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 new method to convert unleveled marine seismic data to leveled splitspread data

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4 

A theoretical and experimental approach to the geophoneground coupling problem based on acoustic reciprocity

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5 

The reflectivity operator for curved interfaces

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6 

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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7 

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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8 

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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9 

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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10 

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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11 

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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12 

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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13 

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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14 

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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15 

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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16 

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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17 

Highresolution reservoir characterization by an acoustic impedance inversion of a Tertiary deltaic clinoform system in the North Sea
Fluviodeltaic sedimentary systems are of great interest for explorationists because they can form prolific hydrocarbon plays. However, they are also among the most complex and heterogeneous ones encountered in the subsurface, and potential reservoir units are often close to or below seismic resolution. For seismic inversion, it is therefore important to integrate the seismic data with higher resolution constraints obtained from well logs, whereby not only the acoustic properties are used but also the detailed layering characteristics. We have applied two inversion approaches for poststack, timemigrated seismic data to a clinoform sequence in the North Sea. Both methods are recursive tracebased techniques that use well data as a priori constraints but differ in the way they incorporate structural information. One method uses a discrete layer model from the well that is propagated laterally along the clinoform layers, which are modeled as sigmoids. The second method uses a constant sampling rate from the well data and uses horizontal and vertical regularization parameters for lateral propagation. The first method has a low level of parameterization embedded in a geologic framework and is computationally fast. The second method has a much higher degree of parameterization but is flexible enough to detect deviations in the geologic settings of the reservoir; however, there is no explicit geologic significance and the method is computationally much less efficient. Forward seismic modeling of the two inversion results indicates a good match of both methods with the actual seismic data.

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18 

Surface and borehole groundpenetratingradar developments
During the past 80 years, groundpenetrating radar (GPR) has evolved from a skeptically received glacier sounder to a full multicomponent 3D volumeimaging and characterization device. The tool can be calibrated to allow for quantitative estimates of physical properties such as water content. Because of its high resolution, GPR is a valuable tool for quantifying subsurface heterogeneity, and its ability to see nonmetallic and metallic objects makes it a useful mapping tool to detect, localize, and characterize buried objects. No tool solves all problems; so to determine whether GPR is appropriate for a given problem, studying the reasons for failure can provide an understanding of the basics, which in turn can help determine whether GPR is appropriate for a given problem. We discuss the specific aspects of borehole radar and describe recent developments to become more sensitiveto orientation and to exploit the supplementary information in different components in polarimetric uses of radar data. Multicomponent GPR data contain more diverse geometric information than singlechannel data, and this is exploited in developed dedicated imaging algorithms. The evolution of these imaging schemes is discussed for groundcoupled and aircoupled antennas. For aircoupled antennas, the measured radiated wavefield can be used as the basis for the wavefield extrapolator in linearinversion schemes with an imaging condition, which eliminates the sourcetime function and corrects for the measured radiation pattern. A handheld GPR system coupled with a metal detector is ready for routine use in mine fields. Recent advances in modeling, tomography, and fullwaveform inversion, as well as Green's function extraction through correlation and deconvolution, show much promise in this field.

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19 

A perspective on 3D surfacerelated multiple elimination
Surfacerelated multiple elimination (SRME) is an algorithm that predicts all surface multiples by a convolutional process applied to seismic field data. Only minimal preprocessing is required. Once predicted, the multiples are removed from the data by adaptive subtraction. Unlike other methods of multiple attenuation, SRME does not rely on assumptions or knowledge about the subsurface, nor does it use event properties to discriminate between multiples and primaries. In exchange for this “freedom from the subsurface,” SRME requires knowledge of the acquisition wavelet and a dense spatial distribution of sources and receivers. Although a 2D version of SRME sometimes suffices, most field data sets require 3D SRME for accurate multiple prediction. All implementations of 3D SRME face a serious challenge: The sparse spatial distribution of sources and receivers available in typical seismic field data sets does not conform to the algorithmic requirements. There are several approaches to implementing 3D SRME that address the data sparseness problem. Among those approaches are preSRME data interpolation, onthefly data interpolation, zeroazimuth SRME, and trueazimuth SRME. Field data examples confirm that (1) multiples predicted using trueazimuth 3D SRME are more accurate than those using zeroazimuth 3D SRME and (2) onthefly interpolation produces excellent results.

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20 

Tutorial on seismic interferometry: Part 2 — Underlying theory and new advances
In the 1990s, the method of timereversed acoustics was developed. This method exploits the fact that the acoustic wave equation for a lossless medium is invariant for time reversal. When ultrasonic responses recorded by piezoelectric transducers are reversed in time and fed simultaneously as source signals to the transducers, they focus at the position of the original source, even when the medium is very complex. In seismic interferometry the timereversed responses are not physically sent into the earth, but they are convolved with other measured responses. The effect is essentially the same: The timereversed signals focus and create a virtual source which radiates waves into the medium that are subsequently recorded by receivers. A mathematical derivation, based on reciprocity theory, formalizes this principle: The crosscorrelation of responses at two receivers, integrated over different sources, gives the Green's function emitted by a virtual source at the position of one of the receivers and observed by the other receiver. This Green's function representation for seismic interferometry is based on the assumption that the medium is lossless and nonmoving. Recent developments, circumventing these assumptions, include interferometric representations for attenuating and/or moving media, as well as unified representations for waves and diffusion phenomena, bending waves, quantum mechanical scattering, potential fields, elastodynamic, electromagnetic, poroelastic, and electroseismic waves. Significant improvements in the quality of the retrieved Green's functions have been obtained with interferometry by deconvolution. A tracebytrace deconvolution process compensates for complex source functions and the attenuation of the medium. Interferometry by multidimensional deconvolution also compensates for the effects of onesided and/or irregular illumination.

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