Jerry M. Harris

Seismic attenuation tomography using the frequency shift method

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 signals 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.

Time-lapse imaging of saline-tracer transport in fractured rock using difference-attenuation radar tomography

[1] Accurate characterization of fractured-rock aquifer heterogeneity remains one of the most challenging and important problems in groundwater hydrology. We demonstrate a promising strategy to identify preferential flow paths in fractured rock using a combination of geophysical monitoring and conventional hydrogeologic tests. Cross-well differenceattenuation ground-penetrating radar was used to monitor saline-tracer migration in an experiment at the U.S. Geological Survey Fractured Rock Hydrology Research Site in Grafton County, New Hampshire. Radar data sets were collected every 10 min in three adjoining planes for 5 hours during each of 12 tracer tests. An innovative inversion method accounts for data acquisition times and temporal changes in attenuation during data collection. The inverse algorithm minimizes a combination of two functions. The first is the sum of weighted squared data residuals. Second is a measure of solution complexity based on an a priori space-time covariance function, subject to constraints that limit radarattenuation changes to regions of the tomograms traversed by high difference-attenuation ray paths. The time series of tomograms indicate relative tracer concentrations and tracer arrival times in the image planes; from these we infer the presence and location of a preferential flow path within a previously identified zone of transmissive fractures. These results provide new insights into solute channeling and the nature of aquifer heterogeneity at the site. INDEX TERMS: 0910 Exploration Geophysics: Data processing; 0915 Exploration Geophysics: Downhole methods; 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 1894 Hydrology: Instruments and techniques; KEYWORDS: radar tomography, fractured rock, ground-penetrating radar, geophysics, hydrogeophysics

Mapping permeability in heterogeneous aquifers using hydrologic and seismic data

A new method is presented for identification of the permeability distribution in near-surface aquifers. In addition to using the usual sparsely sampled pressure and permeability data, the method incorporates densely sampled seismic data, as obtained from a reflection or tomography survey, along with empirical relationships between seismic and hydraulic properties. The procedure is to first estimate by hydrologic inversion a pressure field. Then the velocity-permeability-pressure relationship is used to map the inverted pressure and measured seismic data to multivalued estimates of the permeability. Of those, the most probable value, based on the hydrologic inversion, is selected. In synthetic case studies the tremendous increase in coverage offered by the seismic data leads to dramatically better results in terms of both accuracy and resolution. An appealing feature is the use of relatively easy to acquire pressure data; a second is the incorporation of geophysical data which can sample an entire aquifer remotely without the need for an expensive and invasive drilling program.

High-resolution crosswell imaging of a West Texas carbonate reservoir; Part 1, Project summary and interpretation

A carbon dioxide flood pilot is being conducted in a section of Chevron’s McElroy field in Crane County, west Texas. Prior to CO2 injection, two high‐frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high‐resolution reservoir delineation and CO2 monitoring. These preinjection profiles provide the baseline for time‐lapse monitoring. Profile #1 was recorded between an injector well and an offset observation well at a nominal well‐to‐well distance of 184 ft (56 m). Profile #2 was recorded between a producing well and the observation well at a nominal distance of 600 ft (183 m). The combination of traveltime tomography and stacked CDP reflection amplitudes demonstrates how high‐frequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells: Transmission traveltime tomography is used to image the large scale velocity variations; CDP reflection imaging is then used to image smaller scale impedance heterogeneities. The...

Time-lapse inversion of crosswell radar data

The combination of differential radar tomography with conventional tracer and/or hydraulic tests facilitates high-resolution characterization of subsurface heterogeneity and enables the identification of preferential flow paths. In dynamic imaging, each tomogram is typically inverted independently, under the assumption that data sets are collected quickly relative to changes in the imaged property (e.g., attenuation or velocity); however, such “snapshot” tomograms may contain large errors if the imaged property changes significantly during data collection. Acquisition of less data over a shorter time interval could ameliorate the problem, but the resulting decrease in ray density and angular coverage could degrade model resolution. To address these problems, we propose a new sequential approach for time-lapse tomographic inversion. The method uses space-time parameterization and regularization to combine data collected at multiple times and to account for temporal variation. The inverse algorithm minimizes the sum of weighted squared residuals and a measure of solution complexity based on an a priori space-time covariance function and a spatiotemporally variable mean. We demonstrate our approach using a synthetic 2-D time-lapse (x; z; t) data set based loosely on a field experiment in which difference-attenuation radar tomography was used to monitor the migration of a saline tracer in fractured rock. We quantitatively show the benefits of space-time inversion by comparing results for snapshot and time-lapse inversion schemes. Inversion over both space and time results in superior estimation error, model resolution, and data reproduction compared to conventional snapshot inversion. Finally, we suggest strategies to improve time-lapse cross-hole inversions using ray-based inversion constraints and a modified survey design in which different sets of rays are collected in alternating time steps.

Permeability dependence of seismic amplitudes

Can permeability be determined from seismic data? This question has been around since Maurice Biot, working for Shell in the 1950s, introduced the idea that seismic waves induce fluid flow in saturated rocks due to fluid-pressure equilibration between the peaks and troughs of a compressional wave (or due to grain accelerations in the case of a shear wave). Biot (1956) established a frequency-dependent analytical relation between permeability and seismic attenuation. However, laboratory, sonic log, crosswell, VSP, and surface seismic have all demonstrated that Biots predictions often greatly underestimate the measured levels of attenuation—dramatically so for the lower-frequency measurements. Yet, if an unresolved link truly exists between seismic amplitudes and permeability, the potential benefit to the oil industry is enormous. For this reason, the Department of Energy (DOE) brought together 15 participants from industry, national laboratories, and universities to concentrate for two days on whether permeability information is conceivably contained in and retrievable from seismic data. The present article represents much of the workshop discussion (which took place 5–6 December 2001 in Berkeley, California), but is not strictly limited to it. Not all connections between hydrological and seismic properties are considered. Three-dimensional seismic images and time-lapse seismic monitoring are routinely used by reservoir engineers in constructing and constraining their reservoir model. Such imaging applications of seismic surveys to hydrological modeling are not discussed. Furthermore, in fractured reservoirs it is reasonable to postulate that any locally determined seismic anisotropy defines a symmetry class for the geologic material that must also be satisfied by the permeability tensor. Neither are such material-symmetry constraints discussed. The focus here is only on whether the permeability of the rocks through which seismic waves propagate directly influences the decay of the wave amplitudes with distance. Key to addressing this question is an up-to-date discussion of the likely attenuation …

Inferring the relation between seismic slowness and hydraulic conductivity in heterogeneous aquifers

Cross-well seismic tomography can be used to develop high-resolution seismic slowness (1/velocity) estimates along planes through aquifers. Unfortunately, the relation between seismic slowness and hydraulic conductivity is poorly understood, resulting in poor characterization of hydraulic properties from seismic data. This relation is generally developed from laboratory measurements, but slowness values measured with very high frequencies in the lab are often poorly correlated with lower frequency cross-well and surface seismic slowness values. To address this problem, we developed an approach to infer the relation between slowness and hydraulic conductivity using field scale geophysical and hydrogeologic measurements. We first develop an a priori relation between the conductivity measurements and the cross-well slowness estimates. Multiple three- dimensional slowness realizations, conditioned on the cross-well estimates, are then generated and remapped into log conductivity fields using the a priori slowness to log conductivity relation. We simulate groundwater flow and tracer transport through these conductivity fields and calculate the residuals between measured and simulated concentration arrival time quantiles and drawdown. The slope and intercept of the relation between slowness and log hydraulic conductivity and the dispersivity are then estimated for each slowness realization to minimize the sum of these squared residuals. We demonstrate this approach for the Kesterson aquifer, California, where seismic tomography provided valuable information about aquifer properties. The groundwater flow and tracer transport simulations, through the estimated conductivity fields, yield reasonable fits to the observed tracer concentration histories for two multiple-well tracer tests (one of which was not used in the inversion) and to the measured drawdown. This approach provides estimates of seismic slowness and hydraulic conductivity, and information about the relation between slowness and log conductivity for a field site.

Elastic wave equation traveltime and waveform inversion of crosswell data

A method is presented for reconstructing P‐ and S‐velocity distributions from elastic traveltimes and waveforms. The input data consist of crosswell hydrophone records generated by a piezoelectric borehole source. Borehole effects are partially accounted for by using a low‐frequency Greens function to simulate the pressure generated in the fluid‐filled receiver well. The tube waves in the borehole are ignored, on the assumption that they can be removed from the field data by median filtering. In addition, the source‐radiation pattern is partially taken into account by inverting for the equivalent stress components acting on the earth at the source location. The elastic wave equation traveltime and waveform inversion (WTW) method is applied to both synthetic crosswell data and the McElroy field crosswell data. As predicted by theory, results show that elastic WTW tomograms provide a sharper interface image than delineated in the traveltime tomograms. The spatial resolution of the McElroy traveltime tomogr...

Terrestrial Sequestration of CO2 – An Assessment of Research Needs

Publisher Summary This chapter provides a brief review of major characteristics of reservoir structures and lithologies serving as a guide to reservoir selection for CO 2 disposal. The chapter focuses on existing experience and uncertainties in reservoir characterization and response to CO 2 injection and long-term containment of sequestration sites. Special issues germane to CO 2 disposal arise in the assessment of depleted reservoirs, whose properties are known to have changed during single or repeated pore-pressure drawdown and fluid redistribution. Oil and gas reservoirs and aquifers share some common geometric elements. Generally, both are tabular bodies in which the fluid flow is constrained by upper and lower less-permeable lithologies. Primary aspects of CO 2 sequestration in geologic formations include the geohydrologic characterization, injection behavior, and long-term containment of supercritical CO 2 for storage in aquifers and reservoirs. The efficiency of a CO 2 enhanced oil-recovery flood depends strongly on the equilibrium phase behavior of mixtures of CO 2 with the oil.

Multi-component wavefield simulation in viscous extensively dilatancy anisotropic media

Abstract A finite difference method for the simulation of multi-component wavefield in viscous extensively dilatancy anisotropic (EDA) media is presented. Transformation of the stress and strain relation from frequency domain to time domain reveals that the viscous effect in EDA media is embedded into the terms of the third derivatives of the strain with respect to time. Numerical examples for viscous EDA media with dry and saturated cracks are calculated, respectively. In the calculation of the wavefields, the absorbing boundary conditions are used to suppress the artificial boundary reflection, the grid dispersion is suppressed by flux corrective transformation (FCT) technique. Snapshots and seismic records show that the existence of cracks and the material contents in the cracks exhibits significant influences on the wave propagation, especially on the radiation pattern and attenuated factor.