EAGE/SEG Research Workshop 1990

One of the most commonly used tool for obtaining stacking velocities is the velocity spectrum. An expert system for aid in the interpretation of velocity spectra on a profile is presented. The raw velocity spectra computed by conventional seismic data processing software and a simplified description of the geological and geophysical context of the survey are input data for the system. In the expert system, the reasoning of the geophysicist picking a velocity spectrum is expressed in the form of rules using the objects of the velocity spectrum (spectrum extrema) and the information about the survey context. The generating of the search space and its filtering are carried out with heuristics specific to geophysical criteria and adapted to the velocity analysis problem.

In mid-1989, Shell Canada acquired a 3D seismic survey, Ram 3D, in the foreland basin of the Canadian Rocky Mountains. The surveyed area includes steeply dipping layers from surface outcrop to Precambrian basement. The azimuths of these layers vary greatly throughout.

For some time the seismic processing industry has possessed the algorithmic capability to accurately migrate seismic data from areas of structural complexity. What has stopped this ability being used in practice is the fact that the required depth migrations are themselves very sensitive to the supplied velocity depth model.

In this paper a macro model estimation technique is presented based on non-recursive wave field extrapolation. By shot record redatuming, using an initial macro model, true Common Depth Point (CDP-) gathers are generated at grid points along one or more lateral positions (such as potential boreholes).

Time domain processing has been extensively developed during the eighties to the stage that, with DMO, good quality images can be formed for most situations. However, in structurally complex areas, converting the time image to a correct depth image has been a serious problem. Recently, prestack depth migration has attracted more and more attention because of its capability of placing the image at the correct location, laterally and vertically.

There are basically two approaches to the dipping layers problem: the combination of normal move-out (NMO) and dip move-out (DMO) followed by post-stack migration and depth conversion, and full pre-stack depth migration.

Several methods for dip moveout (DMO) and prestack Imaging (PSI) have reduced the operations to invariant convolutlons by resampling the data.

Summary not available

In order to obtain from seismic data a good image of complex geologic structures we must meet several demands. First among these is a correct velocity model whose detail is related to the complexity of the structures and which is geologically plausible. In order to limit our search for the velocity model to part of the model space we should incorporate in our model construction a priori geophysical and geological constraints on the model. My presentation deals with these limits on the velocity model.

A velocity analysis process, SIVA, is described which combines the information in CMP gathered trace data with interpreted seismic data (eg, time maps) to create a model of structure and P wave interval velocity.

The accurate determination of seismic velocities requires a new approach to velocity analysis. In many areas of geological interest the assumptions underlying conventional velocity analysis fail; the raypaths cannot be taken as being straight, and lateral variation of the velocity structure becomes significant. Seismic reflection datasets collected in such areas often feature moveout with offset curves which are not hyperbolae. This leads to ambiguity in the determined stacking velocities and hence in the derived interval velocities.

In most seismic applications, the traveltimes of the primary reflections constitute the main information about the subsurface. The traveltimes may be picked and used directly. or the parameters in a traveltime approximation may be estimated from the seismic data. For a given geological model the traveltimes may be computed by raytracing methods. Traveltime inversion consists of computing the geological structure and the velocity functions from the observed traveltimes.

The retrieval of 2- and 3-D seismic velocity fields from surface, or borehole observations may be formulated as a non-linear inverse problem.

The anomalous spatial oscillations of stacking velocity due to local inhomogeneities have been studied over the last decade (e.g. Rocca and Toldi, 1982; Loinger, 1983), but only recently have they been exploited as a quantitative tool for measuring the variations of seismic wave propagation velocity in the Earth. Harlan (1989) introduced an algorithm for tomographic inversion of stacking velocity anomalies using reflected rays, which is tested in this paper for information on its advantages and lirnits.

Summary not available

In this study, it is demonstrated that sonic pulses can be emitted from a borehole energy source concurrently while drilling oil and gas welIs, and that these pulses are of sufficient strength and suitable frequencies to be very useful.

Data acquired using Walkaway VSP techniques are used to image subsurface structure. The migration techniques involved require some prior knowledge of the structure and velocity distribution to generate a background model. Although VSP surveys provide accurate interval velocities along the borehole we cannot always assume to have a good estimate of layer velocities away from the well, even in relatively smoothly varying structure. Velocity inaccuracies can arise in the presence of transverse isotropy, laterally varying velocities or incorrect estimates of overburden dip. Velocity perturbations are applied to a background model in order to examine the sensitivity of the imaging of a fault structure. Using a case study we show that inaccurate estimates of velocities produce significant time shifts but do not have much effect on the lateral positioning of a fault edge.

In the following case study, cross-well seismic tomography was used to image both structural and stratigraphic features in Gulf Coast Miocene sediments. A piezoelectric downhole source was used to produce seismic waves which were recorded by hydrophones between wells 250 apart. Traveltime tomography was then applied to a set of over 5000 picks. In addition to the targeted fault delineation, the tomography successfully imaged a number of porous sandstone layers previously identified from type logs for the Miocene. Two different methods of traveltime picking and tomographic inversion were used. The two resulting velocity tomograms show similar velocity variations that lead to a consistent geological interpretation. Borehole gravity meter (BHGM) data, obtained in both wells following the tomographic survey, were used to associate densityvariations with the velocity features imaged by the tomography. The lD BHGM-derived low density zones were found to coincide with the 2D seismically imaged low velocity zones, thus supporting the identification of porous sandstone layers.

A useful method of estimating the velocities of geological structures between boreholes is traveltime tomography. Measurements of the traveltimes along different paths can be used to form an image of the velocity distribution. This approach is valid when seismic velocities are the same regardless of the direction in which waves propagate. Thus, the target media is required to be isotropic for the method to work correctly. However, in many environments, velocities to depend on the direction of wavepropagation. This effect, known as seismic anisotropy, can lead to distortions in tomographic images. The distortions may be serious enough to cause the images to be misinterpreted.

Shear-wave velocity logs are useful for a variety of seismic interpretation applications, including bright spot analyses, amplitude-versus-offset analyses and multicomponent seismic interpretations. Measured shear wave velocity logs are, however, often unavailable.

Laboratory measurements of seismic velocities play an important role in understanding seismic wave propagations in reservoir rocks. These seismic velocities are widely used in seismic data processing, correlations of seismic stratigraphy, seismic and weIl logging interpretations, lithology identification, and most recently in reservoir description and surveillance. In order to make fuIl use of the seismic velocity data, we must understand relationships between seismic velocity and a variety of reservoir and physical parameters.

Velocity and attenuation of seismic waves are of great potential use in deducing lithological parameters from seismic data if we understand the physical laws governing dissipation of seismic energy and the accompanying velocity dispersion.

Using laboratory measurements, we investigate the relationship between static and dynamic bulk moduli in 43 tight gas sandstones with porosity ranging from 2 to 14 percent and clay content ranging from 0 to 66 percent. Dynamic moduli are computed using compressional and shear velocities measured on the sandstones in air-dry condition.

Sediments at the sea bottom of ten contain gas - normally methan, nitrogen or carbon dioxide (see f.i. Wheeler, 1986, Boden 1987). The presence of gas in a sediment affects its mechanical properties, and may thus have an impact on the stability of oil platforms resting on the sea bottom. Also, pockets of gas may cause dangerous situations during oil drilling, especially if the driller is not prepared for it. Seismic data often represent the only available information from the sediment prior to planning of platform sites, and drilling. Thus, it is important to be able to decide from the seismic data - with largest possible certainty - whether a sediment contains gas or not.

Knowledge of perrneability of in situ rocks is of great importance in studies of nuclear waste disposal, underground water movement, petroleum and geothermal energy exploitation from reservoirs and in civil engineering applications such as dam constructions. Great difficulties have been experienced in predicting permeability of in situ rocks because of the limited information on discontinuities of all scales and of the pores of rocks, which together contribute to the permeability.

Shear wave investigations of the sea floor in combination with conventional reflection seismic measurements yield information on the subsurface structures as well as on the elastic muduli of the sediments.

Common practice is to perform steep-dip time migration followed by an image-ray correction. This paper examines the image-ray correction and finds it lacking. In particular, this correction is valid only for the small-dip portion of the migrated image.

Summary not available

Interval velocities have been derived from a dense grid of non-exclusive seismic data in the Southern Gas Basin of the North Sea. The analysis of around 8000 velocity spectra has enabled detailed interval velocity maps to be drawn down to Top Zechstein level. A comparison of the seismically derived velocities with known well-derived velocities indicates that in most circumstances the "Dix" interval velocities are a good approximation to the well velocities - particularly for the Tertiary and Upper Cretaceous Chalk intervals. Consequently, the interval velocity maps so derived are useful for the purposes of depth conversion. In addition , these maps reveal interesting trends and anomalies many of which can be interpreted as having distinct geological causes. This poster paper concentrates on the interpretation of velocity anomalies in the Upper Cretaceous Chalk.

Large sums of money and lots of time is put into seismic acquisition, processing and interpretation to obtain the best possible results. From these seismic interpretations, with the use of sophisticated gridding contouring and mapping software, finally depth and isopachmaps are produced on which basis expensive drilling deelslons will be taken.

Seismic exploration within the oil industry has three main objectives: i) Identify targets for future petroleum production. ii) Give a geometric description of reservoirs in order to obtain reliable estimates of hydrocarbon volumes in place. iii) Contribute to a physical characterization of sedimentary bodies with respect to lithology, porosity, pore fluid, pore pressure, etc.

The vector analysis of shear-waves provides an opportunity to improve upon existing seismic methods for imaging the subsurface, to include a detailed investigation of the internal structure of reservoir rock and patterns of flow behaviour.

Commencing with Mayne's (1962) benchmark paper on common midpoint stacking (CMP), conventional stacking velocity analysis evolved rapidly as did realization of the problems inherent in the derived interval velocities (see reviews by Robinson, 1983; Cordier, 1985).

Velocity dispersions are calculated in several rocks saturated with water, normal decane (n-decane), and a heavy oil (in petroleum engineering, heavy oil is referred as crude oil with viscosity higher than 0.5 Pa.s), respectively.