Analyses of sonic logs in a horizontal well provide new information about mechanical properties of rocks, made possible by recent developments in our understanding of acoustic wave propagation in prestressed formations. Most sections of this horizontal well exhibit azimuthal shear isotropy, indicating isotropic stresses in the plane perpendicular to the well trajectory, leading to stable wellbore conditions. However, two sections show dipole dispersion crossovers that confirm the presence of stress‐induced shear anisotropy caused by a difference between the maximum and minimum stresses in the plane perpendicular to the well trajectory. The two dipole dispersions are obtained by processing the recorded waveforms by a modified matrix pencil algorithm. The fast‐shear direction is estimated from Alford rotation of the cross‐dipole waveforms. One section of the well exhibits the fast‐shear direction parallel to the overburden stress as the maximum stress direction, whereas the other section has the fast‐shear direction parallel to the horizontal stress that is larger than the overburden stress. The cause of this change in the fast‐shear direction is believed to be the well’s penetration into a 3-ft-thick bed with lower porosity and permeability and significantly higher elastic stiffnesses than those in the other part of the homogeneous, high‐permeability reservoir. A stiff bed is likely to have greater stresses in its plane than perpendicular to it, which would make the horizontal stresses greater than the vertical.

Vertical seismic profiles are usually acquired by deploying downhole seismic sensors below a wireline logging cable. A seismic source is triggered at surface while recording the downhole vibration via the wireline cable. In this paper, an alternative approach based on distributed vibration measurement is tested using wireline deployment for the first time. Local axial strain of a multi‐kilometre fibre optic line is measured at intervals of approximately 1 m and processed to a spatial resolution of 10 m with sub nanometer strain resolution by an optical interrogation device (distributed vibration/acoustic sensing). When deployed in a well, the optical fibre line should be mechanically coupled to the borehole wall to generate valid seismic records. A conventional vertical seismic profile was acquired with three‐component sensors in a vertical well near Bottesford, UK. The impulsive seismic source was a novel portable airgun tank. Clear seismic reflections are observed within and below the borehole, in agreement with surface seismic data. A single shot generated equivalent data with an experimental optical wireline logging cable and an adequate optical interrogator at the surface. The main difference between the two records is the presence of a strong tube wave in the optical profile, which can be easily removed with conventional velocity filter processing. Corridor stacks from both conventional and optical profiles match each other and provide a reasonable tie to a nearby surface seismic line.

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A B S T R A C TWe present the analysis of a multi-azimuth vertical seismic profiling data set that has been acquired in a tight gas field with the objective of characterizing fracture distributions using seismic anisotropy. We investigate different measurements of anisotropy, which are shear-wave splitting, P-wave traveltime anisotropy and azimuthal amplitude variation with offset. We find that for our field case shear-wave splitting is the most robust measure of azimuthal anisotropy, which is clearly observed over two distinct intervals in the target. We compare the results of the vertical seismic profiling analysis with other borehole data from the same well. Cross-dipole sonic and Formation MicroImager data from the reservoir section suggest that no open fractures intersect the well or are present within half a metre of the borehole wall. Furthermore, a detailed dispersion analysis of the sonic scanner data provides no indication of stress-induced seismic anisotropy along the logged borehole section. We therefore explain the azimuthal anisotropy measured in the vertical seismic profiling data with a model that contains discrete fracture corridors, which do not intersect the well itself but lie within the vertical seismic profiling investigation radius. We show that such a model can reproduce some basic characteristics of azimuthal anisotropy observed in the vertical seismic profiling data. The model is also consistent with well test data that suggest the presence of a fracture corridor away from the well. With this study we demonstrate the necessity of integrating different data types that investigate different scales of rock volume and can provide complementary information for understanding the characteristics of fracture networks in the subsurface.

A number of electromagnetic instruments with fixed frequency transmitterreceiver has been developed by Centre de Recherches Geophysiques (Garchy, France) in order to measure magnetic susceptibility and electrical resistivity of ground. A dipole-dipole induction tool (SH3) was built for some meter penetration from the surface (spacing=1.5m, frequency=8kHz) and has allowed electrical resistivity and magnetic susceptibility mapping to be applied to archaeological exploration. A "ROMULUS" probe (spacing 0.85m, frequency=4kHz) has followed as a borehole version (diameter 0.04m) for mineral exploration by Bureau de Recherches Geologiques et Minieres (Orleans, France). Another borehole instrument called "ERIC" was then designed on the same principle (frequency=1kHz) but with variable transmitter-receiver spacing (5m, 10 m, 20m) for ten meter scale investigation of conductive bodies around the hole. The method, its field of application and design of equipments is described. Several examples of shallow investigation surface maps and borehole induction logs are presented.