Showing papers in "Geophysics in 2002"

Applications of plane-wave destruction filters

Abstract: Plane‐wave destruction filters originate from a local plane‐wave model for characterizing seismic data. These filters can be thought of as a time–distance (T‐X) analog of frequency‐distance (F‐X) prediction‐error filters and as an alternative to T‐X prediction‐error filters. The filters are constructed with the help of an implicit finite‐difference scheme for the local plane‐wave equation. Several synthetic and real data examples show that finite‐difference plane‐wave destruction filters perform well in applications such as fault detection, data interpolation, and noise attenuation.

Seismic anisotropy in sedimentary rocks, part 2: Laboratory data

Abstract: Part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug. This method saves at least two‐thirds of the time for preparing core samples and measuring velocities in transversely isotropic (TI) rocks. Using this method, we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world. We present some of the data in this paper, which include seismic velocity and anisotropy in 17 brine‐saturated shale samples, 1 gas‐ and brine‐saturated coal sample, 8 brine‐saturated sands, 12 gas‐saturated sands, 32 gas‐saturated carbonate samples, and 25 brine‐saturated carbonate samples. The results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy. Very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands, sandstones, and carbonates under reservoir conditions. In contrast, all shales were found seismically anisotropic: anisotr...

Remote sensing of hydrocarbon layers by seabed logging (SBL)Results from a cruise offshore Angola

Abstract: Detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry. Seismic methods have traditionally been used in this context, but the results can be ambiguous. Another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids. Modeling presented by Eidesmo et al. (2002) demonstrates that by using seabed logging (SBL), a special application of frequency domain controlled source electromagnetic (CSEM) sounding, the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified. Such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying.

Computation of linear elastic properties from microtomographic images: Methodology and agreement between theory and experiment

Abstract: Elastic property‐porosity relationships are derived directly from microtomographic images. This is illustrated for a suite of four samples of Fontainebleau sandstone with porosities ranging from 7.5% to 22%. A finite‐element method is used to derive the elastic properties of digitized images. By estimating and minimizing several sources of numerical error, very accurate predictions of properties are derived in excellent agreement with experimental measurements over a wide range of the porosity. We consider the elastic properties of the digitized images under dry, water‐saturated, and oil‐saturated conditions. The observed change in the elastic properties due to fluid substitution is in excellent agreement with the exact Gassmann's equations. This shows both the accuracy and the feasibility of combining microtomographic images with elastic calculations to accurately predict petrophysical properties of individual rock morphologies. We compare the numerical predictions to various empirical, effective medium ...

Fast structural interpretation with structure-oriented filtering

Abstract: This paper addresses a new approach to structural interpretation of 3-D seismic data. It is complemented by a second paper which has been submitted to Geophysics dealing with the technical aspects of structure-oriented filtering. The method and workflow should be viewed against a background of recent changes in the working environment for seismic interpretation. For instance, there has been constant pressure to reduce turnaround time or at least to quickly come up with preliminary results that can influence investment decisions. Integrated teams have been set up to analyze problems in parallel rather than sequential workflows. For the seismic interpreter, this means changing from a one-time transmission of final results to a repetitive process of feeding data into iterative team processes—starting with a crude structural model and followed by increasingly detailed, sophisticated models. Another change is that geologists have more and more replaced geophysicists in seismic interpretation. This trend has improved the geologic feasibility of the average interpretation—in particular with noisy data where defendable answers to ambiguity can be found only by testing alternative geologic scenarios. However, the change has also led to frequent overinterpretation of noisy data. For example, structural and stratigraphic phenomena have been wrongly seen in coherent noise patterns (see convincing demonstration by Hesthammer, 1999). The problem occurs because geologist interpreters tend to accept 3-D seismic data delivered to them by seismic processors too readily as the best possible product and often lack sufficient processing knowledge to identify spurious reflection patterns as artifacts. Thus, acceleration of interpretation and handling of noisy data would be helped by functionality to stabilize and simplify 3-D seismic data. Shell has developed such techniques on the basis of image processing techniques and implemented them at the fingertips of interpreters, to make them empirically find the best solution for a given task. Seismic …

Offset and angle-domain common image-point gathers for shot-profile migration

Abstract: Prestack depth migration of shot profiles by downward continuation is a practical imaging algorithm that is especially cost-effective for sparse-shot wide-azimuth geometries. The interpretation of offset as the displacement between the downward-propagating (shot) wavefield and upward-propagating (receiver) wavefield enables us to extract offset-domain common image-point (CIP) gathers during shot-profile migration. The offset-domain gathers can then be transformed to the angle domain with a radial-trace mapping originally introduced for shot-geophone migration. The computational implications of this procedure include both the additional cost of multioffset imaging and an implicit transformation from shot-geophone to midpoint-offset coordinates. Although this algorithm provides a mechanism for imaging angle-dependent reflectivity via shot-profile migration, for sparse-shot geometries the fundamental problem of shot-aliasing may severely impact the quality of CIP gathers.

A stable and efficient approach of inverse Q filtering

Abstract: Stability and efficiency are two issues of general concern in inverse Q filtering. This paper presents a stable, efficient approach to inverse Q filtering, based on the theory of wavefield downward continuation. It is implemented in a layered manner, assuming a depth-dependent, layered-earth Q model. For each individual constant Q layer, the seismic wavefield recorded at the surface is first extrapolated down to the top of the current layer and a constant Q inverse filter is then applied to the current layer. When extrapolating within the overburden, instead of applying wavefield downward continuation directly, a reversed, upward continuation system is solved to obtain a stabilized solution. Within the current constant Q layer, the amplitude compensation operator, which is a 2-D function of traveltime and frequency, is approximated optimally as the product of two 1-D functions depending, respectively, on time and frequency. The constant Q inverse filter that compensates simultaneously for phase and amplitude effects is then implemented efficiently in the Fourier domain.

Characterization of fluid transport properties of reservoirs using induced microseismicity

Abstract: We systematically describe an approach to estimate the large-scale permeability of reservoirs using seismic emission (microseismicity) induced by fluid injection. We call this approach seismicity-based reservoir characterization (SBRC). A simple variant of the approach is based on the hypothesis that the triggering front of hydraulically-induced microseismicity propagates like a diffusive process (pore pressure relaxation) in an effective homogeneous anisotropic poroelastic fluid-saturated medium. The permeability tensor of this effective medium is the permeability tensor upscaled to the characteristic size of the seismically active heterogeneous rock volume. We show that in a homogeneous medium the surface of the seismicity triggering front has the same form as the group-velocity surface of the low-frequency anisotropic, second-type Biots wave (i.e., slow wave). Further, we generalize SBRC for 3-D mapping of the permeability tensor of heterogeneous reservoirs and aquifers. For this we apply an approach similar to the geometric optics approximation. We derive an equation describing kinematic aspects of triggering-front propagation in a way similar to the eikonal equation for seismic wavefronts. In the case of isotropic heterogeneous media, the inversion for the hydraulic properties of rocks follows from a direct application of this equation. In the case of an anisotropic heterogeneous medium, only the magnitude of a global effective permeability tensor can be mapped in a 3-D spatial domain. We demonstrate the method on several field examples and also test the eikonal equation-based inversion.

3‐D magnetic inversion with data compression and image focusing

Abstract: We develop a method of 3‐D magnetic anomaly inversion based on traditional Tikhonov regularization theory. We use a minimum support stabilizing functional to generate a sharp, focused inverse image. An iterative inversion process is constructed in the space of weighted model parameters that accelerates the convergence and robustness of the method. The weighting functions are selected based on sensitivity analysis. To speed up the computations and to decrease the size of memory required, we use a compression technique based on cubic interpolation.Our method is designed for inversion of total magnetic anomalies, assuming the anomalous field is caused by induced magnetization only. The method is applied to synthetic data for typical models of magnetic anomalies and is tested on real airborne data provided by ExxonMobil Upstream Research Company.

Extended elastic impedance for fluid and lithology prediction

Abstract: Constant angle projections of seismic sections can be designed to provide maximum discrimination between fluids or lithologies. The optimum projection for a noise‐free, isotropic environment can be obtained using an extension to the elastic impedance concept, which itself is an extension of acoustic impedance (AI) to nonzero angles of incidence. To achieve this, we modify the definition of elastic impedance (EI) beyond the range of physically meaningful angles by substituting tanχ for sin2 θ in the two‐term reflectivity equation. The primary variable now becomes χ rather than θ. We allow it to vary between −90° and +90°, which gives an extension of EI for any combination of intercept and gradient. We refer to this form of elastic impedance as extended elastic impedance (EEI).In this paper we demonstrate that EEI can be tuned using different χ values to be approximately proportional to a number of elastic parameters, and we give EEI expressions for shear impedance (SI), bulk modulus, shear modulus, Lame's ...

IP interpretation in environmental investigations

Abstract: The induced polarization (IP) response of rocks and soils is a function of lithology and fluid conductivity. IP measurements are sensitive to the low‐frequency capacitive properties of rocks and soils, which are controlled by diffusion polarization mechanisms operating at the grain‐fluid interface. IP interpretation typically is in terms of the conventional field IP parameters: chargeability, percentage frequency effect, and phase angle. These parameters are dependent upon both surface polarization mechanisms and bulk (volumetric) conduction mechanisms. Consequently, they afford a poor quantification of surface polarization processes of interest to the field geophysicist. A parameter that quantifies the magnitude of surface polarization is the normalized chargeability, defined as the chargeability divided by the resistivity magnitude. This parameter is proportional to the quadrature conductivity measured in the complex resistivity method. For nonmetallic minerals, the quadrature conductivity and normalize...

Accurate interpolation with high-resolution time-variant Radon transforms

Abstract: It is well known that a sparse hyperbolic Radon transform (RT) can be used to extend the aperture of aperture limited data, filter noise, and fill gaps. In the same manner, an elliptical RT can achieve similar results when applied to slant stack sections. A problem with these transformations is that they have a time-variant kernel that results in slow implementation. By defining a model space in terms of an irregularly sampled velocity space to minimize the number of unknowns during the inversion and using sparse matrices, however, the computation time can be kept low enough for practical application. We implement hyperbolic and elliptical time domain RTs by using inversion via weighted conjugate gradient methods with a sparseness constraint. The hyperbolic RT performs accurate interpolation in common-midpoint (CMP) gathers, while the elliptical RT attenuates sampling artifacts in slant stack sections obtained from CMP gathers with poor sampling and gaps.

Estimation of quality factors from CMP records

Abstract: Estimates of the quality, Q, factor are commonly obtained from vertical seismic data or stacked surface seismic data. This paper describes a method that allows Q-factor to be estimated directly from common midpoint (CMP) gathers. Absorption of the wavefield is dependent on three parameters: frequency, traveltime in the medium, and medium Q-factor. Assuming that the amplitude spectrum of the seismic source signature may be modeled by that of a Ricker wavelet, we derive an analytical relation between Q-factor and seismic data peak frequency variation both along offset and vertical time direction. The Q-factor is estimated from CMP gathers using a layer-stripping approach.

Geopressure prediction using seismic data: Current status and the road ahead

Abstract: The subject of seismic detection of abnormally high‐pressured formations has received a great deal of attention in exploration and production geophysics because of increasing exploration and production activities in frontier areas (such as the deepwater) and a need to lower cost without compromising safety and environment, and manage risk and uncertainty associated with very expensive drilling. The purpose of this review is to capture the “best practice” in this highly specialized discipline and document it. Pressure prediction from seismic data is based on fundamentals of science, especially those of rock physics and seismic attribute analysis. Nonetheless, since the first seismic application in the 1960s, practitioners of the technology have relied increasingly on empiricism, and the fundamental limitations of the tools applied to detect such hazardous formations were lost. The most successful approach to seismic pressure prediction is one that combines a good understanding of rock properties of subsurf...

3D edge-preserving smoothing and applications

Abstract: This paper presents a new algorithm for reducing noise in seismic-impedance cubes while preserving structural and stratigraphic discontinuities or edges. The method divides the vicinity of every location in a 3D impedance cube into a number of blocks as the analysis point moves throughout the volume. At an interior discontinuity location with any 3D orientation, the process does not average values across the edge because this would blur the feature. Instead, the smoothest neighboring value is used. The main advantage of this 3D smoothing algorithm is that it preserves both lateral and vertical edges (i.e., the impedance boundaries) and improves the contrast in coherence attributes derived from impedance data.

Elastic impedance normalization

Abstract: The elastic impedance, or EI, function has enabled far-offset-angle stack data to be inverted using technology developed for acoustic impedance inversion. An undesirable feature of the EI function has been that its dimensionality varies with incidence angle θ and provides numerical values that change significantly with θ . These problems have been overcome by modifying the EI function with constants α o , β o , and ρ o . These modifications allow for a direct comparison between elastic impedance values across a range of angles in a manner that was not available with the previous formulation. The modifications neither improve nor degrade the accuracy of the reflectivity that can be derived from the EI function.

Converted-wave seismic exploration: Methods

Abstract: Multicomponent seismic recording (measurement with vertical‐ and horizontal‐component geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than conventional single‐element techniques. In the last several years, multicomponent surveying has developed rapidly, allowing creation of converted‐wave or P‐S images. These make use of downgoing P‐waves that convert on reflection at their deepest point of penetration to upcoming S‐waves. Survey design for acquiring P‐S data is similar to that for P‐waves, but must take into account subsurface VP/VS values and the asymmetric P‐S ray path. P‐S surveys use conventional sources, but require several times more recording channels per receiving location. Some special processes for P‐S analysis include anisotropic rotations, S‐wave receiver statics, asymmetric and anisotropic binning, nonhyperbolic velocity analysis and NMO correction, P‐S to P‐P time transformation, P‐S dip moveout, prestack migration with two velocities and wa...

Neural networks as an intelligence amplification tool: A review of applications

Abstract: The sophisticated algorithms we use to process, analyze, and interpret geophysical data automate tasks we used to do by hand, transform data into domains where patterns are more obvious, and allow us to calculate quantities where we used to rely on intuition or back‐of‐envelope estimates. But, the crux of the exploration problem is still interpretation—associating abstract patterns with geologic formations of economic value. Artificial neural networks are able to couple the speed and efficiency of the computer with the pattern recognition and association capabilities of the brain to aid the exploration process. The key concept to understand in the application of neural network technology is that they should not be used as an artificial intelligence tool to replace a human interpreter; rather, neural networks are an intelligence amplification toolkit that allows the interpreter to focus on the important information.More than 102 neural network papers have been published by SEG since 1988, and more than 550...

Detecting high overpressure

Abstract: Normal pressure is pore fluid pressure that equals the hydrostatic pressure of a column of formation water extending to the surface. Overpressure is pore fluid pressure greater than normal pressure. However, no standard definition exists for what constitutes high overpressure. What can be said is that high overpressure often means trouble. For an explorationist, it could mean blown reservoir seals; for a driller, it could mean excessive time spent fighting formation fluid influxes and/or drilling fluid losses. A practical upper limit for pore pressure is the overburden stress. Pore pressures in this range are on the verge of opening fractures that can vent fluid and bleed off pressure like a pressure relief valve. Therefore, criteria for defining high overpressure are sometimes expressed in terms of a percentage of the overburden stress, say, pore pressure greater than 90% of the overburden stress. In this article, high overpressure will be defined simply as pore pressure that approaches the overburden stress. All but one potential cause of overpressure can produce high pressure. Fortunately, the mechanism that cannot generate high pressure is the most common cause of overpressure. Therefore, detecting high overpressure basically boils down to determining where extraordinary overpressure mechanisms may be encountered. Overpressure detection is based on the premise that pore pressure affects compaction-dependent geophysical properties such as density, resistivity, and sonic velocity. Shales are the preferred lithology for pore pressure interpretation because they are more responsive to overpressure than most rock types. Consequently, overpressure detection centers around shale deformation behavior. For stress ranges of practical interest, shale compaction is controlled by the difference between total applied stress and pore fluid pressure. This difference, termed the effective stress , represents the portion of the total stress carried by the rock grains. Figure 1 illustrates the effective stress concept with laboratory data for …

Comparison between permeability anisotropy and elasticity anisotropy of reservoir rocks

Abstract: We developed new experimental and theoretical tools for the measurement and the characterization of arbitrary elasticity tensors and permeability tensors in rocks. They include an experimental technique for the 3‐D visualization of hydraulic invasion fronts in rock samples by monitoring the injection of salt solutions by X‐ray tomography, and a technique for inverting the complete set of the six coefficients of the permeability tensor from invasion front images. In addition, a technique for measuring the complete set of the 21 elastic coefficients, a technique allowing the identification and the orientation in the 3‐D space of the symmetry elements (planes, axes), and a technique for approximating the considered elastic tensor by a tensor of simpler symmetry with the quantification of the error induced by such an approximation have been developed.We apply these tools to various types of reservoir rocks and observed quite contrasted behaviors. In some rocks, the elastic anisotropy and the hydraulic anisotr...

A 3-D resistivity investigation of a contaminated site at Lernacken, Sweden

Abstract: A contaminated site at Lernacken in southern Sweden, formerly used for sludge disposal, was investigated using a 3-D resistivity imaging technique. The data acquisition was carried out using a roll-along technique for 3-D data acquisition that allows using standard multielectrode equipment designed for engineering and environmental applications. The technique allows for the measurement of large true 3-D resistivity data sets, and data were measured using two perpendicular electrode-orientation directions with only one layout of the cables. The data were plotted as two sets of pseudo depth slices using the two electrode orientation directions, which resulted in markedly different plots. The complete data set was inverted to form a resistivity-depth model of the ground using a 3-D least-squares smoothness constrained inversion technique. The results obtained were compared to other geophysical and background data, and a good agreement was found. The results show that the 3-D roll-along technique in combination with 3-D inversion can be highly useful for engineering and environmental applications. However, multichannel measurement equipment is necessary to speed up the data acquisition process for routine application.

Seismic anisotropy in sedimentary rocks, part 1: A single‐plug laboratory method

Abstract: A single-plug method for measuring seismic velocities and transverse isotropy in rocks has been rigorously validated and laboratory tested. The method requires only one sample to measure the velocities needed to derive the five independent elastic constants for transversely isotropic materials. In this method, piezoelectric transducers are fitted to the top, bottom, and sides of the cylindrical sample. Laboratory velocity and anisotropy can be measured as functions of pressure, temperature, fluid saturation, and fluid displacement. Because this method uses a horizontal core plug that has much higher permeability than a vertical core plug, it is especially suitable for low-permeability shale measurements. It reduces the sample preparation and velocity measurement time by more than two-thirds.

Simultaneous inversion of prestack seismic data for rock properties using simulated annealing

Abstract: A new prestack inversion algorithm has been developed to simultaneously estimate acoustic and shear impedances from P‐wave reflection seismic data. The algorithm uses a global optimization procedure in the form of simulated annealing. The goal of optimization is to find a global minimum of the objective function, which includes the misfit between synthetic and observed prestack seismic data. During the iterative inversion process, the acoustic and shear impedance models are randomly perturbed, and the synthetic seismic data are calculated and compared with the observed seismic data. To increase stability, constraints have been built into the inversion algorithm, using the low‐frequency impedance and background Vs/Vp models. The inversion method has been successfully applied to synthetic and field data examples to produce acoustic and shear impedances comparable to log data of similar bandwidth. The estimated acoustic and shear impedances can be combined to derive other elastic parameters, which may be use...

Biot–Gassmann theory for velocities of gas hydrate‐bearing sediments

Abstract: Elevated elastic velocities are a distinct physical property of gas hydrate‐bearing sediments. A number of velocity models and equations (e.g., pore‐filling model, cementation model, effective medium theories, weighted equations, and time‐average equations) have been used to describe this effect. In particular, the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate‐bearing sediments. A weakness of the weighted equation is its use of the empirical relationship of the time‐average equation as one element of the equation. One drawback of the effective medium theory is its prediction of unreasonably higher shear‐wave velocity at high porosities, so that the predicted velocity ratio does not agree well with the observed velocity ratio. To overcome these weaknesses, a method is proposed, based on Biot–Gassmann theories and assuming the formation velocity ratio (shear to compressional velocity) of an unconsolidated sediment is related to the...

Interactive seismic facies classification using textural attributes and neural networks

Abstract: In this study, we present an application of textural analysis to 3D seismic volumes. Specifically, we combine image textural analysis with a neural network classification to quantitatively map seismic facies in three-dimensional data. Key advantages of this approach are: 1. it produces a detailed 3D facies classification volume (whereas manual seismic facies classifications are typically 2D maps), 2. it enables rapid and quantitative anlaysis of the increasingly large seismic volumes available to the interpreter, and 3. it eliminates many time-consuming tasks, thereby freeing the interpreter to focus on determining seismic facies and integrating them into a geologic framework. Finally, we extend our textural analysis-based seismic facies classification technique to interpretation of AVO attribute volumes, such as “A + B” (AVO intercept + gradient), to reduce the inherent nonuniqueness of seismic facies to geologic and lithologic facies, and simplify the facies analysis of complex, mixed-impedance reservoirs. Seismic facies analysis is a powerful qualitative technique used in stratigraphic analysis from seismic data and in hydrocarbon exploration. Seismic facies are groups of seismic reflections whose parameters (such as amplitude, continuity, reflection geometry, and frequency) differ from those of adjacent groups. Seismic facies analysis involves two key steps—(1) seismic facies classification (i.e., seismic facies are defined, and lateral/vertical extents delineated) and (2) interpretation (i.e., analysis of vertical/lateral associations, map patterns, and calibration to wells) to produce a geologic and depositional interpretation. This interpretation step is required because there is a nonunique relationship between seismic data, seismic facies, and depositional environment or rock property relationships (Figure 1). Figure 1. Examples of seismic facies and potential associated geologic fill. A seismic facies can be defined as a stratigraphic region in the seismic data volume that has a characteristic reflection pattern distinguishable from those of other areas on the basis of reflection amplitude, continuity, geometry, and/or internal configuration of reflectors. Inherent in …

Three‐dimensional induction logging problems, Part I: An integral equation solution and model comparisons

Abstract: A 3‐D frequency‐domain solution based on a volume integral equation approach has been implemented to simulate induction log responses. In our treatment of the problem, we assume that the electrical properties of the bedding as well as the borehole and invasion zones can exhibit transverse anisotropy. The solution process uses a Krylov subspace iteration to solve the scattering equation, which is based on the modified iterative dissipative method. Internal consistency checks and comparisons with mode matching and finite‐difference solutions for vertical borehole models demonstrate the accuracy of the solution.There are no known analytical solutions for induction log responses arising from deviated boreholes intersecting horizontal bed boundaries. To simulate such responses requires the numerical solution of Maxwell's equations in three dimensions along with independent tests to validate the solution approach and its accuracy. In this paper, we compare two independent 3‐D frequency‐domain solutions for the ...

Structural uncertainties: Determination, management, and applications

Abstract: Structural uncertainties have a direct impact in exploration, development, and production, and in drilling decisions. In this paper, we present an approach for determining and handling structural uncertainties. We first examine the magnitude of the different sources of uncertainty, and explain how to estimate their direction and correlation length. This task requires a huge geophysical input. This information is then used in a general scheme to generate multiple realizations of the structural model consistent with structural uncertainties. The technique is based on geostatistical concepts. Finally, we illustrate the application of this scheme in examples relevant for exploration, development and production, and drilling.The structural model is described as a set of horizons represented by triangulated surfaces cut by faults. The relationships between horizons and faults are expressed as a set of constraints. On a horizon, each source of uncertainty (typically migration, picking, and time‐to‐depth conversi...

An adaptive finite-difference method for traveltimes and amplitudes

Abstract: The point-source traveltime field has an upwind singularity at the source point. Consequently, all formally high-order, finite-difference eikonal solvers exhibit first-order convergence and relatively large errors. Adaptive upwind finite-difference methods based on high-order Weighted Essentially NonOscillatory (WENO) Runge-Kutta difference schemes for the paraxial eikonal equation overcome this difficulty. The method controls error by automatic grid refinement and coarsening based on a posteriori error estimation. It achieves prescribed accuracy at a far lower cost than does the fixed-grid method. Moreover, the achieved high accuracy of traveltimes yields reliable estimates of auxiliary quantities such as take-off angles and geometric spreading factors.

Investigating peatland stratigraphy and hydrogeology using integrated electrical geophysics

Abstract: Hydrology has been suggested as the mechanism controlling vegetation and related surficial pore-water chemistry in large peatlands. Peatland hydrology influences the carbon dynamics within these large carbon reservoirs and will influence their response to global warming. A geophysical survey was completed in Caribou Bog, a large peatland in Maine, to evaluate peatland stratigraphy and hydrology. Geophysical measurements were integrated with direct measurements of peat stratigraphy from probing, fluid chemistry, and vegetation patterns in the peatland. Consistent with previous field studies, ground-penetrating radar (GPR) was an excellent method for delineating peatland stratigraphy. Prominent reflectors from the peat-lake sediment and lake sediment-mineral soil contacts were precisely recorded up to 8 m deep. Two-dimensional resistivity and induced polarization imaging were used to investigate stratigraphy beneath the mineral soil, beyond the range of GPR. We observe that the peat is chargeable, and that IP imaging is an alternative method for defining peat thickness. The chargeability of peat is attributed to the high surface-charge density on partially decomposed organic matter. The electrical conductivity imaging resolved glaciomarine sediment thickness (a confining layer) and its variability across the basin. Comparison of the bulk conductivity images with peatland vegetation revealed a correlation between confining layer thickness and dominant vegetation type, suggesting that stratigraphy exerts a control on hydrogeology and vegetation distribution within this peatland. Terrain conductivity measured with a Geonics EM31 meter correlated with confining glaciomarine sediment thickness and was an effective method for estimating variability in glaciomarine sediment thickness over approximately 18 km 2 . Our understanding of the hydrogeology, stratigraphy, and controls on vegetation growth in this peatland was much enhanced from the geophysical study.