Dennis E. Willen

The construction of subsurface illumination and amplitude maps via ray tracing

Editors note: This article was selected as the Best Poster Paper at SEGs 1999 Annual Meeting. Due to its quantitative nature, the 1999 Best Student Poster Paper will not be published in TLE but in a future issue of Gℯℴ𝓅𝒽𝓎𝓈𝒾𝒸𝓈, without undergoing peer-review. The goal of a seismic survey is to illuminate subsurface geologic formations that may hold hydrocarbon accumulations. Conventional seismic survey design relies on the assumption that uniform midpoint coverage will lead to uniform illumination in the subsurface as long as each midpoint is hit by a sufficient range of offsets. In areas of complex velocity structure, severe wavefield distortions lead to irregular subsurface illumination patterns, even if surface midpoint maps show a uniform distribution. A more appropriate approach is to design seismic surveys to ensure illumination of key subsurface horizons. The difference between midpoint coverages and subsurface illumination patterns is particularly large in salt-prone areas (Muerdter et al., 1997). Due to severe wave distortion through complex, high-velocity salt bodies, conventional design methods that result in relatively uniform surface coverage (Figure 1) generate uneven amplitudes and shadow zones on subsalt horizons, an effect that is shown clearly by ray-trace modeling of an entire seismic survey (Figure 2). Figure 1. Total hits in each surface bin resulting from a 3-D seismic survey collected along east-west lines. Distances are in kilofeet. Figure 2. Amplitude of a subsurface …

Massively parallel electrical-conductivity imaging of hydrocarbons using the IBM Blue Gene/L supercomputer

Large-scale controlled source electromagnetic (CSEM) three-dimensional (3D) geophysical imaging is now receiving considerable attention for electrical conductivity mapping of potential offshore oil and gas reservoirs. To cope with the typically large computational requirements of the 3D CSEM imaging problem, our strategies exploit computational parallelism and optimized finite-difference meshing. We report on an imaging experiment, utilizing 32,768 tasks/processors on the IBM Watson Research Blue Gene/L (BG/L) supercomputer. Over a 24-hour period, we were able to image a large scale marine CSEM field data set that previously required over four months of computing time on distributed clusters utilizing 1024 tasks on an Infiniband fabric. The total initial data misfit could be decreased by 67 percent within 72 completed inversion iterations, indicating an electrically resistive region in the southern survey area below a depth of 1500 m below the seafloor. The major part of the residual misfit stems from transmitter parallel receiver components that have an offset from the transmitter sail line (broadside configuration). Modeling confirms that improved broadside data fits can be achieved by considering anisotropic electrical conductivities. While delivering a satisfactory gross scale image for the depths of interest, the experiment provides important evidence for the necessity of discriminating between horizontal and vertical conductivities for maximally consistent 3D CSEM inversions.

CSEM inversion: Impact of anisotropy, data coverage, and initial models

While 3-D inversion is a particularly effective tool for interpreting marine CSEM data, 3-D data acquisition is needed to image the resistivity structure of an anisotropic subsurface. Tests on synthetic models show resistivity artifacts when anisotropic data are forced through isotropic inversion and when offline data are excluded from anisotropic inversions. Similar artifacts appear also when the inversion starting model differs too much from the actual subsurface. As a result, inadequate attention to anisotropy or inadequate data coverage can lead directly to misinterpretation of the subsurface resistivity structure. Anisotropic imaging with good data coverage, accurate receiver orientation, and good initial resistivity models are necessary to quantitatively image resistive anomalies in an anisotropic earth.

Inversion study of a large marine CSEM survey

The Brazil RC Marine CSEM survey was collected in April of 2004 for the ExxonMobil Remote Reservoir Resistivity Mapping (RM) Project. The portion of the survey reported here consisted of a total of 735 km of transmitter towlines arranged approximately on a 5 km x 5 km rectangular grid (see Figure 1). Vertical and horizontal electric field measurements were recovered at a total of 23 seafloor locations from a deployment of 36 seafloor instruments. Imaging of these CSEM data into full three-dimensional conductivity volumes represents a formidable challenge due to the subtle effects of reservoir targets, the volume of data and its large dynamic range. In this presentation, we report on an initial round of inversion results obtained using both isotropic and anisotropic (VTI) imaging methods. Our results support the need for an anisotropic model to accurately represent subsurface resistivity.

Identifying, removing, and imaging P-S conversions at salt-sediment interfaces

The large velocity contrast between salt and the surrounding sediments generates strong conversions between P‐ and S‐wave energy. The resulting converted events can be noise on P‐wave migrated images and should be identified and removed to facilitate interpretation. On the other hand, they can also be used to image a salt body and its adjacent sediments when the P‐wave image is inadequate. The converted waves with smaller reflection and transmission angles and much larger critical angles generate substantially different illumination than does the P‐wave.In areas where time migration is valid, the ratio between salt thickness in time and the time interval between the P‐wave and the converted‐wave salt base on a time‐migrated image is about 2.6 or 1.3, depending upon whether the seismic wave propagates along one or both of the downgoing and upcoming raypaths in salt as the S‐wave, respectively. These ratios can be used together with forward seismic modeling and 2D prestack depth migration to identify the co...

Differential reduction of magnetic anomalies to the pole on a massively parallel computer

A finite‐impulse‐response filter was implemented on a computer with massively parallel processors to reduce a magnetic anomaly map to the magnetic pole, allowing each grid node to have a different inclination and declination (differential reduction to the pole, DRTP). The dramatic speed improvement of such an implementation for the filter design and application via space‐domain convolution makes DRTP a practical tool for hydrocarbon and mineral exploration.Application of this tool to magnetic anomalies in east China reveals that the northward shift in position of the anomaly maximum generated by DRTP is 6 km for anomalies with dominant wavelengths of approximately 25 km in the northernmost part of the study area. The shift increases as the anomaly wavelength increases. Shifts for all anomaly wavelengths are even larger in the southern part of the study area, where the magnetic inclination is lower. The shift in position of the anomaly maximum for anomalies of wavelengths 25 km in the northernmost area pro...

Marine CSEM Data Processing Techniques

Summary Marine CSEM data processing techniques have evolved rapidly to accommodate the proliferation of marine controlled-source electromagnetic (CSEM) surveys. Processing methods developed for marine MT surveys are not directly applicable to marine CSEM data. Basic marine CSEM data processing eliminates the signature of the acquisition system and extracts the normalized spectral response of the earth from the receiver time series data. More sophisticated methods have been developed to simplify interpretation and to ensure that the highestquality data are available for inversion. These methods include determining the 3D receiver orientations by inversion, extending the range of useful offsets by noise suppression, and minimizing the effect of air waves by model-based subtraction.

Equivalence of Controlled-source Electric and Magnetic Data for Resistivity Inversion

We use a combination of theory and synthetic surveys to understand whether horizontal magnetic data offer additional information over horizontal electric data alone for CSEM inversions. Our results show that the two data types are almost equally effective at detecting reservoirtype resistivity anomalies in the subsurface. We note that the magnetic data produce somewhat better-behaved inversions close to the seafloor, but this advantage is at least partially offset in the field by the smaller dynamic range available in magnetic data. The practical impact is that we may freely substitute magnetic data for electric data on instruments with faulty electric channels when inverting CSEM surveys.

Remote Reservoir Resistivity Mapping - Breakthrough Geophysics For The Upstream

Abstract Marine controlled-source electromagnetic surveying has emerged as a new tool for remotely detecting reservoired hydrocarbons offshore. The technology was pioneered by university and government researchers over the past 25 years, and recently has benefited from development by contractors and the oil industry, including ExxonMobil. When integrated carefully with other geoscience information, primarily seismic, marine CSEM shows promise for adding considerable value in Upstream applications. Remote reservoir resistivity detection and imaging results from a recent ExxonMobil survey demonstrate this technology. Early Marine CSEM Development Offshore CSEM research began in the 1920s with studies of seafloor power cables, and saw the first commercial minerals survey offshore Cornwall, England a decade later. The research that led to todays commercial methods began in earnest much later, primarily at the British Geological Survey and UCSD Scripps Institution of Oceanography in the 1970s. Many other university and government groups started research on the technology about that time, and many continue to the present. Exxon began investigating marine CSEM in early 1981 using theory and computer modeling, and scoped the feasibility of field tests. Results looked promising for very deep water applications. However, the lack of suitable acquisition equipment, limited deep water opportunities, and the emphasis on the (then) new 3D marine seismic technology resulted in deferring further marine CSEM research.

A path‐integral approach to depth migration

Seismic depth migration can be cast as a weighted sum over forward paths connecting data measured on the surface to image locations in the subsurface. The integral forms of conventional downward-continuation operators have a sum-over-paths or pathintegral interpretation. These path integrals are related to, but distinct from other path integrals that have appeared in the geophysical literature. Kirchhoff migration approximates the path sum by the contributions from a few, special raypaths. This connection to Kirchhoff migration may permit the computational domain to be trimmed sufficiently to apply path integrals to target-oriented, 3-D prestack depth migration. The effective use of path integrals for depth migration will require an appropriate Monte Carlo integration method, presumably developed around stationary or near-stationary points in the travel time field.