The 7th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2003)

The extended Born or localized nonlinear (LN) approximation of integral equation (IE) solutions has been applied to inverting crosshole EM data using a cylindrically symmetric model. The LN approximation is less accurate than a full solution but much superior to the simple Born approximation. Moreover, when applied to the cylindrically symmetric model with a vertical magnetic dipole source, the LN approximation works well because electric fields are scalar and continuous everywhere. One of the most important steps in the inversion is the selection of a proper regularization parameter for stability. The LN solution provides an efficient means for selecting an optimum regularization parameter, because Green’s functions, the most time consuming part in IE methods, are repeatedly re-usable throughout the selection process. In addition, the IE formulation readily contains a sensitivity matrix, which can be revised at each iteration at little expense. This fast inversion scheme has been tested on its stability and efficiency using synthetic and field data.

Naturally fractured reservoirs have attracted increasing interest in exploration and production geophysics in recent years. The presence of fractures can affect the AVO response of a reservoir. In this paper we analyse this effect of fractures in a porous reservoir on seismic data, and estimate the errors in the estimation of reservoir properties due to ignoring the presence of these fractures. The effect of fractures on elastic properties of a porous rock is studied using the model of fractures as linear-slip interfaces in an isotropic porous background. The effect of fluid on the elastic properties of this medium is modelled using equations of anisotropic poroelasticity. This yields explicit analytical expressions for the elastic properties of the fractured porous medium saturated with a given fluid. We use this theory to model the seismic response of a reservoir with vertical fractures. We then invert this response for reservoir properties (porosity, net-to-gross) using a workflow based on the above theory, and compare the predictions with the results of the standard industry workflow based on the isotropic Gassmann equation. The comparison yields an estimate of the potential error in the prediction of rock properties using an isotropic workflow for a fractured reservoir. These errors can be as large as±3% in predicted porosity, and ±40% in predicted netto-gross.

Time lapse seismic data provides an ability to track the progress of migrating CO2 during the injection process. When supercritical CO2 is injected into a formation, the formation's structure, seal, reservoir mineralisation, temperature and pressure dictates the fluid's migration pattern, and its state-of-phase. Understanding the basic rock physics and interaction of CO2 with the associated PT relationships, mineralisation and seal status is a prerequisite for seismic monitoring, geophysically tracking the CO2 as it moves. This paper uses the conditions of Sleipner in the North Sea. As with any chemical mix, the rock matrix may be change state as a function of the presence of CO2. The result is a dynamic process requiring a seismic methodology for tracking CO2 migration within any reservoir. Consequently while 3-D seismic is widely regarded as the preferred but expensive tool, other tools are emerging which may be more cost effective, but a knowledge of reservoir conditions is still a prerequisite to determining the optimum tool.

The Permanent Scatterers technique uses data from satellite mounted Synthetic　Aperture Radars like ERS1, ERS2, ENVISAT and Radarsat to extract the location of　scatterers that have strong and stable signature so that they will not be corrupted by　clutter. Then, it is possible to determine the spatial position of the PS with a sub - metric precision and their progressive motion (one measurement per month, approximately) with sub - millimetric precision. The spatial density of the PS decreases from several hundred /km2 in towns to just a few in agricultural environment. However, a metal pipe can be sufficient to make a PS, and detect an illegal well from the seasonal subsidence. If the PS density is high enough, an estimate is possible of the Atmospheric Phase Screen that rotates the radar signals, initiating the bootstrap process that finally yields relative motions and locations. The minimum length of the data series necessary to estimate PS is about 10 - 15 takes. Applications go from building stability analyses and collapse predictions, subsidence analysis, pre seismic motion analysis, landslides studies. More than 2800 images have been processed in more than 60 sites around the world. Data from Tokyo and the Isu province in Japan will be shown.

Shallow refraction seismic technique was applied to investigate piece-wise lateral inhomogeneities and buried sand bodies within a sequence of sedimentary succession. Traveltime parameters, include layer reciprocal time, principle of parallelism, apparent refractor velocities and the intercept time have been applied to identify the lateral inhomogeneities. In-line reversed profiling technique is essential for the required data. Synthetic data and different field examples have been applied to illustrate this algorithm.

Three pairs of diamond drillholes were surveyed at Voisey’ Bay, Labrador, Canada, in the Eastern Deeps deposit. The surveys were carried out in NQ diameter drillholes, which were approximately 800-850 m in length. The holes were nearly vertical and hole-to-hole distances ranged from 45 m to 80 m. The p-wave velocity tomography surveys were designed to outline the morphology and continuity of the massive sulphide zone. Reflection imaging was also applied; to confirm whether this method could be used in conjunction with transmission tomography to expand the panel images to a volume. Confidence in the continuity of the massive sulphide zone is crucial since the relatively small massive zone contains a large portion of the metal in the Eastern Deeps deposit. Based on the results of these surveys, further sulphide delineation using surface exploration drillholes may be possible before underground exploration and development.

Field Magnetic susceptibility measurement has been carried out for various soil/sediment in northeastern Tunisia, Mediterranean coast of North Africa, where industrial/urban zones have been rapidly expanded in late 20th century. The variation of magnetic susceptibility is compared with the environmental geochemical data, the concentration of potentially toxic elements (PTEs) possibly derived from anthropogenic activities. There is a correlation between the magnetic susceptibility and the concentration of PTEs, As, Cd, Co, Cr, Cu, Ni, Pb, and Zn, where remarkable anomaly could be observed in and around Tunis industrial/urban zone. The result of principal component analysis (PCA) also showed that some of elements, As, Cu, P, and Se, significantly correspond to magnetic susceptibility value. It suggests us the magnetic susceptibility measurement is applicable for a low-cost and quick field screening of soil/sediment contamination by PTEs.

In Taiwan area, the instrumental observation of earthquakes has been initiated since 1898, and up to 2000, about 180,000 earthquakes have been accumulated in the synthetic catalog organized by the Central Weather Bureau of Taiwan (abbr. CWB). In this study, we aim to have a systematical comprehension from preliminary statistical analysis of seismicity in terms of Taiwan’s seismotectonics. We firstly applied a completeness test for the CWB seismicity catalog for determining the reliable records which can be considered to correspond with Taiwan’s tectonic activity. Afterward, b-value and z-value have been realized after such reliable records. The b-value analysis has extracted the relationship between the seismic behaviors and tectonic setting. The application of the z-test demonstrated a series of rate changes before and after the Chi-Chi earthquake.

3-D seismic survey is recognized as a very powerful exploration tool and the usage is widely made to obtain clearer subsurface images. However, there is the case in which the acquisition of 3-D seismic data is not allowed economically or in exploration strategy. In such a case only existing 2-D data give the subsurface information. However, the interpretation of multi-line 2-D seismic data is always ambiguous under the complex geological structure. The main reason is that the sideswipe reflections are not separated on single 2-D seismic data. The only way to improve the interpretation accuracy is that the sideswipe energy contained in multi-line 2-D data set is located in the correct reflection position and shapes the reflector image by means of 3-D pre-stack depth migration (PSDM). Our final goal of the study is to obtain a 3-D subsurface image from multi-line 2-D seismic data by means of 3-D PSDM. The critical issue of the approach is that the subsurface spatial sampling is too sparse to obtain a meaningful PSDM image. Therefore, the elaborated interpretation technique is necessary to compensate the lack of spatial sampling. In this paper we will discuss an interpolation technique to produce a pseudo 3-D data set from sparse multi-line 2-D data using t-x-y domain operator.