摘要:

AbstractGeophysical surveys and chemical analyses on cores were carried out in three Ontario peatlands, from which we have gained a better understanding of the peat properties that control the geophysical responses. The electrical conductivity depends linearly on the concentration of total dissolved solids in the peat pore waters and the pore waters in turn bear the ionic signatures of the underlying mineral sediments. The ionic concentration, and thus the electrical conductivity, increase linearly from the surface to basement. The average bulk electrical conductivity of peatlands at Ellice Marsh, near Stratford, and at Wally Creek Area Forest Drainage Project, near Cochrane, are of the order of 25 mS/m. The Mer Bleue peatland, near Ottawa, has extremely high electrical conductivity, reaching levels of up to 380 mS/m near the base of the peat. The Mer Bleue peatland water has correspondingly high values of total dissolved solids, which originate from the underlying Champlain Sea glaciomarine clays. The dielectric permittivity in peats is largely controlled by the bulk water content. Ground penetrating radar can detect changes in water content greater than 3%, occurring within a depth interval less than 15 cm. The principal peatland interfaces detected are the near‐surface aerobic to anaerobic transition and the peat to mineral basement contact. The potential for the successful detection of the basement contact using the radar can be predicted using the radar instrument specifications, estimates of the peatland depth, and either the bulk peat or the peat pore water electrical conductivities. Predicted depths of penetration of up to 10 m for Ellice Marsh and Wally Creek exceed the observed depths of 1 to 2 m. At Mer Bleue, on the other hand, we observe that, as predicted, a 100 MHz signal will penetrate to the base of a 2 m thick peat but a 200 MHz signal will no

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00205.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractWe present a layer‐stripping method of migration for irregularly layered media in which first‐order velocity discontinuities separate regions of constant or smoothly varying velocity. We use the reverse‐time method to migrate seismic data layer by layer, from the surface downwards. As part of the migration of a given layer, the bottom boundary of the layer is defined based on power in the migrated signal, and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated for each layer, with the final image formed from the individual layer images. Layer‐stripping migration consists of three steps: (1) layer definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme when used with reverse‐time extrapolation is similar to datuming with an imaging condition. The reverse‐time method uses an explicit fourth‐order time, tenth‐order space, finite‐difference approximation to the scalar wave equation.The advantages of layer‐stripping reverse‐time migration are: (1) it preserves the benefits of the reverse‐time method by handling strong velocity contrasts between layers and steeply dipping structures; (2) it reduces computer memory and saves computation time in high‐velocity layers, and (3) it allows interpretational control of the image.Post‐stack layer‐stripping reverse‐time migration is illustrated with a synthetic CMP data example. Prestack migration is illustrated with a synthetic data set and with a marine seismic reflection profile across the Santa Maria Basin and the Hos

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00206.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe investigation of water in salt‐rock formations is of particular relevance to underground nuclear waste repositories. In the Asse salt‐mine (Germany) a study into the relationship ofin situresistivity to water content has been made. Measurements were carried out in older rock‐salt using an electrode array in boreholes, an electrode profile in a drift and small resistivity sensors in and around a drift seal. Further measurements were made on moist zones in a contact area of younger rock‐salt and carnallitite and also in older rock‐salt with anhydrite bands using electrode profiles in the drifts. The resistivities range from 102Ωm to 106Ωm. Corresponding probes have water contents from 0.01% to 1.3%. A definite relationship between resistivity and water content is revealed which can be described by Archie's law using a cementation factormof 1.9. Porosities are between 0.08% and 1.4% and the saturations vary considerably. An explicit influence of saturation on resistivity cannot be discovered using the present data. The results enable us to estimate thein situwater content and the order of thein situporosity using resistivity surveys at different scales. This increases significantly the safety of a nuclear repo

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00207.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractMinimization of seismic residuals does not guarantee uniqueness of the model, and this implies ambiguities in the inversion. Amplitude vs. offset (AVO) inversion does not lead to a unique solution of single elastic interface parameters unless converted and S‐wave or critical angle reflections are available. Given the ambiguity of AVO inversion, this paper discusses the interaction between AVO and velocity estimation. The number of independent parameters necessary to describe an isolated reflection with AVO behaviour and residual velocity error is determined. Statistical analysis allows the establishment of an approximate equivalence of the effects of AVO and slight velocity variations; this equivalence cannot be solved without geologicala prioriinformation (kinematic equivalence). The data are then decomposed into compound events (i.e. sequences ofNinterfaces that follow each other at a fixed time lag). The decomposition is obtained by extrapolating the results of the analysis from narrowband to wideband data. Compound events decomposition demonstrates that AVO inversion is ambiguous, not only in the physical parameter space (P‐ and S‐wave velocities, and density) but also kinematically. As an example of compound event decomposition, a medium is derived. This medium is geologically implausible but is kinematically equiv

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00208.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractKnowledge of the declination and inclination of the total and induced magnetization vectors is normally required for the interpretation and analysis of magnetic anomalies. A new method of estimating the direction of the total magnetization vector of magnetized rocks from magnetic anomalies is proposed. The unknown declination and inclination (D*TandI*T) can be found by applying a reduction‐to‐the‐pole operator to the measured anomalies for different couples of total magnetization direction parameters (DTandIT) and by observing the variation of the anomaly minimum as a function of bothDTandI*T.andD*Tare estimated using the maximum of this function. Comparing our method to previous methods, one advantage is that our estimates are not zero‐level dependent; furthermore, the method allows inclinations to be well estimated, with the same accuracy as declinations; finally declinations are not underestimated. Our method is applied to a real case and meaningful results are obtained; it is shown that the feasibility of the method is improved by removing the low‐frequency c

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00209.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00210.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY