Aeromagnetic compensation for the magnetic effects (noise) of the aircraft is now a significant limiting factor in the final data quality. Fundamentally, the residual noise from the aircraft movement after compensation is still 100's of times larger than the signal to noise capabilities of a modern cesium sensor. Users who require highly accurate data for interpretation of subtle features can utilize acquisition systems capable of the required sensitivity and software capable of utilizing the required sensitivity but the final data is flawed by relatively high noise from the aircraft movement effects. Surveys over magnetic terrains face the additional problem that the aircraft attitude required for the compensation techniques are often incorrect. This is due to the measurement of the aircraft attitude via the use of a vector fluxgate magnetometer and the assumption that the Earth's field is uniform. This problem is envisaged to increase as surveying heights get lower and lower, increasing the anomalous magnetic field sensed by both the vector fluxgate and the airborne magnetometer. Non-magnetic orientation devices, such as the use of three GPS receivers on the aircraft are an obvious solution to this problem. The results of this work demonstrates that it is possible to use three well spaced GPS receivers on an aircraft to measure the aircraft attitude to the precision required to compensate for the aircraft effects. The experiments with the different highpass filters used on the data and with the different solvers indicate that with the proper selection of filters and solvers, better compensation results can be obtained.
Lu, Richard S. (ExxonMobil Upstream Research Company) | Lovell, Richard W. (ExxonMobil Development Company) | Dawson, D. Cas (ExxonMobil Development Company) | Yu, Yang (ExxonMobil Upstream Research Company)
The amplitude and frequency loss caused by attenuation through the gas cap of a reservoir (Res A) in offshore west Africa has been recovered using viscoacoustic wave-equation migration (Q-migration) that enhances the seismic resolution beneath the gas cap. Amplitude and frequency attributes below the gas cap match those outside of the gas gap in the Q-migrated volumes. The Q-migration results are being used for well planning and to create a geologic model for reservoir characterization and simulation.
This paper addresses the effects of electrical anisotropy toward the 3D inversion of single-well induction logging data, when anisotropy is not considered. Of concern are possible artifacts that may lead to an incorrect interpretation of the formation about the borehole. Comparison is made of 3D isotropic inversion on a suite of model data, with and without anisotropy, consisting of an infinite layer and layer terminated at the borehole. In both cases, the layer dip (or well deviation) is varied. Inversion of the anisotropic data resulted in an overestimate of the layer conductivity, and the lateral extent of the layer to be confined about the borehole.
Numerical simulations of fluid flow through 3D pore space can provide accurate estimations for permeability. A digital volume required for these numerical experiments may be obtained directly by microtomography or statistically reconstructed from 2D thin sections. Such a digital pore volume has to be statistically representative of the original rock. However, only small rock fragments, such as drill cuttings, and only 2D images of those may be available in the field. To address this practical constraint, we investigate how permeability can be estimated from small 2D images. We select a number of natural and artificial medium-to-high porosity well-sorted sandstones. 3D microtomography volumes are obtained from each of these physical samples. Then, analogous to making thin sections of drill cuttings, we select a large number of small 2D slices from a 3D scan. As a result, a single physical sample is used to produce hundreds of virtual-drill-cutting 2D images. Corresponding 3D pore space realizations are statistically generated from these 2D images, fluid flow is simulated in 3D, and the absolute permeability is computed. As expected, this permeability does not match the measured permeability of a physical sample, which is due to inherent variations of pore-space geometry among the small images. However, for all the physical samples, a single and clear trend is formed by cross-plotting the simulated permeability versus porosity. This trend is typical for clean sandstone. The simulated permeability of under-representative sandstone fragments does not match the physically measured data. Instead it provides a valid permeability-porosity transform which can be used to estimate permeability if porosity is independently known from well log or seismic measurement
Converted-wave exploration has played a more and more important role in oil field prospecting and development phase. In order to accelerate the gas development in Sulige gas field and promote the application of converted-wave exploration, BGP has carried out some 2D converted-wave projects in Sulige gas field in recent years, a plenty of results has been acquired and a well-rounded work flow, including acquisition, data processing and interpretation has come into being d and this is good for the next step of exploration and development in this area.
In this paper we present a case history for mapping electrical conductivity using Controlled Source Audio Magnetotellurics (CSAMT). We have completed a first pass 1D inversion of a 3D CSAMT data set collected over a geothermal reservoir located in western Idaho. We compare the inversion results of data combinations including apparent resistivity, phase, and direct inversion of the electric field component. The 1D profiles from the first pass inversion are combined to generate 2D subsurface conductivity images. We include our interpretation of the subsurface conductivity structure to determine the pathways of the fluid conduits in the geothermal reservoir. Our eventual goal is to obtain a 3D conductivity model by inverting these data using 3D CSAMT inversion.
This paper introduces a method which spectrally decomposes a seismic trace by solving an inverse problem. In our technique, the reverse wavelet transform with a library of complex wavelets serves as a forward operator. The inversion reconstructs the wavelet coefficients that represent the seismic trace and satisfy an additional constraint. The constraint is needed as the inverse problem is non-unique. We show synthetic and real examples with three different types of constraints: 1) minimum L2 norm, 2) minimum L1 norm, and 3) sparse spike, or minimum support constraint. The sparse-spike constraint has the best temporal and frequency resolution. While the inverse approach to spectral decomposition is slow compared to other techniques, it produces solutions with better time and frequency resolution than popular existing methods.
Chalks and marl reservoirs may maintain high porosity at more than two kilometers of burial depths due to overpressure and the presence of hydrocarbons. As a result, formation stability is compromised considerably and wells are is danger of being lost. Analyses of the variations in velocity, attenuation, and static compression modulus in carbonates allow us to better interpret seismic measurements in terms of subsurface petrophysical parameters and to understand the failure and damage potential. We present a study of variations in dynamic moduli in various carbonate reservoirs. Our study includes log and laboratory data from velocity, porosity, permeability, and attenuation measurements.
We describe the advantages of using dual-azimuth streamer data for complex model building and imaging in the Gulf of Mexico Deep Water. We iteratively image two datasets separately while building a unique common velocity model. Tomography and salt interpretation can take advantage of two illumination patterns that complement each other while each migration is performed along the native shooting direction. The results show improvements compared to the monoazimuth exploration vintage image and provides a step change needed for the field development.