  
Time Domain ◽  
Large Scale ◽  
Reference Data ◽  
Least Square ◽  
Model Parameters ◽  
Fast Approach ◽  
Suggested Technique ◽  
Short Time ◽  
Good Agreement

Abstract. In this paper we address a fast approach for an accurate eigenfrequency extraction, taken into consideration the evaluated electric field computations in time domain of a superconducting resonant structure. Upon excitation of the cavity, the electric field intensity is recorded at different detection probes inside the cavity. Thereafter, we perform Fourier analysis of the recorded signals and by means of fitting techniques with the theoretical cavity response model (in support of the applied excitation) we extract the requested eigenfrequencies by finding the optimal model parameters in least square sense. The major challenges posed by our work are: first, the ability of the approach to tackle the large scale eigenvalue problem and second, the capability to extract many, i.e. order of thousands, eigenfrequencies for the considered cavity. At this point, we demonstrate that the proposed approach is able to extract many eigenfrequencies of a closed resonator in a relatively short time. In addition to the need to ensure a high precision of the calculated eigenfrequencies, we compare them side by side with the reference data available from CEM3D eigenmode solver. Furthermore, the simulations have shown high accuracy of this technique and good agreement with the reference data. Finally, all of the results indicate that the suggested technique can be used for precise extraction of many eigenfrequencies based on time domain field computations.

Depth of investigation in electromagnetic sounding methods

Geophysics ◽  
1989 ◽  
Vol 54 (7) ◽  
pp. 872-888 ◽  
Author(s):  
Brian R. Spies
Keyword(s):  
Magnetic Field ◽  
Field Measurements ◽  
Sedimentary Basins ◽  
Step Response ◽  
High Signal ◽  
Depth Of Penetration ◽  
Transient Electromagnetic ◽  
Magnetic Field Measurements ◽  
Depth Of Investigation ◽  
Power Of Source

The time or frequency at which the electromagnetic (EM) response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section; it is relatively independent of the type of source or receiver and their separation. The ability to make measurements at this time or frequency, however, depends on the sensitivity and accuracy of the instrumentation, the signal strength, and the ambient noise level. These factors affect different EM sounding systems in surprisingly different ways. For the magnetotelluric (MT) method, it is possible to detect a buried half‐space under about 1.5 skin depths of overburden. The maximum depth of investigation is virtually unbounded because of high signal strengths at low frequencies. Transient electromagnetic (TEM) soundings, on the other hand, have a limited depth of penetration, but are less affected by static shift errors. For TEM, a buried inhomogeneity can be detected under about one diffusion depth of overburden. For conventional near‐zone sounding in which induced voltage is measured (impulse response), the depth of investigation is proportional to the [Formula: see text] power of the source moment and ground resistivity. By contrast, if the receiver is a magnetometer (step response system), the depth of investigation is proportional to the [Formula: see text] power of source moment and is no longer a function of resistivity. Magnetic‐field measurements may, therefore, be superior for exploration in conductive areas such as sedimentary basins. Far‐zone, or long‐offset, TEM soundings are traditionally used for deep exploration. The depth of investigation for a voltage receiver is proportional to the [Formula: see text] power of source moment and resistivity and is inversely proportional to the source‐receiver separation. Magnetic‐field measurements are difficult to make at long offsets because instrumental accuracy limits the measurement of the very slow decay of the magnetic field. Frequency‐domain controlled‐source systems are ideally suited for sounding at the very shallow depths needed for engineering, archaeological, and groundwater applications because of the relative ease of extending the measurements to arbitrarily high frequencies, and also because geometric soundings can be made at low induction numbers.

Optimal CSEM survey design for CO2 monitoring at Smeaheia offshore Norway

2021 ◽  
Author(s):  
Peder Eliasson ◽  
Anouar Romdhane ◽  
Romina Gehrmann ◽  
Joonsang Park ◽  
Hanbo Chen
Keyword(s):  
Electric Field ◽  
Large Scale ◽  
Survey Design ◽  
Geophysical Methods ◽  
Vertical Component ◽  
Significant Loss ◽  
Model Parameters ◽  
Storage Site ◽  
Design Strategies ◽  
Target Region

<p>Geophysical monitoring is essential for CO<sub>2</sub> storage projects and mandatory Measurements, Monitoring, and Verification (MMV) plans. Various geophysical methods can be used to estimate, from measured data, selected properties (e.g. velocity, density, and resistivity) of the subsurface. Accurate knowledge of those properties in turn makes it possible to quantify important reservoir parameters such as CO<sub>2</sub> saturation and pore pressure, giving the operator valuable information for predictable CO<sub>2</sub> injection and storage.</p><p>A combination of seismic and non-seismic technologies is usually part of the CO<sub>2</sub> monitoring plan throughout the project lifecycle (pre-injection, injection, and post-injection phases). The EM4CO2 project investigates whether marine Controlled Source Electro-Magnetics (CSEM) can be a cost-efficient complement to seismic in such monitoring plans. The main focus of the project is on demonstrating sufficient sensitivity of the technology and on further developing CSEM for time-lapse applications in areas with potentially interfering infrastructure. While improved data processing, imaging, and inversion techniques is often the subject of large research efforts, less attention is usually paid to developing better survey design strategies (rather relying on conventional methods). The work described here relates to the development and demonstration of new strategies for optimization of 4D survey design. Such optimization could decrease the large costs associated with acquisition of geophysical data (in this case CSEM), which could otherwise be a hurdle when proposing large-scale CO<sub>2</sub> storage as a means to mitigate climate change.</p><p>Conceptually, survey design aims at selecting the data acquisition that optimally resolves the subsurface model parameters of interest while maintaining the cost as low as possible. In other words, it consists of finding the best trade-off between data value and data collection cost. In this work, the CSEM survey design strategy is based on the analysis of the eigenvalue spectrum of the data misfit Hessian. A Python notebook was implemented for interactive prototyping and testing of various optimization strategies. A few examples of survey design are given for a model of the Sleipner storage site, showing how well a given regular survey can be decimated without significant loss of information. The main part of the work is, however, focused on survey optimization for potential CO<sub>2</sub> storage in the Smeaheia formation at about 1000 m depth below the sea surface, offshore Norway. This study shows how to determine the most valuable electric field components, most important frequencies, and source/receiver positions to use for reliable monitoring of a target region of choice. Initial results indicate that measuring the vertical component in addition to the horizontal electric field adds relevant information, and that lower frequencies (0.1-0.5 Hz) carry more information than higher (0.75-5 Hz) about the target depth. It is also clear that the method identifies sources and receivers distributed mainly above the target region as the most important.</p>

An Algorithm of 3-D ADI-R-FDTD Based on Non-Zero Divergence Relationship

2011 ◽  
Vol 317-319 ◽  
pp. 1172-1176
Author(s):  
Xiu Hai Jin ◽  
Yi Wang Chen ◽  
Pin Zhang
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Time Domain ◽  
Finite Difference Time Domain ◽  
Fdtd Method ◽  
Memory Requirement ◽  
Alternating Direction ◽  
Relationship Of ◽  
Difference Time

In this letter, an alternating-direction reduced finite-difference time-domain (ADI-R-FDTD) method is presents. It is proven that the divergence relationship of electric-field and magnetic-field is non-zero even in charge-free regions, when the electric-field and magnetic-field are calculated with alternating-direction finite-difference time-domain (ADI-FDTD) method in 3 dimensions case, and the expression of the divergence relationship is derived. Based on the non-zero divergence relationship, the ADI-FDTD method is combined with the reduced finite-difference time-domain (R-FDTD) method. In the proposed method, the memory requirement of ADI-R-FDTD is reduced by1/12 of the memory requirement of ADI-FDTD averagely in 3D case. The formulation is presented and the accuracy and efficiency of the proposed method is verified by comparing the results with the conventional results.

Two‐dimensional transient electromagnetic responses

Geophysics ◽  
1985 ◽  
Vol 50 (12) ◽  
pp. 2849-2861 ◽  
Author(s):  
Jopie I. Adhidjaja ◽  
Gerald W. Hohmann ◽  
Michael L. Oristaglio
Keyword(s):  
Magnetic Field ◽  
Half Space ◽  
Time Domain ◽  
The Body ◽  
Two Dimensional ◽  
Total Field ◽  
Secondary Field ◽  
Field Solution ◽  
Transient Electromagnetic ◽  
Simple Boundary

The time‐domain electromagnetic (TEM) modeling method of Oristaglio and Hohmann is reformulated here in terms of the secondary field. This finite‐difference method gives a direct, explicit time‐domain solution for a two‐dimensional body in a conductive earth by advancing the field in time with DuFort‐Frankel time‐differencing. As a result, solving for the secondary field, defined as the difference between the total field and field of a half‐space, is not only more efficient but is also simpler and eliminates several problems inherent in the solution for the total field. For example, because the secondary field varies slowly both in space and time, it can be modeled on a coarse grid with large time steps. In addition, for a simple body the field is local; therefore, because the field can be assumed to satisfy a simple boundary condition in the earth computation is greatly simplified. Our tests show that for the same accuracy, the secondary‐field solution is roughly five times faster than the total‐field solution. We compute and analyze the magnetic field impulse response for a suite of models, most of which consist of a thin body embedded in a conductive half‐space—with or without overburden. The results indicate the conductive half‐space will both delay and attenuate the response of the body and even obscure it if the conductivity contrast is small. The results also suggest that the conductive host can alter the decay rate of the response of the body from its free‐space counterpart. Our results for multiple bodies illustrate the importance of early‐time measurements to obtain resolution, particularly for measurements of the horizontal magnetic field. The vertical magnetic field, however, can be used to infer the dip direction of a dipping body by studying the migration of the crossover. The results for models which include overburden show that the effect of a conductive overburden, in addition to the half‐space effect, is to delay the response of the body, because the primary current initially tends to concentrate and slowly diffuse through the overburden, and does not reach the body until later time. This effect also complicates the early‐times profiles, becoming more severe as the conductivity of the overburden is increased.

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