1 

Characterization of a heterogeneous landfill using seismic and electrical resistivity data
Understanding the processes occurring inside a landfill is important for improving the treatment of landfills. Irrigation and recirculation of leachate are widely used in landfill treatments. Increasing the efficiency of such treatments requires a detailed understanding of the flow inside the landfill. The flow depends largely on the heterogeneous distribution of density. It is, therefore, of great practical interest to determine the density distribution affecting the flow paths inside a landfill. Studies in the past have characterized landfill sites but have not led to highresolution, detailed quantitative results. We performed an Swave reflection survey, multichannel analysis of surface waves (MASW), and electrical resistivity survey to investigate the possibility of delineating the heterogeneity distribution in the body of a landfill. We found that the highresolution Swave reflection method offers the desired resolution. However, in the case of a very heterogeneous landfill and a high noise level, the processing of highresolution, shallow reflection data required special care. In comparison, MASW gave the general trend of the changes inside the landfill, whereas the electrical resistivity (ER) survey provides useful clues for interpretation of seismic reflection data. We found that it is possible to localize finescale heterogeneities in the landfill using the Swave reflection method using a highfrequency vibratory source. Using empirical relations specific to landfill sites, we then estimated the density distribution inside the landfill, along with the associated uncertainty considering different methods. The final interpretation was guided by supplementary information provided by MASW and ER tomography.

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2 

Sensitivity of the nearsurface vertical electric field land ControlledSource Electromagnetic monitoring
We investigate potential benefits of measuring the vertical electric field component in addition to the routinely measured horizontal electric field components in onshore timelapse controlledsource electromagnetics. Synthetic electromagnetic data based on a model of the Schoonebeek onshore oil field are used. We confirm that the vertical electric field component is more sensitive to small changes in the reservoir than the horizontal components, yet its amplitudes are small. Accordingly, optimal sourcereceiver geometry and precise knowledge of the verticality of the receiver dipole will be required for successful utilization of the vertical electric field.

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3 

Experimental verification of stressinduced anisotropy

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4 

Biangular decomposition of seismic data

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5 

3D Surfacewave estimation and separation: A closedloop approach
Surfacewaves are often dominant in seismic data in a shallow water and land environment. Separating them out from the seismic data is of great importance for either removing them as noise for reservoir characterization, or extracting them as signal for nearsurface characterization. However, their complex properties make the surfacewave separation significantly challenging in seismic processing. To address the challenges and adopt recent advances in seismic processing, we propose a methodology of surfacewave estimation and separation using a closedloop approach. The methodology was successfully demonstrated on real 3D seismic data with mudroll that is often dominant in a shallow water environment.

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6 

Combining intersource seismic interferometry and sourcereceiver interferometry for deep local imaging
The virtual source method has been applied successfully to retrieve the impulse response between pairs of receivers in the subsurface. This method is further improved by an updown separation prior to the crosscorrelation to suppress the reflections from the overburden and the free surface. In a reversed situation where the sources are in the subsurface and receivers are on the surface, in principle, one can apply the same logic to retrieve the virtual response between pairs of sources by sourcereceiver reciprocity, turning the physical borehole sources into virtual receivers. However, since the updown separation is not applicable on the source side, the simple crosscorrelation of the total fields results in spurious events due to the incomplete receiver coverage around the sources. We show with a numerical example that for this configuration of borehole sources and surface receivers, one can replace such an updown separation at the source side by that of the direct and reflected waves as a first order approximation. This procedure produces the virtual receiver data that is adequate for local imaging below the source depth and is completely independent of the accuracy of the overburden velocity model. We implement this intersource type of interferometry by multidimensional deconvolution (MDD). Further, if the conventional surface survey data is available, we test the methodology from sourcereceiver interferometry (SRI) for this reverse configuration with borehole sources to retrieve the virtual receiver data with reflections coming from above, using also only the separation of the direct and reflected waves. By migrating the two sets of virtual receiver data, one can create a local image around the borehole sources in a deep area with better focusing and localization without a sophisticated velocity model.

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7 

Seismoelectric interface response signal behaviour in thinbed geological settings
Increasing industrial and societal challenges demand a continuous need for improved imaging methods. In recent years, quite some research has been performed on using seismoelectric phenomena for geophysical exploration and imaging. Like the other methods, the seismoelectric technique also has its drawbacks. Besides the fact that the physical phenomenon is very complex, one of itsmain challenges is the very low signalto noise ratio of the coupled signals, especially the secondorder interface response fields. From seismics, it is wellknown that anonamously high amplitudes can arise due to amplitudetuning effects which can occur when a seismic signal travels through a package of thinlayers with appropriate amplifying thickness. Using numerical seismoelectric wave propagation experiments through packages of thinbeds, we show that thinbed geological settings can improve the signaltonoise ratio of the interface response fields. Whether a certain package of thinbeds results in a net strengthening or weakening of the signal, is determined by the contrast in and the order of the coupling coefficients of the different thinlayer media. Formulated differently, we show that the seismoelectric method is sensitive to the medium parameters of thinbed geological structures far below the seismic resolution, and that due to natural strengthening of the seismoelectric interface response signal, the method might be already suitable for certain geological settings.

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8 

Seismoelectric wave propagation modeling for typical laboratory configurations: A numerical validation
The seismoelectric effect can be of importance for hydrocarbon exploration as it is complementary to conventional seismics. Besides enabling seismic resolution and electromagnetic sensitivity at the same time, the seismoelectric method can also provide us with additional, highvalue information like porosity and permeability. However, very little is still understood of this complex physical phenomenon. Therefore, it is crucial to be able to perform numerical modeling experiments to carefully investigate the effect and the parameters that play a role. Over the last couple of years, several seismoelectric laboratory experiments have been carried out in an attempt to validate the underlying theory of the phenomenon and to better understand this complex physical phenomenon. We have recently extended our analytically based, numerical seismoelectric modeling code ’ESSEMOD’ to be able to model seismoelectric wave propagation in arbitrarily layered Earth geometries with fluid / porous medium / (fluid) interfaces. In this way, we are capable of effectively simulating full seismoelectric wave propagation, i. e. all existing seismoelectric and electroseismic sourcereceiver combinations, in typical laboratory configurations. We present the underlying theory that is required for the extension towards arbitrary fluid / porous medium / (fluid) geometries and an effective way to incorporate this in a general seismoelectric layered Earth modeling code. We then validate the underlying global reflection scheme by comparing it with an independently developed layered Earth modeling code for purely electromagnetic fields. The results show a perfect match in both amplitude and phase, indicating that ESSEMOD is correctly modeling the electromagnetic parts of the seismoelectric wave propagation in horizontally layered media with fluid / porous medium / fluid transitions. We finalize with a seismoelectric reciprocal modeling experiment, proving that also the full seismoelectric wave propagation through fluid / porous medium transitions is modeled consistently.

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9 

An approximate 3D computational method for realtime computation of induction logging responses
Over many years, induction logging systems have been used to create well formation logs. The major drawback for the utilization of these tools is the long simulation time for a single forward computation. We proposed an efficient computational method based on a contrasttype of integralequation formulation, in which we applied an approximation for the 3D electromagnetic field. We assumed that the dominant contribution in the integral equation is obtained by the contribution around the singularity of Green’s kernel. It is expected that the approximation yields reliable results when the (homogeneous) background conductivity around the logging tool is close to the actual conductivity at the location of the tool. We have developed a datadriven method to determine this background conductivity from the dominant part of the measured coaxial magnetic fields, which are mainly influenced by the conductivity at the tool sensors. For a synthetic model, the results were compared to the ones of a rigorous solution of the integral equation and show a good simulation response to smallscale variations in the medium. Further, the method was used to simulate the response of a realistic reservoir model. Such a model is created by a geological modeling program. We concluded that our approximate method was able to improve the approximation results in highly heterogeneous structures compared to the Born approximation and provide an effective mediumgradient around the tool. Our method, based on the wavefield approximation, also estimates the error, and hence yields a warning when the method becomes unreliable.

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10 

Retrieval of reflections from ambient noise recorded in the Mizil area, Romania
We applied seismic interferometry (SI) by crosscorrelation to ambientnoise panels recorded in the Mizil area, Romania, aiming to retrieve bodywave reflections. To achieve this goal, surface waves in the noise panels input to SI should be suppressed. We did this by selecting for input to SIonly noise panels that are not dominated by surface waves; the selection was either after visual inspection in the time domain or after automatic slowness evaluation on crosscorrelated panels. The latter used the slowness of arrivals passing through the virtualsource position at time 0 s. We discovered that the automatic slownessevaluation method allows better retrieval of reflections. From the retrieved reflection gathers, we obtained stacked images of the subsurface. Comparing the SI images to a stacked image from activesource data, we concluded that some retrieved events correspond to reflectors in the active seismic section, including known geologic markers. In a previous application of SI to ambient noise, the retrieved reflections exhibited frequency content lower than that of active data. In our results, the frequency content of the SI retrieved data was comparable to the one of the active data.

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11 

Datadriven wavefield focusing and imaging with multidimensional deconvolution: Numerical examples for reflection data with internal multiples
Standard imaging techniques rely on the single scattering assumption. This requires that the recorded data do not include internal multiples, i.e., waves that have bounced multiple times between reflectors before reaching the receivers at the acquisition surface. When multiple reflections are present in the data, standard imaging algorithms incorrectly image them as ghost reflectors. These artifacts can mislead interpreters in locating potential hydrocarbon reservoirs. Recently, we introduced a new approach for retrieving the Green’s function recorded at the acquisition surface due to a virtual source located at depth. We refer to this approach as datadriven wavefield focusing. Additionally, after applying sourcereceiver reciprocity, this approach allowed us to decompose the Green’s function at a virtual receiver at depth in its downgoing and upgoing components. These wavefields were then used to create a ghostfree image of the medium with either crosscorrelation or multidimensional deconvolution, presenting an advantage over standard prestack migration. We tested the robustness of our approach when an erroneous background velocity model is used to estimate the firstarriving waves, which are a required input for the datadriven wavefield focusing process. We tested the new method with a numerical example based on a modification of the Amoco model.

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12 

Marchenko imaging
Traditionally, the Marchenko equation forms a basis for 1D inverse scattering problems. A 3D extension of the Marchenko equation enables the retrieval of the Green’s response to a virtual source in the subsurface from reflection measurements at the earth’s surface. This constitutes an important step beyond seismic interferometry. Whereas seismic interferometry requires a receiver at the position of the virtual source, for the Marchenko scheme it suffices to have sources and receivers at the surface only. The underlying assumptions are that the medium is lossless and that an estimate of the direct arrivals of the Green’s function is available. The Green’s function retrieved with the 3D Marchenko scheme contains accurate internal multiples of the inhomogeneous subsurface. Using sourcereceiver reciprocity, the retrieved Green’s function can be interpreted as the response to sources at the surface, observed by a virtual receiver in the subsurface. By decomposing the 3D Marchenko equation, the response at the virtual receiver can be decomposed into a downgoing field and an upgoing field. By deconvolving the retrieved upgoing field with the downgoing field, a reflection response is obtained, with virtual sources and virtual receivers in the subsurface. This redatumed reflection response is free of spurious events related to internal multiples in the overburden. The redatumed reflection response forms the basis for obtaining an image of a target zone. An important feature is that spurious reflections in the target zone are suppressed, without the need to resolve first the reflection properties of the overburden.

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13 

Nonlinear imaging condition and the effect of source illumination: Imaging fractures as nonwelded interfaces
Fluid flow through a fractured reservoir is often controlled by the large fractures. Seismically imaging these large fractures has the potential to illuminate their hydraulic properties.We derived a nonlinear imaging condition considering a medium containing nonwelded interfaces such as fractures for highresolution fracture imaging. This was achieved by using the general correlationtype representation theorem relating the wavefield between two different states representing different fracture compliances. We numerically tested the imaging condition to investigate the effect of the nonlinear term and that of the onesided source illumination. Assuming a dry fracture, we calculated the wavefield from a nonwelded interface. We obtained a Pwave imaging result from Pwave sources. In the case of perfect source illumination, we found that introducing the nonlinear term in the imaging condition enhances the image because the nonwelded interface is imaged as a thin layer in an otherwise homogeneous medium. Investigating the effect of onesided illumination by horizontally aligned sources revealed interesting limitations and possibilities. The imaging result for a horizontal fracture showed a volumetric distribution of nonzero amplitudes around the polarity change at the fracture that can be misinterpreted as a welded thick layer boundary. However, when the fracture was not horizontal, the imaging result was quite good and was closer to that with a perfect source illumination. These led to a new possibility of imaging subvertical fractures from surface seismic measurements, or subhorizontal fractures from vertical seismic profiling data, assuming that we successfully estimated the perturbed wavefield from the receiver responses.

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14 

A seismic vertical vibrator driven by linear synchronous motors
A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSMdriven vibrator, we successfully designed and built a multiLSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the lowfrequency regime, down to 2 Hz, rather easily.

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15 

Studying CO2 storage with ambientnoise seismic interferometry: A combined numerical feasibility study and fielddata example for Ketzin, Germany
Seismic interferometry applied to ambientnoise measurements allows the retrieval of the seismic response between pairs of receivers. We studied ambientnoise seismic interferometry (ANSI) to retrieve timelapse reflection responses from a reservoir during CO2 geologic sequestration, using the case of the experimental site of Ketzin, Germany. We applied ANSI to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of CO2. The modeled data represented global transmission responses from bandlimited noise sources randomly triggered in space and time. We found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the timelapse signal as long as sufficient sourcelocation repeatability was observed between the base and the repeat passive survey. To illustrate the potential of the technique, ANSI was applied to three days of passive field data recorded in 2012 at Ketzin. Comparison with the modeled results illustrated the potential to retrieve key reflection events using ANSI on field data from Ketzin. This study supports the idea that the geologic setting and characteristics of ambient noise at Ketzin may be opportune to monitor CO2 sequestration.

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16 

Imaging scatterers in landfills using seismic interferometry
A significant problem with landfills is their aftercare period. A landfill is considered to be safe for the environment only after a relatively long period of time. Until it reaches such a condition, it has to be periodically treated. Not only are treatments very expensive, but they could be dangerous as well; for example, when barriers limiting the waste break. So far, there is no established technique that can predict the leachate and gasemission potential of a landfill, especially in timelapse monitoring. This potential depends on the channeling of fluids due to the presence of highdensity waste areas and the redistribution of the channels with time. We propose to use seismic interferometry (SI) applied to active reflection seismics to help improve the image of the waste areas (scatterers) and to monitor the subsurface changes in time. Normally, application of SI to reflection recordings from active sources at the surface would result in an erroneous retrieved result, but secondary illumination of the receivers from strongly scattering subsurface, like a landfill, would remedy this problem. We conduct modeling studies to examine the possible benefits of this approach compared to using the conventional seismic reflection method. We show that the reflections retrieved from SI can be used to obtain a clearer image of the shallower scatterers. In addition, we illustrate that timelapse monitoring using reflections retrieved by SI shows a more repeatable result than the conventional approach in case of source nonrepeatability.

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17 

Green's function retrieval with Marchenko equations: A sensitivity analysis
Recent research showed that the Marchenko equation can be used to construct the Green’s function for a virtual source position in the subsurface. The method requires the reflection response at the surface and an estimate of the direct arrival of the wavefield, traveling from the virtual source location to the acquisition surface. In this paper, we investigate the sensitivity of this method. We demonstrate its robustness with respect to significant amplitude and phase errors in the direct arrival. The erroneous operators introduce low amplitude artefacts. The main reflections and internal multiples are still presents and disturbing ghost events are not introduced. In case the reflection data is modeled in a medium with losses, ghost events seem to be visible in the upgoing wavefield, but not in the downgoing wavefield.

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18 

Autofocus imaging: Image reconstruction based on inverse scattering theory
Conventional imaging algorithms assume single scattering and therefore cannot image multiply scattered waves correctly. The multiply scattered events in the data are imaged at incorrect locations resulting in spurious subsurface structures and erroneous interpretation. This drawback of current migration/imaging algorithms is especially problematic for regions where illumination is poor (e.g., subsalt), in which the spurious events can mask true structure. Here we discuss an imaging technique that not only images primaries but also internal multiples accurately. Using only surface reflection data and directarrivals, we generate the up and downgoing wavefields at every image point in the subsurface.
An imaging condition is applied to these up and downgoing wavefields directly to generate the image. Because the above algorithm is based on inversescattering theory, the reconstructed wavefields are accurate and contain multiply scattered energy in addition to the primary event. As corroborated by our synthetic examples, imaging of these multiply scattered energy helps eliminate spurious reflectors in the image. Other advantages of this imaging algorithm over existing imaging algorithms include more accurate amplitudes, targetoriented imaging, and a highly parallelizable algorithm.

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19 

Closing the performance gap between an iterative frequencydomain solver and an explicit timedomain scheme for 3D migration on parallel architectures
Threedimensional reversetime migration with the constantdensity acoustic wave equation requires an efficient numerical scheme for the computation of wavefields. An explicit finitedifference scheme in the time domain is a common choice. However, it requires a significant amount of disk space for the imaging condition. The frequencydomain approach simplifies the correlation of the source and receiver wavefields, but requires the solution of a large sparse linear system of equations. For the latter, we use an iterative Krylov solver based on a shifted Laplace multigrid preconditioner with matrixdependent prolongation. The question is whether migration in the frequency domain can compete with a timedomain implementation when both are performed on a parallel architecture. Both methods are naturally parallel over shots, but the frequencydomain method is also parallel over frequencies. If we have a sufficiently large number of compute nodes, we can compute the result for each frequency in parallel and the required time is dominated by the number of iterations for the highest frequency. As a parallel architecture, we consider a commodity hardware cluster that consists of multicore central processing units (CPUs), each of them connected to two graphics processing units (GPUs). Here, GPUs are used as accelerators and not as an independent compute node. The parallel implementation of the 3D migration in frequency domain is compared to a timedomain implementation. We optimize the throughput of the latter with dynamic load balancing, asynchronous I/O, and compression of snapshots. Because the frequencydomain solver uses matrixdependent prolongation, the coarsegrid operators require more storage than available on GPUs for problems of realistic size. Due to data transfer, there is no significant speedup using GPUaccelerators. Therefore, we consider an implementation on CPUs only. Nevertheless, with the parallelization over shots and frequencies, this approach could compete with the timedomain implementation on multiple GPUs.

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20 

Seismic reflector imaging using internal multiples with Marchenkotype equations
We present an imaging method that creates a map of reflection coefficients in correct oneway time with no contamination from internal multiples using purely a filtering approach. The filter is computed from the measured reflection response and does not require a background model. We demonstrate that the filter is a focusing wavefield that focuses inside a layered medium and removes all internal multiples between the surface and the focus depth. The reflection response and the focusing wavefield can then be used for retrieving virtual vertical seismic profile data, thereby redatuming the source to the focus depth. Deconvolving the upgoing by the downgoing vertical seismic profile data redatums the receiver to the focus depth and gives the desired image. We then show that, for oblique angles of incidence in horizontally layered media, the image of the same quality as for 1D waves can be constructed. This step can be followed by a linear operation to determine velocity and density as a function of depth. Numerical simulations show the method can handle finite frequency bandwidth data and the effect of tunneling through thin layers.

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