摘要:

AbstractMany conventional schemes of automated residual statics estimate time lags between prestack traces (sorted as CDP gathers) and a model section, and transform the collected lags into surface‐consistent residuals. The method discussed in this paper aims at improving the lag estimator. ‘Externally generated’ reference traces are avoided and a principle of localized stack optimization is introduced whereby application of a multichannel filter to the stack and evaluation of the normalized power of every filtered trace gives a measure of the stack quality. One may consider the power as a function of all variable (inconsistent) shifts applied to the prestack traces. To obtain a set of optimal lag estimates the power function must be maximized. This power function is complex and the number of its variables prohibits a straightforward search for the maximum. Thus an iterative method must be employed, and steepest descent schemes have proved the most satisfactory. In the actual calculation, the repeated evaluation of the objective can be replaced by the computation of certain cross‐correlations. At the last iteration (after five to ten coordinate sweeps), the global behaviour of this correlation gives some indications of how well a prestack trace is adapted to the filtered stack. This information is used to compute a weighting factor to be stored with the lag estimate. At this stage simple statistical procedures are run to eliminate the most unlikely estimates. The remaining ones are transformed to surface‐consistent residuals by means of a weighted least‐squares inversion according to a model which takes into account the fact that the lags have no fixed reference datum.The efficiency of the method is demonstrated by a field data example into which synthetic anomalies were introduced, and the effect of the new process is compared with that of a ‘classical’ production program using field data with genuine

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01864.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractConversion of borehole Stoneley waves to channel waves was observed in data from a seismic cross‐borehole experiment conducted between wellbores penetrating a thin coal layer at 2022 m depth, near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves underwent partial conversion to channel waves at the coal layer. The channel waves were detected directly in an adjacent borehole 35 m away at receiver positions within the coal. Stoneley waves, subsequently produced by partial conversion of channel waves, were also detected at receiver positions located up to 50 m above and below the coal layer in the adjacent borehole. We infer the channel wave to be the first‐higher Rayleigh mode by comparing the observed group velocity with theoretically derived dispersion curves. Identifying the conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing a transmitter or receiver in the coal its

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01865.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractUbiquitous splitting of seismic shear‐waves indicates that most rocks in the upper half of the crust are pervaded by stress‐aligned fluid‐filled inclusions, called EDA‐cracks. These inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures. At depths where the overburden stress is sufficiently large (typically below a few hundred metres), this minimum stress is usually horizontal, so that the EDA‐cracks and hydraulic fractures are typically aligned vertically, striking parallel, or subparallel, to the direction of maximum compression. This is confirmed by the polarizations of the split shear‐waves along raypaths at depth in the crust. At the free surface, however, the vertical stress is zero (or approximately zero) and cracks (and hydraulic fractures) at shallow depths in intact rock tend to be horizontal. Thus, the directions of minimum stress, and the orientations of hydraulic fractures, are likely to swing through 90° near the surface of the Earth. Since the behaviour of cracks and stress is often crucial to drilling operations, the rotation of the crack‐ and stress‐geometry near‐surface has important implications, particularly for optimizing hydrocarbon production and geothermal reservoir management. Consequently, evidence gained from experiments, for example in hot‐dry‐rock geothermal heat extraction, in inappropriate crack geometries at shallow depths, may not be valid when applied to other crack‐ and stress‐geome

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01866.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractInversion of multicomponent seismic data can be subdivided in three main processes: (1) Surface‐related preprocessing (decomposition of the multicomponent data into ‘primary’ P‐and S‐wave responses). (2) Prestack migration of the primary P‐ and S‐wave responses, yielding the (angle‐dependent) P‐P, P‐S, S‐P and S‐S reflectivity of the subsurface. (3) Target‐related post‐processing (transformation of the reflectivity into the rock and pore parameters in the target). This paper deals with the theoretical aspects of surface‐related preprocessing.In a multicomponent seismic data set the P‐ and S‐wave responses of the subsurface are distorted by two main causes: (1) The seismic vibrators always radiate a mixture of P‐ and S‐waves into the subsurface. Similarly, the geophones always measure a mixture of P‐ and S‐waves. (2) The free surface reflects any upgoing wave fully back into the subsurface. This gives rise to strong multiple reflections, including conversions.Therefore, surface‐related preprocessing consists of two steps: (1)Decomposition of the multicomponent data (pseudo P‐ and S‐wave responses) into true P‐ and S‐wave responses. In practice this procedure involves (a) decomposition per common shot record of the particle velocity vector into scalar upgoing P‐ and S‐waves, followed by (b) decomposition per common receiver record of the traction vector into scalar downgoing P‐ and S‐waves. (2) Elimination of the surface‐related multiple reflections and conversions. In this procedure the free surface is replaced by a reflection‐free surface. The effect is that we obtain ‘primary’ P‐and S‐wave responses, that contain internal multiples only.An interesting aspect of the procedure is that no knowledge of the subsurface is required. In fact, the subsurface may have any degree of complexity. Both the decomposition step and the multiple elimination step are fully determined by the medium parameters at the free surface only. After surface‐related preprocessing, the scalar P‐ and S‐wave r

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01867.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA mathematical formulation for the electric potential from point current‐sources coaxial with a metal casing has been obtained. The excitation caused by the axial point‐sources will produce currents in the pipe. By assuming that the pipe can be divided into many cylindrical ring segments with constant axially‐directed current, the solution of the fields inside and outside the pipe can be formulated in an integral form. The integral equation applied to the segmented pipe yields a set of simultaneous linear equations which are solved for the currents in the pipe; these are then used to calculate the potentials anywhere outside the pipe in the medium.This solution has been used to study the distribution of the potentials in a half‐space for a single current‐source at and beyond the bottom of a finite length of casing. For a casing 0.1 m in radius and 0.006 m in wall thickness with a conductivity of 106S/m, in a half‐space of 10‐2S/m, it was found that only in a region very near the pipe does the pipe exert substantial influence on the fields of a point‐source 100 casing diameters beyond the end of the pipe. It appears that cross‐hole resistivity surveys can be implemented without corrections for the casing if the source is located at least 50–100 casing diameters beyond the end of the casing. Hole‐to‐surface surveys are much more affected by the pipe. For a pipe‐source separation of 100 casing diameters, the surface measurements must not be closer than a half pipe length for a 5%

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01868.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY