High-Resolution 3-D and 4-D Imaging of the Near Surface III: Posters
Presiding: C Bank, Colorado College; A Pidlisecky, Stanford University
NS51A-01 0830h
Semblance-based Topographic Migration (SBTM): a Method for Identifying Fracture Zones in 3-D Georadar Data
Steep-dipping fracture zones are generally difficult to delineate using traditional ground-penetrating radar (georadar) techniques. Evidence for their presence in standard georadar images may be either completely absent or limited to diffractions and/or chaotic reflection patterns. To address this issue, we present a novel 3-D migration scheme based on computations of semblance. This new approach, which accounts for undulating surface topography, emphasizes diffractors while markedly reducing the effects of specular reflectors. After demonstrating the efficiency of the technique on 3-D synthetic data, we apply it to a 3-D georadar data set acquired across an unstable mountain slope in the Swiss Alps. This region is characterized by rugged topography and numerous shallow- to steep-dipping fracture zones and faults. Only the shallow- to moderate-dipping structures are imaged as reflectors in conventionally migrated versions of the georadar data. Our semblance-based topographic migration (SBTM) scheme produces a 3-D volume containing clouds of high-semblance values. Application of morphology image processing to these clouds reveals the presence of geologically meaningful structures, most of which are very steeply dipping (>75°). Several of the steep-dipping structures project to open fracture zones and associated lineaments at the surface, thus demonstrating the capability of the combined SBTM and morphology procedure for mapping near-vertical fracture zones.
NS51A-02 0830h
High Resolution Near Surface 3D Seismic Experiments: A Carbonate Platform vs. a Siliciclastic Sequence
Interest in high-resolution 3D seismic experiments for imaging shallow targets has increased over the past years. Many case studies presented, show that producing clear seismic images with this non-evasive method, is still a challenge. We use two test-sites where nearby outcrops are present so that an accurate geological model can be built and the seismic result validated. The first so-called natural field laboratory is located in Boulonnais (N. France). It is an upper Jurassic siliciclastic sequence; age equivalent of the source rock of N. Sea. The second one is located in Cap Blanc,to the southwest of the Mallorca island(Spain); depicting an excellent example of Miocene prograding reef platform (Llucmajor Platform); it is a textbook analog for carbonate reservoirs. In both cases, the multidisciplinary experiment included the use of multicomponent and quasi- or 3D seismic recordings. The target depth does not exceed 120m. Vertical and shear portable vibrators were used as source. In the center of the setups, boreholes were drilled and Vertical Seismic Profiles were shot, along with core and borehole measurements both in situ and in the laboratory. These two geologically different sites, with different seismic stratigraphy have provided us with exceptionally high resolution seismic images. In general seismic data was processed more or less following standard procedures, a few innovative techniques on the Mallorca data, as rotation of horizontal components, 3D F-K filter and addition of parallel profiles, have improved the seismic image. In this paper we discuss the basic differences as seen on the seismic sections. The Boulonnais data present highly continuous reflection patterns of extremenly high resolution. This facilitated a high resolution stratigraphic description. Results from the VSP showed substantial wave energy attenuation. However, the high-fold (330 traces ) Mallorca seismic experiment returned a rather discontinuous pattern of possible reflectors, opposing to the predicted seismic stratigraphy/geology of the area. The Llumajor Platform has been buried only a few meters at most, therefore primary and secondary porocity remains intact, creating a fractal like environment of scatterers and diffractors. We have interpreted two possible reflections, the top of the reef and the water table; the former is nicely coupled with the VSP. The seismic wave attenuation observed is believed to be predominantly due to the scattering effects.
NS51A-03 0830h
Seismic amplitude extraction techniques for the recognition of deepwater sediment dispersal patterns in the Western Gulf of Mexico
Acquisition of large 3D seismic volumes over the slope and abyssal plane of the Western and Southern Gulf of Mexico is providing deep inside, not only to its subsurface hydrocarbon potential, but also, into its present geomorphology and shallow geology. Detailed analysis of the sea floor morphology, combined with sea floor seismic attribute extraction, provides valuable information to understand present sediment dispersal patterns over the slope and abyssal plain. Geomorphologic features of the Perdido, Mexican Ridges, Deepwater Salina and Campeche Provinces include: large active canyons, scarps, massive sediment slides, sea floor folding and faulting, expulsion craters and sediment dispersal patterns, among other. These features indicate the highly active nature of the present depositional systems of the western and southern deepwater Gulf of Mexico. Based on these analogs and careful seismic amplitude extraction techniques, several sediment dispersal patterns and the concentration of sand prone facies have been proposed for Paleocene, Eocene, and Miocene sequences. Clear identification of a major deepwater fan dominated by individual and amalgamated, meandric channels that characterize the lower slope and abyssal plane of the Eocene, are clearly visible. Large linear trends of sediment transport and accumulation over the Miocene upper slope are also evident. To predict the presence of hydrocarbons related to confined and unconfined facies, AVO analysis and amplitude extraction over near and far offset volumes were obtained. Map of differences between near and far amplitudes combined with AVO allowed us to identify the most attractive areas, as well as, risk and rank the numerous hydrocarbon prospects. Superposition of sediment dispersal patterns and sand prone accumulation areas with structural maps of key horizons allows estimate potential hydrocarbon volumes, and helps to define final wells position and preferred trajectory.
NS51A-04 0830h
The Application Of Second-Generation Wavelets In Seismic Data Denoising
In 1994 Swelden first proposed the wavelet transform based on lifting steps. Lifting scheme is a kind of flexible method of wavelet construction and it uses linear or nonlinear operators to implement wavelet transform and make sure the transform is reversible. Lifting theme is independent of Fourier transform and the wavelet transform based on lifting theme is also called Second-generation wavelet transform. Its features include: Keep the multi-resolution feature of first-generation wavelet; Independent of Fourier Transform; Fast arithmetic is easily to be achieved; Reverse transform is easier; Non-linear wavelet transform is available, etc. The paper discusses the principle and procedures of second-generation wavelet transform and apply it to the de-noising of seismic data. Examples on synthetic as well as field data prove that it is a new effective de-noising method. Assuming that the original signal cj is decomposed into low frequency approximate signal cj-1 and high frequency detail signal dj-1 by lifting steps. The transform includes three steps: split, predict and update: (1)Split: The original signal is split into two non-intersectant subsets cj-1 and dj-1. The more correlative cj-1 and dj-1 are, the better the split effect is. Commonly, we divide a signal sequence into even sequence and odd sequence. (2) Predict: By means of the correlativity of the data, we can predict dj-1 from cj-1 by using a predict operator P. The resulting difference is the wavelet coefficient d[n], and it reflects the approaching degree of the two data sequences. (3) Update: After the two steps above, some characters of the resulting data sequence cj-1 are not consistent with the original data, so update step is necessary. We can use an update operator U to generate a better data sequence c[n], and make it keep the characters of the original data sequence. The three steps above constitute a lifting step, and by iterative lifting steps we can obtain approximate signal cj-n and high frequency detail signal dj-n. After n times of decompositions, the original data can be represented as cj-n, dj-n, dj-n+1, dj-1. By changing operation orders and signs, we can get the reconstruction formulas very conveniently. The de-noising by second-generation wavelet transform is divided into three steps: wavelet decomposition, wavelet coefficient reduction and data reconstruction. The common wavelet de-noising methods include soft threshold and hard valve methods. Soft valve method is used in this paper. In the paper, wavelet Deslauriers-Dubuc(4, 2) is used to make wavelet transform. Based on the transform methods above, we can get different levels of approximation and detail signals. At each level, we use soft threshold method to reduce wavelet coefficients, and then reconstruct data. As a consequence, the noise can be reduced apparently. Second-generation wavelet transform is the further development of traditional wavelet theory. The research about its theory and application should be done more deeply. In the paper, we discuss the principles and transform process of Second-generation wavelet, and apply it to seismic data de-noising. Examples on synthetic as well as real data prove that it is a new effective de-noising method.
NS51A-05 0830h
Development of an Innovative Downhole Seismic Source
MSE Technology Applications, Inc. (MSE) previously designed, built, and tested an innovative downhole seismic source. The design criteria included a size limitation (the source needed to fit into a 2-inch diameter well casing), the source would use .22 caliber power loads as the energy source, it would have the ability to fire at least 12 times before reloading, it would be able to function under water (depth is limited by internal pressure from the .22 caliber power loads, which must be greater than pressure exerted by water column), and it would use no more than 24-volt dc current. MSE developed the design criteria from a need for a downhole seismic source suitable for high-resolution seismic tomography applications. Tomographic methods may provide detailed information at waste sites for both characterization and monitoring. Since borehole diameters are kept to a minimum (i.e., 2-inches or less) to reduce waste volumes from drill cuttings, or the borehole may be installed using a direct push technology such as a GeoprobeT or cone penetrometer, a small diameter source is desirable. Additionally, the use of .22 caliber power loads reduces the amount of supporting equipment required to operate the source as compared to other downhole seismic sources (e.g., air guns and piezoelectric sources). MSE tested and evaluated the completed seismic source to assess the effectiveness of the .22 caliber power loads as energy sources and to assess the operational ease of using the source. Results of the testing indicated that the power loads provided energy suitable for high-resolution cross-well seismic tomography applications. Operation of the source required significantly less supporting equipment than other downhole sources tested. However, the testing suggested the system could be improved if the number of mechanical components were reduced. Subsequent research suggested that the power loads could be fired using an electric current. As a result, MSE believes that the entire operation of the seismic source could be accomplished using electronic components. This would eliminate mechanical components currently used inside the seismic source. Planned borehole seismic source modifications include converting the source from primarily mechanical operation to electrical operation and completing the firing control system. The firing control system will allow the user to turn the source on and off, initiate the firing process, and track the number of power loads fired. Testing of the modified seismic source will be limited to bench testing the electronic firing mechanism and the firing control system since we are not modifying the energy source used (.22 caliber power loads) performance of the source in this respect will not change. Acknowledgements Work is being conducted through the Savannah River Operations office at the Western Environmental Technology Office under DOE Contract Number DE-AC09-96EW96405.
NS51A-06 0830h
Finite-Difference Method For Electromagnetic Logging In 3D Anisotropic Media
We consider the problem of computing the electromagnetic field in 3D anisotropic media for electromagnetic logging applications. The proposed finite-difference scheme for Maxwell equations has the following new features: coercivity, i.e., the complete discrete analogy of all continuous equations in every grid cell; a special conductivity averaging that does not require the grid to be small compared to layering or fractures; a spectrally optimal grid refinement minimizing the error at the receiver locations and optimizing the approximation of the boundary conditions at infinity. All these features significantly reduce the grid size and accelerate the computation of electromagnetic logs in 3D geometries without sacrificing the accuracy. The problem is solved in both Cartesian and cylindrical coordinate systems. We use Spectral Lanczos Decomposition Method and its pre-conditioned modification as solvers. We present examples of modeling a triaxial induction tool response. Both metal and insulating tool details are included in the model.
NS51A-07 0830h
Three-dimensional sensitivity distribution of low-induction-number frequency-domain electromagnetic instruments
Low-induction-number frequency-domain (LIN FEM) instruments operate in three dimensions, but analyses have traditionally been done only in one. For proper site-selection and planning of field surveys, it is critical that users be aware of the complex variability of instrument sensitivity in three dimensions and be able to predict the manner in which the electrical conductivity (EC) of a given environment affects the spatial distribution of sensitivity. The objectives of this study were to examine the three-dimensional sensitivity of LIN FEM instruments and to describe the implications of this for field surveys. Simulations were carried out to map the local sensitivity in three-dimensional, homogeneous-half-space environments with EC of the ground varying from 1 to 100 mS/m. Local variations in instrument sensitivity were complex and included regions having opposite polarities. The largest variations in sensitivity were within a few meters of the instrument. Changing the orientation of the transmitter and receiver dipoles from horizontal to vertical resulted in significant differences in the local sensitivity distribution. The results of this study can be used to identify potential sources of error in field applications and design. For example, gross errors in interpretation of target location can occur if it is assumed that instrument sensitivity is uniform throughout the sample volume or even if it is assumed that sensitivity drops off monotonically as a function of distance from the instrument. In heterogeneous environments, there also could be near-surface effects. High near-surface EC can make it difficult to measure EC at greater depths. Temporal variations in water content of upper layers can, by changing EC, make exploration depth variable with time. Addressing the three-dimensional sensitivity distribution will result in better assessment of the suitability of the method to the target or process under investigation.