摘要:

ABSTRaCTMultiple coverage reflection seismic data provide an important source of information concerning the subsurface. However, due to the stacking and migration techniques used in the processing, the first arrivals are muted and details about the upper part of the sections are generally lost.This paper describes a computerized method for the inverse modelling of laterally varying velocities and shallow depths which are not sufficiently resolved in the reflection seismic processing. The method minimizes, in a least‐squares manner, the difference between the observed first arrivals, picked from the reflection traces, and a set of synthetic traveltimes, calculated by ray tracing in a cell model. An initial model, e.g. froma prioriknowledge or the application of a conventional interpretation method, is refined iteratively until no further essential improvement can be achieved. Traditional first‐arrival inversion methods cannot, in general, provide such flexible modelling. The technique is successfully tested on synthetic data as well as on first arrivals picked automatically from the records of a reflection seismic survey in North Jutland, Denm

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02217.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractStatic corrections for reflection seismic data shot on high velocity crystalline rock can be calculated directly from the data using a global, iterative stackpower optimization. The reflections consist of several peaks, creating the possibility of aligning the wrong peak in the signals. Due to misalignments a large number of local maxima in the stackpower exist in a cyclic manner, only slightly smaller than thes stackpower of the‘best’stack. Therefore the search must be of a global nature. A Monte Carlo search requires long run times. A global search method is presented using a varying sequence of parameters within each iteration. The ability for a set of static corrections and the associated stackpower to move from a local maximum to a larger maximum is enhanced. The performance of the iterative process is improved, so a relatively small number (about 20) of iterations is needed to obtain the optimum set of corrections. The risk of misalignment of traces by using the wrong peak in a signal consisting of several peaks is diminished, and it is less important that the initial set of corrections is close to the final set. The method is illustrated by a synthetic example and on a data set shot on the granites of Dalarna, Swe

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02218.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractWe analyse the stacking process within the framework of imaging techniques. Our results show how the NMO stretch, traditionally looked upon as giving a negative contribution, can be utilized to improve the vertical resolution of the stacked data from a source deficient in low frequencies. The added bandwidth is provided by the spatial coherency of the energy emitted by a point source.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02219.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTIn order to make 3D prestack depth migration feasible on modern computers it is necessary to use a target‐oriented migration scheme. By limiting the output of the migration to a specific depth interval (target zone), the efficiency of the scheme is improved considerably. The first step in such a target‐oriented approach is redatuming of the shot records at the surface to the upper boundary of the target zone. For this purpose, efficient non‐recursive wavefield extrapolation operators should be generated. We propose a ray tracing method or the Gaussian beam method. With both methods operators can be efficiently generated for any irregular shooting geometry at the surface. As expected, the amplitude behaviour of the Gaussian beam method is better than that of the ray tracing based operators.The redatuming algorithm is performed per shot record, which makes the data handling very efficient. From the shot records at the surface‘genuine zero‐offset data’are generated at the upper boundary of the target zone. Particularly in situations with a complicated overburden, the quality of target‐oriented zero‐offset data is much better than can be reached with a CMP stacking method at the surface. The target‐oriented zero‐offset data can be used as input to a full 3D zero‐offset depth migration scheme, in order to obtain a depth secti

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02220.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTGround‐penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions. The need to better understanding overburden conditions for activities such as geochemical sampling, geotechnical investigations, and placer exploration, as well as the factors controlling groundwater flow, has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible.The areas of application for ground‐penetrating radar are diverse. The method has been used successfully to map ice thickness, water depth in lakes, bedrock depth, soil stratigraphy, and water table depth. It is also used to delineate rock fabric, detect voids and identify karst features. The effective application of the radar for the high‐resolution definition of soil stratigraphy and fractures in bedrock is highlighted.The basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories. One example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m. Two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine. Another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination. The corroboration of the radar results by borehole investigations demonstrates the power and utility of the high‐resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes. With the advent of new instrumentation and field procedures, the routine application of the radar method is becoming economically viable and the method will see expanded use in the

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02221.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low‐level, low‐frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three‐dimensional bodies. We determine the characteristic anomalies associated with square, plate‐like conductors, which are excellent models of many base metal mineral deposits.The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half‐plane. The dipole and half‐plane models are used to bracket the behaviour of plates of finite size.The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations.The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined.The ability of the integral equation method to model plate‐like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the obs

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02222.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02223.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY