Showing papers in "Geophysics in 1997"

Anisotropic parameters and P-wave velocity for orthorhombic media

Abstract: Although orthorhombic (or orthotropic) symmetry is believed to be common for fractured reservoirs, the difficulties in dealing with nine independent elastic constants have precluded this model from being used in seismology. A notation introduced in this work is designed to help make seismic inversion and processing for orthorhombic media more practical by simplifying the description of a wide range of seismic signatures. Taking advantage of the fact that the Christoffel equation has the same form in the symmetry planes of orthorhombic and transversely isotropic (TI) media, we can replace the stiffness coefficients by two vertical (P and S) velocities and seven dimensionless parameters that represent an extension of Thomsen's anisotropy coefficients to orthorhombic models. By design, this notation provides a uniform description of anisotropic media with both orthorhombic and TI symmetry. The dimensionless anisotropic parameters introduced here preserve all attractive features of Thomsen notation in treatin...

Seismic attenuation tomography using the frequency shift method

Abstract: We present a method for estimating seismic attenuation based on frequency shift data. In most natural materials, seismic attenuation increases with frequency. The high-frequency components of the seismic signal are attenuated more rapidly than the low-frequency components as waves propagate. As a result, the centroid of the signal's spectrum experiences a downshift during propagation. Under the assumption of a frequency-independent Q model, this downshift is proportional to a path integral through the attenuation distribution and can be used as observed data to reconstruct the attenuation distribution tomographically. The frequency shift method is applicable in any seismic survey geometry where the signal bandwidth is broad enough and the attenuation is high enough to cause noticeable losses of high frequencies during propagation. In comparison to some other methods of estimating attenuation, our frequency shift method is relatively insensitive to geometric spreading, reflection and transmission effects, source and receiver coupling and radiation patterns, and instrument responses. Tests of crosswell attenuation tomography on 1-D and 2-D geological structures are presented.

Velocity anisotropy in shales: A petrophysical study

Abstract: Using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content, clay mineralogy, and porosity at a wide range of effective pressure, we find that elastic anisotropy of shales increases substantially with compaction. The effect is attributed to both porosity reduction and smectite- to-illite transformation with diagenesis. A means of kerogen content mapping using velocity versus porosity crossplot for shales is shown. Matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0.4 at total organic carbon (TOC)= 15–20%. A strong chemical softening effect was found in shales containing even minor amounts of swelling (smectite) clay when saturated with aqueous solution. This effect results in a significant P -wave anisotropy reduction as compared to dry and oil-saturated shales. Since mature black shales are normally oil wet, this effect can only have a local significance restricted to the wellbore wall. Accurate measurements of phase velocities, including velocities at a 45° direction to the bedding plane, allow us to immediately calculate elastic stiffnesses and anisotropic parameters. Intrinsic (high pressure) properties of shales display an e > δ > 0 relation. Introduction of the bedding-parallel microcracks in overpressured shales results in a δ decrease when fully fluid saturated and a δ increase when partially gas saturated, with a characteristic effect on the shape of the P -wave velocity surface at small angles of incidence. Filtering the contribution of the intrinsic anisotropy of shales, it is possible to estimate the pore fluid phase, microcrack density, and aspect ratio parameters using seismic anisotropy measurements.

An inverse-scattering series method for attenuating multiples in seismic reflection data

Abstract: We present a multidimensional multiple‐attenuation method that does not require any subsurface information for either surface or internal multiples. To derive these algorithms, we start with a scattering theory description of seismic data. We then introduce and develop several new theoretical concepts concerning the fundamental nature of and the relationship between forward and inverse scattering. These include (1) the idea that the inversion process can be viewed as a series of steps, each with a specific task; (2) the realization that the inverse‐scattering series provides an opportunity for separating out subseries with specific and useful tasks; (3) the recognition that these task‐specific subseries can have different (and more favorable) data requirements, convergence, and stability conditions than does the original complete inverse series; and, most importantly, (4) the development of the first method for physically interpreting the contribution that individual terms (and pieces of terms) in the inv...

P-wave reflection coefficients for transversely isotropic models with vertical and horizontal axis of symmetry

Abstract: The study of P-wave reflection coefficients in anisotropic media is important for amplitude variation with offset (AVO) analysis. While numerical evaluation of the reflection coefficient is straightforward, numerical solutions do not provide analytic insight into the influence of anisotropy on the AVO signature. To overcome this difficulty, I present an improved approximation for P-wave reflection coefficients at a horizontal boundary in transversely isotropic media with vertical axis of symmetry (VTI media). This solution has the same AVO-gradient term describing the low-order angular variation of the reflection coefficient as the equations published previously, but is more accurate for large incidence angles. The refined approximation is then extended to transverse isotropy with a horizontal axis of symmetry (HTI), which is caused typically by a system of vertical cracks. Comparison of the approximate reflection coefficients for P-waves incident in the two vertical symmetry planes of HTI media indicates that the azimuthal variation of the AVO gradient is a function of the shear-wave splitting parameter y, and the anisotropy parameter describing P-wave anisotropy for nearvertical propagation in the vertical plane containing the symmetry axis.

Automatic conversion of magnetic data to depth, dip, and susceptibility contrast using the SPI (TM) method

Abstract: The Source Parameter Imaging (SPI™) method computes source parameters from gridded magnetic data The method assumes either a 2-D sloping contact or a 2-D dipping thin-sheet model and is based on the complex analytic signal Solution grids show the edge locations, depths, dips, and susceptibility contrasts The estimate of the depth is independent of the magnetic inclination, declination, dip, strike and any remanent magnetization; however, the dip and the susceptibility estimates do assume that there is no remanent magnetization Image processing of the source-parameter grids enhances detail and provides maps that facilitate interpretation by nonspecialists The SPI method tests successfully on synthetic profile and gridded data SPI maps derived from aeromagnetic data acquired over the Peace River Arch area of northwestern Canada correlate well with known basement structure and furthermore show that the Ksituan Magmatic Arc can be divided into several susceptibility subdomains

Estimation of multiple scattering by iterative inversion, Part I: Theoretical considerations

Abstract: A review has been given of the surface-related multiple problem by making use of the so-called feedback model. From the resulting equations it has been concluded that the proposed solution does not require any properties of the subsurface. However, source-detector and reflectivity properties of the surface need be specified. Those properties have been quantified in a surface operator and this operator is estimated as part of the multiple removal problem. The surface-related multiple removal algorithm has been formulated in terms of a Neumann series and in terms of an iterative equation. The Neumann formulation requires a nonlinear optimization process for the surface operator; while the iterative formulation needs a number of linear optimizations. The iterative formulation also has the advantage that it can be integrated easily with another multiple removal method. An algorithm for the removal of internal multiples has been proposed as well. This algorithm is an extension of the surface-related method. Removal of internal multiples requires knowledge of the macro velocity model between the surface and the upper boundary of the multiple generating layer. In Part II (also published in this issue) the success of the proposed algorithms has been demonstrated on numerical experiments and field data examples.

Orthorhombic media: Modeling elastic wave behavior in a vertically fractured earth

Abstract: Vertical fractures and horizontal fine layering combine to form a long-wavelength equivalent orthorhombic medium. Such media constitute a subset of the set of all orthorhombic media. Orthorhombic elastic symmetry is the lowest symmetry for which the slowness surface (the solution of the Christoffel equation) is bicubic rather than sextic. Various properties of orthorhombic media, such as the number and location of conical points and longitudinal directions, may be derived from the slowness surface or, because of its bicubic character, the squared slowness surface, which is a cubic surface. From the occurrence and angular orientation of some of these distinctive features, conclusions can be drawn with respect to the properties of the medium and to the parameters of the assumed underlying causes of the anisotropy. The estimation of these more subtle properties gains greater importance with the proliferation of multiazimuthal seismic surveys and the ability to drill along ever-more complicated 3-D well trajectories.

Estimation of multiple scattering by iterative inversion; Part II, Practical aspects and examples

Abstract: A surface-related multiple-elimination method can be formulated as an iterative procedure: the output of one iteration step is used as input for the next iteration step (part I of this paper). In this paper (part II) it is shown that the procedure can be made very efficient if a good initial estimate of the multiple-free data set can be provided in the first iteration, and in many situations, the Radon-based multiple-elimination method may provide such an estimate. It is also shown that for each iteration, the inverse source wavelet can be accurately estimated by a linear (least-squares) inversion process. Optionally, source and detector variations and directivity effects can be included, although the examples are given without these options. The iterative multiple elimination process, together with the source wavelet estimation, are illustrated with numerical experiments as well as with field data examples. The results show that the surface-related multiple-elimination process is very effective in time gates where the moveout properties of primaries and multiples are very similar (generally deep data), as well as for situations with a complex multiple-generating system.

Seismic attribute technology for reservoir forecasting and monitoring

Abstract: During the 1970s and 1980s, the seismic attributes most used in petroleum exploration were amplitude‐based instantaneous attributes. However, in the 1990s, seismic attribute technology has dramatically advanced in several directions; techniques now range from single‐trace instantaneous event attribute computations to more complex multitrace windowed seismic event attribute extractions to the generation of seismic attribute volumes. Applications run from simple amplitude anomaly detection to monitoring fluid front movement over time.

Principles of AVO crossplotting

Abstract: Hydrocarbon related “AVO anomalies” may show increasing or decreasing amplitude variation with offset. Conversely, brine‐saturated “background” rocks may show increasing or decreasing AVO.

Velocity analysis using nonhyperbolic moveout in transversely isotropic media

Abstract: P-wave reflections from horizontal interfaces in transversely isotropic (TI) media have nonhyperbolic moveout. It has been shown that such moveout as well as all time-related processing in TI media with a vertical symmetry axis (VTI media) depends on only two parameters, V{sub nmo} and {eta}. These two parameters can be estimated from the dip-moveout behavior of P-wave surface seismic data. Alternatively, one could use the nonhyperbolic moveout for parameter estimation. The quality of resulting estimates depends largely on the departure of the moveout from hyperbolic and its sensitivity to the estimated parameters. The size of the nonhyperbolic moveout in TI media is dependent primarily on the anisotropy parameter {eta}. An effective version of this parameter provides a useful measure of the nonhyperbolic moveout even in v(z) isotropic media. Moreover, effective {eta}, {eta}{sub eff}, is used to show that the nonhyperbolic moveout associated with typical TI media (e.g., shales, with {eta} {approx_equal} 0.1) is larger than that associated with typical v(z) isotropic media. The departure of the moveout from hyperbolic is increased when typical anisotropy is combined with vertical heterogeneity. Although estimation of {eta} using nonhyperbolic moveouts is not as stable as using the dip-moveout method of Alkhalifah and Tsvankin, particularlymore » in the absence of large offsets, it does offer some flexibility. It can be applied in the absence of dipping reflectors and also may be used to estimate lateral {eta} variations. Application of the nonhyperbolic inversion to data from offshore Africa demonstrates its usefulness, especially in estimating lateral and vertical variations in {eta}.« less

3-D magnetic imaging using conjugate gradients

Abstract: A 3-D inversion approach is outlined that determines a distribution of susceptibility that produces a given magnetic anomaly The subsurface model consists of a 3-D array of rectangular blocks, each with a constant susceptibility The inversion incorporates a model norm that allows smoothing and depth-weighting of the solution Since the number of parameters can be many thousands, even for small problems, the linear system of equations is inverted using a preconditioned conjugate gradient approach This reduces memory requirements and avoids large matrix multiplications The method is used to determine the 3-D susceptibility distribution responsible for the Temagami magnetic anomaly in southern Ontario, Canada

Mapping groundwater contamination using dc resistivity and VLF geophysical methods; a case study

Abstract: Geophysical methods can be helpful in mapping areas of contaminated soil and groundwater. Electrical resistivity and very low-frequency electromagnetic induction (VLF) surveys were carried out at a site of shallow hydrocarbon contamination in Utah County, Utah. Previously installed monitoring wells facilitated analysis of water chemistry to enhance interpretation of the geophysical data. The electrical resistivity and VLF data correlate well, and vertical cross-sections and contour maps generated from these data helped map the contaminant plume, which was delineated as an area of high interpreted resistivities.

The effect of a percolation threshold in the Kozeny‐Carman relation

Abstract: One of the most important properties of reservoir rocks, and perhaps the most difficult to predict, is permeability. Laboratory studies have shown that permeability depends on a long list of parameters: porosity, pore size and shape, clay content, stress, pore pressure, fluid type, saturation—a nearly overwhelming complexity. In spite of this, the essential behavior can often be expressed successfully using the remarkably simple Kozeny‐Carman (Kozeny, 1927; Carman, 1937, 1956; Bear, 1972; Scheidegger, 1974) relation κ=Bϕ3S2,where κ is the permeability, ϕ is the porosity, S is the specific surface area (pore surface area per volume of rock), and B is a geometric factor.

Reflection moveout and parameter estimation for horizontal transverse isotropy

Abstract: Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here, I present an exact equation for normal-moveout (NMO) velocities from horizontal reflectors valid for pure modes in HTI media with any strength of anisotropy. The azimuthally dependent P -wave NMO velocity, which can be obtained from 3-D surveys, is controlled by the principal direction of the anisotropy (crack orientation), the P -wave vertical velocity, and an effective anisotropic parameter equivalent to Thomsen9s coefficient δ. An important parameter of fracture systems that can be constrained by seismic data is the crack density, which is usually estimated through the shear-wave splitting coefficient γ. The formalism developed here makes it possible to obtain the shear-wave splitting parameter using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore, γ can be estimated just from the P -wave NMO velocity in the special case of the vanishing parameter e, corresponding to thin cracks and negligible equant porosity. Also, P -wave moveout alone is sufficient to constrain γ if either dipping events are available or the velocity in the symmetry direction is known. Determination of the splitting parameter from P -wave data requires, however, an estimate of the ratio of the P -to- S vertical velocities (either of the split shear waves can be used). Velocities and polarizations in the vertical symmetry plane of HTI media, that contains the symmetry axis, are described by the known equations for vertical transverse isotropy (VTI). Time-related 2-D P -wave processing (NMO, DMO, time migration) in this plane is governed by the same two parameters (the NMO velocity from a horizontal reflector and coefficient η) as in media with a vertical symmetry axis. The analogy between vertical and horizontal transverse isotropy makes it possible to introduce Thomsen parameters of the “equivalent” VTI model, which not only control the azimuthally dependent NMO velocity, but also can be used to reconstruct phase velocity and carry out seismic processing in off-symmetry planes.

The measurement of velocity dispersion and frequency-dependent intrinsic attenuation in sedimentary rocks

Abstract: A series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the Imperial College borehole test site. These experiments fall into four categories: vertical seismic profiles (VSPs) within the frequency range 30-280 Hz, crosshole surveys (0.2-2.3 kHz), sonic logging (8-24 kHz), and laboratory measurements (300-900 kHz). The intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible. Velocity dispersion is observed for both compressional and shear waves. The intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band. The data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio. The modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 (super -3) to 10 (super -4) .

Estimation of permeable pathways and water content using tomographic radar data

Abstract: Near‐surface environmental investigations often require monitoring of the spatial distribution of water content and identification of preferential fluid flow paths. Water content estimates are needed, for example, to model and predict pollutant transport through the vadose zone and to subsequently design an efficient and reliable remediation plan. The characteristics of preferential flow paths, as well as the location and geometry of these features, are necessary to model and predict flow and transport in complex geological media such as fractured or strongly heterogeneous porous systems.

On the resource evaluation of marine gas hydrate deposits using sea‐floor transient electric dipole‐dipole methods

Abstract: Methane hydrates are solid, nonstoichiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the sea floor, and estimates of the total mass available there exceed 1016Kg. Since each volume of hydrate can yield up to 164 volumes of gas, offshore methane hydrate is recognized as a very important natural energy resource. The depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sediment usually increases with increasing depth. The base of the hydrate zone is a phase boundary between solid hydrate and free gas and water. Its depth is determined principally by the value of the geothermal gradient. It stands out on seismic sections as a bright reflection. The diffuse upper boundary is not as well mar...

Effects of pore and differential pressure on compressional wave velocity and quality factor in Berea and Michigan sandstones

Abstract: Compressional‐wave velocity (VP) and quality factor (QP) have been measured in Berea and Michigan sandstones as a function of confining pressure (Pc) to 55 MPa and pore pressure (Pp) to 35 MPa. VP values are lower in the poorly cemented, finer grained, and microcracked Berea sandstone. QP values are affected to a lesser extent by the microstructural differences. A directional dependence of QP is observed in both sandstones and can be related to pore alignment with pressure. VP anisotropy is observed only in Berea sandstone. VP and QP increase with both increasing differential pressure (Pd=Pc-Pp) and increasing Pp. The effect of Pp on QP is greater at higher Pd. The results suggest that the effective stress coefficient, a measure of pore space deformation, for both VP and QP is less than 1 and decreases with increasing Pd.

Pushing the limits of seismic imaging, Part I: Prestack migration in terms of double dynamic focusing

Abstract: In this paper, the author proposes to extend the synthesis of areal sources (controlled emission) to the synthesis of areal detectors (controlled detection) such that the concept of numerical focusing can be formulated as a special version of target‐oriented synthesis. As a consequence, the insight in the complex prestack migration process can be improved significantly by making use of the concepts “focusing in emission” and “focusing in detection.” Focusing in emission transforms shot records into so‐called common focus‐point (CFP) gathers. Focusing in detection transforms CFP gathers into the prestack migration result. If structural information is sought, the focus point in emission is chosen equal to the focus point in detection: confocal version of CFP migration. If rock and pore information is required as well, the focus point in emission is chosen different from the focus point in detection: bifocal version of CFP migration. Errors in the underlying macro velocity model can be better analysed than b...

Hybrid l 1 /l 2 minimization with applications to tomography

Abstract: Least squares or l 2 solutions of seismic inversion and tomography problems tend to be very sensitive to data points with large errors. The l p minimization for 1 ≤ p < 2 gives more robust solutions, but usually with higher computational cost. Iteratively reweighted least squares (IRLS) gives efficient approximate solutions to these l P problems. We apply IRLS to a hybrid l 1 /l 2 minimization problem that behaves like an l 2 fit for small residuals and like an l 1 fit for large residuals. The smooth transition from l 2 to l 1 behavior is controlled by a parameter that we choose using an estimate of the standard deviation of the data error. For linear problems of full rank, the hybrid objective function has a unique minimum, and IRLS can be proven to converge to it. We obtain a robust efficient method. For nonlinear problems, a version of the Gauss-Newton algorithm can be applied. Synthetic crosswell tomography examples and a field-data VSP tomography example demonstrate the improvement of the hybrid method over least squares when there are outliers in the data.

Using AVO for fracture detection: Analytic basis and practical solutions

Abstract: Tight, low‐porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability. Characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature. TLE, for example, had an entire issue (August 1996) on this subject. While the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures, the problem is how to infer fracture parameters from these data.

Ground-roll suppression using the wavelet transform

Abstract: Low-frequency, high-amplitude ground roll is an old problem in land-based seismic field records. Current processing techniques aimed at ground-roll suppression, such as frequency filtering, f - k filtering, and f - k filtering with time-offset windowing, use the Fourier transform, a technique that assumes that the basic seismic signal is stationary. A new alternative to the Fourier transform is the wavelet transform, which decomposes a function using basis functions that, unlike the Fourier transform, have finite extent in both frequency and time. Application of a filter based on the wavelet transform to land seismic shot records suppresses ground roll in a time-frequency sense; unlike the Fourier filter, this filter does not assume that the signal is stationary. The wavelet transform technique also allows more effective time-frequency analysis and filtering than current processing techniques and can be implemented using an algorithm as computationally efficient as the fast Fourier transform. This new filtering technique leads to the improvement of shot records and considerably improves the final stack quality.

Electroseismic investigation of the shallow subsurface: Field measurements and numerical modeling

Abstract: Recent studies have demonstrated that electroseismic phenomena in porous media have the potential to detect zones of high fluid mobility and fluid chemistry contrasts in the subsurface. However, there have only been a few field studies of these phenomena since they were first observed 60 years ago. None of these studies were able to support observations with an explicit comparison to results of full waveform modeling. In this paper, we demonstrate that the electroseismic phenomena in porous media can be observed in the field, explained, and modeled numerically, yielding a good agreement between the field and the synthetic data. We first outline the design of our field experiment and describe the procedure used to reduce noise in the electroseismic data. After that, we present and interpret the field data, demonstrating how and where different electroseismic signals originated in the subsurface. Finally, we model our field experiment numerically and demonstrate that the numerical results correctly simulate arrival times, polarity, and amplitude variation with offset behavior of the electroseismic signals measured in the field.

Inherent power‐law behavior of magnetic field power spectra from a Spector and Grant ensemble

Abstract: The Spector and Grant method, which has been in use for 25 years, relates average depths to source to rate of decay of the magnetic power spectra. This method, which assumes a uniform distribution of parameters for an ensemble of magnetized blocks, leads to a depth-dependent exponential rate of decay. We show that also inherent in this model is a power-law rate of decay that is independent of depth. For most cases, except for extreme depths and small block sizes, the observed power spectrum should be corrected for a power law decay rate of beta approximately 3. If the depth distribution of the magnetic blocks is Gaussian, then the observed power spectrum should be corrected for both a depth independent power law and exponential decay. This power-law decay is very similar to the scaling behavior, supposed as a fractal character, of observed magnetic fields in North America. As a general rule, when beta approximately 3, further information is needed to discriminate between a fractal or Spector and Grant model. However, it is becoming quite clear that magnetic power spectra should be corrected for a power law decay before applying the Spector and Grant method for depth determination.

Gravity inversion of basement relief using approximate equality constraints on depths

Abstract: We present a gravity interpretation method for estimating the relief of an arbitrary interface separating two homogeneous media. The upper medium is discretized into rectangular, juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated from the gravity anomaly. The density contrast of each prism is assumed to be constant and known. To stabilize the inversion, we introduce two kinds of constraints on the depths. The first one requires proximity between the observed and computed depths at isolated points such as those obtained from boreholes (absolute equality constraint). The second one requires that groups of depths approximately follow an established linear relationship among the depths (relative equality constraint). Both kinds of constraints are imposed in the least‐squares sense. We illustrate the method performance by applying it to a synthetic anomaly produced by a simulated basement relief consisting of four narrow and adjacent structural lows...

Comparison of sea‐ice thickness measurements under summer and winter conditions in the Arctic using a small electromagnetic induction device

Abstract: Drillhole-determined sea-ice thickness was compared with values derived remotely using a portable smalloffset loop-loop steady state electromagnetic (EM) induction device during expeditions to Fram Strait and the Siberian Arctic, under typical winter and summer conditions. Simple empirical transformation equations are derived to convert measured apparent conductivity into ice thickness. Despite the extreme seasonal differences in sea-ice properties as revealed by ice core analysis, the transformation equations vary little for winter and summer. Thus, the EM induction technique operated on the ice surface in the horizontal dipole mode yields accurate results within 5 to 10% of the drillhole determined thickness over level ice in both seasons. The robustness of the induction method with respect to seasonal extremes is attributed to the low salinity of brine or meltwater filling the extensive pore space in summer. Thus, the average bulk ice conductivity for summer multiyear sea ice derived according to Archie’s law amounts to 23 mS/m compared to 3 mS/m for winter conditions. These mean conductivities cause only minor differences in the EM response, as is shown by means of 1-D modeling. However, under summer conditions the range of ice conductivities is wider. Along with the widespread occurrence of surface melt ponds and freshwater lenses underneath the ice, this causes greater scatter in the apparent conductivity/ice thickness relation. This can result in higher deviations between EM-derived and drillhole determined thicknesses in summer than in winter.

To Bayes or not to Bayes

Abstract: The goal of geophysical inversion is to make quantitative inferences about the Earth from noisy, finite data. The limitations of noise and the inadequacy of the data mean that geophysical inversion problems are fundamentally problems of statistical inference. We do not invert data to find “models.” as much as we might like to; we invert data to make inferences about models. There will usually be an infinity of models that fit the data. Thus we must look to probability theory for help.

Multiattribute seismic analysis

Abstract: Multiattribute seismic analysis is a broad term that encompasses all geostatistical methods that utilize more than one attribute to predict some physical property of the earth Bivariate geostatistics is obviously the simplest subset of the multivariate techniques and thus the standard technique of cokriging could be called multivariate geostatistics However, in this paper we will use the name to refer to geostatistical methods that use more than two variables Even then, there are many different methods that fall under this heading There are three major subcategories