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 

A microelectromechanical system digital 3C array seismic cone penetrometer
A digital 3C array seismic cone penetrometer has been developed for multidisciplinary geophysical and geotechnical applications. Seven digital triaxial microelectromechanical system accelerometers are installed at 0.25m intervals to make a 1.5mlong downhole seismic array. The accelerometers have a flat response up to 2 kHz. The seismic array is attached to a class 1 digital seismic cone, which measures cone tip resistance, sleeve friction, porepressure, and inclination. The downhole 3C array can be used together with impulsive seismic sources and/or highfrequency vibrators that are suitable for highresolution shallow applications. Results from two field experiments showed that a good data quality, including a constant source function within an array, and a dense depthsampling allowed robust estimation of seismic velocity profiles in the shallow subsoil. Using horizontal and vertical seismic sources, downhole 9C seismic array data can be easily acquired. The quality of the shearwave data is much superior when the surface seismic source is a controlled, highfrequency vibrator in stead of a traditional sledge hammer. A remarkable correlation in depth, in a fine scale, between lowstrain seismic shear wave velocity and highstrain cone tip resistance could be observed. The array measurements of the fullelastic wavefield and the broad spectral bandwidth are useful in investigating frequencydependent seismic wave propagation in the porous nearsurface soil layers, which is informative of the in situ fluidflow properties. Stable estimates of dispersive seismic velocity and attenuation can be obtained.

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4 

A new empirical complex electrical resistivity model
Macroscopic measurements of electrical resistivity require frequencydependent effective models that honor the microscopic effects observable in macroscopic measurements. Effective models based on microscopic physics exist alongside with empirical models. We adopted an empirical model approach to modify an existing physical model. This provided a description of electrical resistivity as a function of not only frequency, but also water saturation. We performed twoelectrode laboratory measurements of the complex resistivity on a number of fine and mediumgrained unconsolidated sand packs saturated with water of three different salinities. For frequencies between 0.1 and 1 MHz, the data were fitted with the new model and compared to fits with Archie’s law. Our model described the relaxation times and DC resistivity values as negative exponential functions with increasing water saturation. All data could be accurately described as a function of frequency and water saturation with nine parameters.

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5 

Increasing the sensitivity of controlledsource electromagnetics with synthetic aperture
Controlledsource electromagnetics (CSEM) has been used as a derisking tool in the hydrocarbon exploration industry. We apply the concept of synthetic aperture to the lowfrequency electromagnetic field in CSEM. Synthetic aperture sources have been used in radar imaging for many years. Using the synthetic aperture concept, big synthetic sources can be constructed by adding the response to small sources (building blocks) in different ways, and consequently, big sources with different radiation patterns can be created. We show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to CSEM data. More challenging targets such as deep reservoirs (4km belowsea floor) can be detected. The synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition.We also show that a pseudovertical source (orthogonally distributed dipole pairs) can be constructed synthetically, and that the detection capability of this pseudovertical source is increased by applying field steering. The synthetic aperture concept opens a new line of research in CSEM, with the freedom to design suitable synthetic aperture sources for a given purpose.

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6 

Quasianalytical method for frequencytotime conversion in CSEM applications
Frequencytotime transformations are of interest to controlledsource electromagnetic methods when timedomain data are inverted for a subsurface resistivity model by numerical frequencydomain modeling at a selected, small number of frequencies whereas the data misfit is determined in the time domain. We propose an efficient, Pronytype method using frequencydomain diffusivefield basis functions for which the timedomain equivalents are known. Diffusive fields are characterized by an exponential part whose argument is proportional to the square root of frequency and a part that is polynomial in integer powers of the square root of frequency. Data at a limited number of frequencies suffice for the transformation back to the time. In the exponential part, several diffusiontime values must be chosen. Once a suitable range of diffusiontime values are found, the method is quite robust in the number of values used. The highest power in the polynomial part can be determined from the source and receiver type. When the frequencydomain data are accurately approximated by the basis functions, the timedomain result is also accurate. This method is accurate over a wider time range than other methods and has the correct latetime asymptotic behavior. The method works well for data computed for layered and 3D subsurface models.

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7 

Focusing the wavefield inside an unknown 1D medium: Beyond seismic interferometry
With seismic interferometry one can retrieve the response to a virtual source inside an unknown medium, if there is a receiver at the position of the virtual source. Using inverse scattering theory, we demonstrate that, for a 1D medium, the requirement of having an actual receiver inside the medium can be circumvented, going beyond seismic interferometry. In this case, the wavefield can be focused inside an unknown medium with independent variations in velocity and density using reflection data only.

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8 

Blended acquisition with dispersed source arrays
Blended source arrays are historically configured with equal source units, such as broadband vibrators (land) and broadband airgun arrays (marine). I refer to this concept as homogeneous blending. I have proposed to extend the blending concept to inhomogeneous blending, meaning that a blended source array consists of different source units. More specifically, I proposed to replace in blended acquisition the traditional broadband sources by narrowband versions — imagine coded single air guns with different volumes or coded single narrowband vibrators with different central frequencies — together representing a dispersed source array (DSA). Similar to what we see in today's audio systems, the DSA concept allows the design of dedicated narrowband source elements that do not suffer from the low versus high frequency compromise. In addition, the DSA concept opens the possibility to use source depths and spatial sampling intervals that are optimum for the low, mid, and highfrequency sources (multiscale shooting grids). DSAs are considered to be an important step in robotizing the seismic acquisition process.

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9 

Combining full wavefield migration and full waveform inversion, a glance into the future of seismic imaging
The next generation seismic migration and inversion technology considers multiple scattering as vital information, allowing the industry to derive significantly better reservoir models — with more detail and less uncertainty — while requiring a minimum of user intervention. Three new insights have been uncovered with respect to this fundamental transition. Unblended or blended multiple scattering can be included in the seismic migration process, and it has been proposed to formulate the imaging principle as a minimization problem. The resulting process yields angledependent reflectivity and is referred to as recursive full wavefield migration (WFM). The full waveform inversion process for velocity estimation can be extended to a recursive, optionally blended, anisotropic multiplescattering algorithm. The resulting process yields angledependent velocity and is referred to as recursive full waveform inversion (WFI). The mathematical equations of WFM and WFI have an identical structure, but the physical meaning behind the expressions is fundamentally different. In WFM the reflection process is central, and the aim is to estimate reflection operators of the subsurface, using the up and downgoing incident wavefields (including the codas) in each gridpoint. In WFI, however, the propagation process is central and the aim is to estimate velocity operators of the subsurface, using the total incident wavefield (sum of up and downgoing) in each gridpoint. Angledependent reflectivity in WFM corresponds with angledependent velocity (anisotropy) in WFI. The algorithms of WFM and WFI could be joined into one automated joint migrationinversion process. In the resulting hybrid algorithm, being referred to as recursive joint migration inversion (JMI), the elaborate volume integral solution was replaced by an efficient alternative: WFM and WFI are alternately applied at each depth level, where WFM extrapolates the incident wavefields and WFI updates the velocities without any user interaction. The output of the JMI process offers an integrated picture of the subsurface in terms of angledependent reflectivity as well as anisotropic velocity. This twofold output, reflectivity image and velocity model, offers new opportunities to extract accurate rock and pore properties at a fine reservoir scale.

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10 

Multiscattering illumination in blended acquisition
In traditional seismic surveys, the firing time between shots is such that the shot records do not interfere in time. However, in the concept of blended acquisition, the records do overlap, allowing denser source sampling and wider azimuths in an economic way. A denser shot sampling and wider azimuths make that each subsurface gridpoint is illuminated from a larger number of angles and will therefore improve the image quality in terms of signaltonoise ratio and spatial resolution. We show that — even with very simple blending parameters like time delays — the incident wavefield at a specific subsurface gridpoint represents a dispersed time series with a “complex code”. For shotrecord migration purposes, this time series must have a stable inverse. In a next step, we show that the illumination can be further improved by utilizing the surfacerelated multiples. This means that these multiples can be exploited to improve the incident wavefield by filling angle gaps in the illumination and/or by extending the range of angles. In this way, the energy contained in the multiples now contributes to the image, rather than decreasing its quality. One remarkable consequence of this property is that the benefits to be obtained from the improved illumination depend on the detector locations in acquisition geometries as well.We show how to quantify the contribution of the blended surface multiples to the illuminating wavefield for a blended source configuration. Results confirm that the combination of blending and multiple scattering increases the illumination energy and, therefore, will improve the quality of shotrecord migration results beyond today’s capability.

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11 

Green's function representations for seismic interferometry
The term seismic interferometry refers to the principle of generating new seismic responses by crosscorrelating seismic observations at different receiver locations. The first version of this principle was derived by Claerbout (1968), who showed that the reflection response of a horizontally layered medium can be synthesized from the autocorrelation of its transmission response. For an arbitrary 3D inhomogeneous lossless medium it follows from Rayleigh's reciprocity theorem and the principle of timereversal invariance that the acoustic Green's function between any two points in the medium can be represented by an integral of crosscorrelations of wavefield observations at those two points. The integral is along sources on an arbitrarily shaped surface enclosing these points. No assumptions are made with respect to the diffusivity of the wavefield. The RayleighBetti reciprocity theorem leads to a similar representation of the elastodynamic Green's function. When a part of the enclosing surface is the earth's free surface, the integral needs only to be evaluated over the remaining part of the closed surface. In practice, not all sources are equally important: The main contributions to the reconstructed Green's function come from sources at stationary points. When the sources emit transient signals, a shaping filter can be applied to correct for the differences in source wavelets. When the sources are uncorrelated noise sources, the representation simplifies to a direct crosscorrelation of wavefield observations at two points, similar as in methods that retrieve Green's functions from diffuse wavefields in disordered media or in finite media with an irregular bounding surface.

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12 

Seismic processing in the inverse data space
Until now, seismic processing has been carried out by applying inverse filters in the forward data space. Because the acquired data of a seismic survey is always discrete, seismic measurements in the forward data space can be arranged conveniently in a data matrix (P). Each column in the data matrix represents one shot record. If we represent seismic data in the temporal frequency domain, then each matrix element consists of a complexvalued number. Considering the dominant role of multiple scattering in seismic data, it is proposed to replace data matrix P by its inverse P–1 before starting seismic processing. Making use of the feedback model for seismic data, multiple scattered energy is mapped onto the zero time axis of the inverse data space. The practical consequence of this remarkable property may be significant: multiple elimination in the inverse data space simplifies to removing data at zero time only. Moving to the inverse data space may cause a fundamental change in the way we preprocess and image seismic data.

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13 

Discrimination between phase and amplitude attributes in timelapse seismic streamer data
Timelapse seismic experiments aim to obtain information about productionrelated effects in hydrocarbon reservoirs to increase the recovery percentage. However, nonrepeatability problems such as acquisition differences, overburden effects, and noise are often significantly stronger than the imprint of production changes in timelapse seismic data sets. Consequently, it is very difficult to appraise the changes in petrophysical reservoir parameters over time. We introduce a 4D monitoring approach based on the spectral ratio method. This method produces two timelapse attributes: the relative change in reflection coefficient and the traveltime shift at reflecting interfaces. These attributes can be used for appraising productionrelated changes in the subsurface. The approach corrects for timeinvariant nonrepeatability effects in the overburden and sourcereceiver coupling problems in timelapse surveys. The validity of the method is limited to structurally simple overburden and reservoirs with weak lateral variations. First, we validate the methodology using a synthetic timelapse seismic experiment. Next, we apply the method to a real timelapse data set from the Troll West gas province in the North Sea. In the real example, we could not detect movement in the fluid contact of 5–15 m. The expected change in amplitude is less than 10%, which is probably below the background noise level for this data set.

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14 

A new elastic model for ground coupling of geophones with spikes
Ground coupling are terms that describe the transfer from seismic ground motion to the motion of a geophone. In previous models, ground coupling was mainly considered as a disk lying on top of a halfspace, not considering the fact that in current practice geophones are spiked and are buried for optimal response. In this paper we introduce a new model that captures the spike added to the geophone and models the effect of geophone burial. The geophone is modeled as a rigid, movable cylinder embedded in a halfspace near or at the surface. The coupling problem is then tackled by a scattering approach using the elastic form of reciprocity; we consider the vertical component only. The main feature in the coupling function is a resonance whose location and shape depend on the different parameters of the geophone and the soil. In accordance with previous models, adding mass reduces the frequency of resonance. However, we show that pure mass loading assumption is too restrictive for standard geophones. Our new model shows that increasing the spike radius and length decreases the frequency of resonance and the resonance is more peaked. Furthermore, burying the geophone decreases the frequency of resonance, but when one takes into account that the soil at depth is more compact, then the behavior is as observed in practice — namely, an increase in frequency of resonance. As for the properties of the soil, the shearwave velocity has the largest effect; when increased, it shifts the frequency of resonance to the highfrequency end as desired.

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15 

3D surfacerelated multiple prediction: A sparse inversion approach
The theory of iterative surfacerelated multiple elimination holds for 2D as well as 3D wavefields. The 3D prediction of surface multiples, however, requires a dense and extended distribution of sources and receivers at the surface. Since current 3D marine acquisition geometries are very sparsely sampled in the crossline direction, the direct Fresnel summation of the multiple contributions, calculated for those surface positions at which a source and a receiver are present, cannot be applied without introducing severe aliasing effects. In this newly proposed method, the regular Fresnel summation is applied to the contributions in the densely sampled inline direction, but the crossline Fresnel summation is replaced with a sparse parametric inversion. With this procedure, 3D multiples can be predicted using the available input data. The proposed method is demonstrated on a 3D synthetic data set as well as on a 3D marine data set from offshore Norway.

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16 

Removal of internal multiples with the commonfocuspoint (CFP) approach. Part 2: Application strategies and data examples
In the past, the surfacemultipleremoval method based on the feedback model has been successfully applied to many different field data sets. The extension of surface to internal multiples can be made by replacing shot records with commonfocuspoint (CFP) gathers, a CFP gather representing focused data with one source in the subsurface and all receivers at the surface (or vice versa for a receiver gather). The internalmultipleremoval algorithm can be formulated in terms of boundaryrelated and layerrelated versions. In the boundaryrelated version, the internal multiples are removed for one downwardscattering reflector at a time. In the layerrelated version, the internal multiples are removed for a sequence of downwardscattering reflectors at a time. An exact velocity model is not required, but proper muting is critical; muting becomes straightforward in the CFP domain. The strategy for applying the two versions of the multipleremoval algorithm is demonstrated on physicalmodel and field data. One can conclude that the layerrelated version is the most appropriate in most situations because it requires less user action and does not need exact knowledge of the multiplegenerating boundary.

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17 

Removal of internal multiples with the commonfocuspoint (CFP) approach. Part 1: Explanation of the theory
Removal of surface and internal multiples can be formulated by removing the influence of downwardscattering boundaries and downwardscattering layers. The involved algorithms can be applied in a modeldriven or a datadriven way. A unified description is proposed that relates both types of algorithms based on wave theory. The algorithm for the removal of surface multiples shows that muted shot records play the role of multichannel prediction filters. The algorithm for the removal of internal multiples shows that muted CFP gathers play the role of multichannel prediction filters. The internal multiple removal algorithm is illustrated with numerical examples. The conclusion is that the layerrelated version of the algorithm has significant practical advantages.

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18 

Passive seismic interferometry by multidimensional deconvolution
We introduce seismic interferometry of passive data by multidimensional deconvolution (MDD) as an alternative to the crosscorrelation method. Interferometry by MDD has the potential to correct for the effects of source irregularity, assuming the first arrival can be separated from the full response. MDD applications can range from reservoir imaging using microseismicity to crustal imaging with teleseismic data.

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19 

Converted waves in a shallow marine environment: Experimental and modeling studies
Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shearwave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of goodquality convertedwave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A highresolution seismic survey was conducted in a shallowwater canal using different types of seismic sources; data were recorded with a fourcomponent waterbottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bottom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shearwave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in airgun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/waterbottom coupled system, kinematically behave as if they were generated by an Swave source placed at the water bottom. In a shallowwater environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shearwave velocity in the sediments.

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20 

Methodology for dense spatial sampling of multicomponent recording of converted waves in shallow marine environments
A widespread use of converted waves for shallow marine applications is hampered by spatial aliasing and field efficiency. Their short wavelengths require dense spatial sampling which often needs to be achieved by receivers deployed on the seabed. We adopted a new methodology where the dense spatial sampling is achieved in the commonreceiver domain by reducing the shot spacing. This is done by shooting one track multiple times and merging the shot lines in an effective manner in a separate processing step. This processing step is essential because positioning errors introduced during the field measurement can become significant in the combined line, particularly when they exceed the distance between two adjacent shot positions. For this processing step, a particular shot line is used as a reference line and relative variations in source and receiver positions in the other shot lines are corrected for using crosscorrelation. The combined shot line can subsequently be regularized for further processing. The methodology is adopted in a field experiment conducted in the Danube River in Hungary. The aim of the seismic experiment was to acquire properly sampled convertedwave data using a multicomponent receiver array. The dense spatial sampling was achieved by sailing one track 14 times. After correcting for the underwater receiver positions using the direct arrival, the crosscorrelation step was applied to merge the different shot lines. The successfully combined result is regularized into a densely sampled data set free of visible spatial aliasing and suitable for convertedwave processing.

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