Jakob Haldorsen

Multichannel Wiener deconvolution of vertical seismic profiles

We describe a technique for performing optimal, least‐squares deconvolution of vertical seismic profile (VSP) data. The method is a two‐step process that involves (1) estimating the source signature and (2) applying a least‐squares optimum deconvolution operator that minimizes the noise not coherent with the source signature estimate. The optimum inverse problem, formulated in the frequency domain, gives as a solution an operator that can be interpreted as a simple inverse to the estimated aligned signature multiplied by semblance across the array. An application to a zero‐offset VSP acquired with a dynamite source shows the effectiveness of the operator in attaining the two conflicting goals of adaptively spiking the effective source signature and minimizing the noise. Signature design for seismic surveys could benefit from observing that the optimum deconvolution operator gives a flat signal spectrum if and only if the seismic source has the same amplitude spectrum as the noise.

Borehole Acoustic Reflection Survey for High Resolution Imaging

Summary Borehole Acoustic Reflection Survey (BARS) data were acquired in the Brent formation in the Norwegian North Sea in an exploration well for Hydro. In total, an interval of approximately 300 m was logged. Initial near-wellbore formation images were obtained within 24 hours. The quality of the sonic waveform data was very good. However, significant uncertainties in the actual tool depth caused by rough weather conditions during the time of logging, affected to some extent the coherence and the range of the formation images. The high-resolution image shows an interface dipping at 5° and clearly visible for at least 45 ft away from the well bore. The dip of the interface is in agreement with the expected local geology at the well location. The highresolution event can be correlated to a 1 m thin coal bed intersecting the borehole and indicated by the petrophysical logs. The coal bed is interpreted as the Top Etive Formation. The BARS images helped characterizing the l o cal g eol og y an d th e res erv oi r. In ad di ti on , t h e hi gh resolution sonic images augment the understanding of the seismic responses in the reservoir section.

Imaging radar maps underground objects in 3-D

City streets cover a complex array of underground electric, gas, and communication lines. Effective maintenance, expansion, and new installation of these networks require accurate information regarding the location of the conduits, cables, and other structures that lie beneath the surface. Underground maps, if they exist, are often inaccurate, incomplete, or out of date, and attempts to find underground lines or obstacles using metal locators often prove disappointing. To help companies create accurate maps of subsurface networks, researchers have developed a new ground-penetrating imaging radar (GPIR) system that creates sharp, three-dimensional (3-D) images of underground lines and objects. Schlumberger Corporation, in conjunction with the Electric Power Research Institute (EPRI) and the Gas Research Institute, has developed a GPIR system that detects, locates, and produces 3D maps of underground features. The new underground imaging system holds the potential to reduce utility operating and maintenance costs by avoiding unneeded excavation and by reducing incidences of costly damage such as ruptured gas lines. Field demonstrations in New York City, San Diego, and other utility locations have proven the ability of the new mapping system to create accurate images of objects in crowded urban areas at depths as great as 10 ft (3 m).

Suppression of high-energy noise using an alternative stacking procedure

Occasionally, seismic data contain transient noise that can range from being a nuisance to becoming intolerable when several seismic vessels try simultaneously to collect data in an area. The traditional approach to solving this problem has been to allocate time slots to the different acquisition crews; the procedure, although effective, is very expensive. In this paper a statistical method called “trimmed mean stack” is evaluated as a tool for reducing the detrimental effects of noise from interfering seismic crews. Synthetic data, as well as field data, are used to illustrate the efficacy of the technique. Although a conventional stack gives a marginally better signal‐to‐noise ratio (S/N) for data without interference noise, typical usage of the trimmed mean stack gives a reduced S/N equivalent to a fold reduction of about 1 or 2 percent. On the other hand, for a data set containing high‐energy transient noise, trimming produces stacked sections without visible high‐amplitude contaminating energy. Equiv...

First Borehole Acoustic Reflection Survey Mapping a Deepwater Turbidite Sand

The South Marlim oil field, discovered in 1987 offshore Campos Basin, Brazil, saw in the 1990s some deepwater production records being established and the development plans for the field should allow reaching a peak production of 420,000 boe/d in 2010. South Marlim is a turbidite reservoir of Upper Oligocene-Lower Miocene age. These reservoirs were initially thought to be homogeneous, widespread turbidite fans, but more recent studies based on later well data and 3D seismic surveys found that they can be rather complex and heterogeneous (Bruhn, 2001).

Fracture imaging from sonic reflections and mode conversion

Summary We have developed a new processing technique to extract improved reflection signals from data acquired in borehole acoustic reflection survey (BARS). The proposed technique utilizes multi-domain focussing and Wiener deconvolution. We applied this technique followed by the prestack Generalized Radon Transform (GRT) migration, to sonic BARS data from a fractured formation for the purpose of imaging the extension of fractures away from the borehole. By interpreting the sonic images in combination with microresistivity images, as well as analyses of Stoneley waves and anisotropy from cross-dipole sonic data, we obtained an improved description of the fractures and the formation.

Wavefield Separation For Borehole Acoustic Reflection Surveys Using Parametric Inversion

A wavefield separation method using parametric inversion and decomposition is developed for data acquired for a borehole acoustic reflection survey (BARS). We start by estimating parameters describing the propagation along the wellbore of compressional, shear, and Stoneley waves, including geometric spreading and complex-valued velocities. We find and subtract the maximum portion of the measured wavefields, compatible with these estimated parameters. A further attenuation of the direct waves are obtained using median filters along the estimated propagation velocities. After removing the direct waves, we are left with a relatively larger presence of components of the wavefield possibly reflected in the formation. The effectiveness of the method is confirmed by application to the real field data.

Ground Probing Radar

Publisher Summary This chapter describes ground probing radar. It refers to the use of radio waves to detect and locate buried objects. Also called “ground-penetrating radar” or GPR, directing radio waves into the ground is becoming a powerful way of mapping the shallow subsurface in civil engineering, environmental studies, forensics, and archaeology. Versatile GPR systems consist of pairs of transmitting and receiver antennas with control electronics are available from several commercial manufacturers. The chapter starts with a conceptual and mathematical description of GPR, including the most useful analytical models of radar propagation on the ground and a simple, but a rigorous model of radar scattering leads to a consistent formalis for 3D imaging. GPRs generate and detect classical EM fields that are described by Maxwells equations. To develop solutions relevant to GPR, a Cartesian coordinate with position vector is used. The chapter concludes with a brief description of practical GPR systems and examples of applications.

Shear wave anisotropy observed in VSP data at the San Andreas Fault Observatory at Depth

The San Andreas Fault (SAF) is the surface expression of a major plate tectonic boundary of the Pacific Plate. While widely known as the San Andreas Fault, it is probably better thought of as an extensive fault system that represents a major transform boundary between the North American and Pacific Plates. The sequence of M=6 earthquakes along the fault near Parkfield, California has stimulated an intense research effort into the seismogenic processes of brittle deformation in the upper crust along the San Andreas Fault. Part of this research effort has led to large network of surface instrumentation in the vicinity of Parkfield including seismometers, strain meters, and a variety of other geologic monitoring equipment. Recently, further instrumentation efforts have focused primarily on subsurface monitoring of seismogenic processes as part of the San Andreas Fault Observatory at Depth (SAFOD) project. In this paper, we review our recent research conducted to determine the local subsurface structure at the SAFOD site. We then present our structural interpretation of vertical seismic profiling data collected during recent crustal profiling studies. We conclude that the observed shear wave anisotropy in our VSP data is the result of aligned fracture sets which are most likely R shears. R shears are one component of conjugate Riedel fracturing associated with shear zone tectonic deformation.

Extending the seismic bandwidth — A simultaneous uncorrelated VSP and surface seismic field test study

A controlled simultaneous VSP and surface seismic field test was conducted to evaluate the evolution of the vibroseis chirp wavelet. We verify that the partitioning of seismic energy is independent of sweep rate for different sweep designs. We demonstrate under what conditions the seismic bandwidth can be extended using the semblanceweighted deterministic deconvolution process in the uncorrelated domain for zero-offset VSP, walk-away VSP, and simultaneous borehole and surface seismic data. The deterministic deconvolution process requires knowledge of the chirp wavelet in order to transfer the harmonic energy from the seismic noise domain into the seismic signal domain. Its performance depends on knowledge of the chirp signature and ambient noise. As expected, the ambient noise conditions in the well were extremely low as compared to the higher noise levels recorded in the surface seismic data. Under low ambient noise conditions, the semblance-weighted deconvolution can extend the recoverable bandwidth beyond the predefined pilot bandwidth for both the VSP and surface seismic data independently. The high level of surface ambient noise conditions in our field test limited the effectiveness of the VSP operators to extend the recoverable bandwidth of the surface seismic data.