David C. Bartel

Time‐domain electromagnetic detection of a hidden target

Numerical modeling of the time‐domain electromagnetic (EM) step response of a vertical tabular target hidden beneath a thin conductive overburden reveals that the target’s presence may be detected only during a well‐defined time window. In a situation where the secondary magnetic field is sensed by an airborne system equipped with horizontal coaxial dipoles, a conductance contrast of about ten between the target and the overburden is needed to ensure target detection. This value will, of course, vary with the size and depth of the target and, to a lesser extent, with the geometry of the system. In general, the time at which the window opens is a function of the geometrical parameters of the target, the height of the system, and the conductance of the overburden. For a given target, its width (defined as the ratio of the time of closure to the time of opening) is only a function of the conductance contrast between the target and the overburden. While the target signal is visible, one observes a maximum val...

Determination of transversely isotropic velocity parameters at the Pluto Discovery, Gulf of Mexico

The collection of both a checkshot and multi-level walkaway vertical seismic profile (VSP) in the initial exploration well at the Pluto Discovery in the Mississippi Canyon area of the Gulf of Mexico provided a unique opportunity to assess and model the velocity anisotropy present in the area. Pre-well depth estimates based on a 3D pre-stack depth migration were found to give horizon depths that were five percent greater than those actually encountered in the well. A transversely isotropic (TI) velocity model was chosen to image the velocity anisotropy, which, for P-wave propagation, depends essentially on three independent parameters the interval stacking velocity and two unitless parameters, delta and eta. The sediment velocity model for the Pluto Discovery area was divided into five layers conformable with the water bottom surface. Constant delta and eta parameters were determined for each layer. These two parameters combined imply a horizontal P-wave velocity that is eight to sixteen percent faster than the vertical P-wave velocity in the top 14,000 feet of sediments. Travel time estimates from the ray tracing program agreed with the checkshot and walkaway VSP travel times to within one percent, when anisotropy was taken into account, as opposed to six percent for isotropic ray tracing. Based on this information, a 3D poststack depth migration using the TI velocity model showed, versus an isotropic velocity model, correct positioning of events in depth and better resolution of high dipping events. These are outcomes that we would expect from a TI velocity model migration. A checkshot and offset VSP were collected in the second exploration well at the Pluto Discovery. The checkshot data showed a similar vertical velocity structure to that indicated in the first well. The receiver depth range used for the offset VSP included greater depths than in the first well, so the anisotropic nature of the area could be explored deeper. For at least this sedimentary sub-basin, there appears to be little lateral change in the velocity or TI parameters. The TI velocity model at the Pluto Discovery is useful in correcting the depth implied from the pre-stack depth migration and in computing the correct travel paths of the seismic energy. Additionally, a post-stack depth migration using the TI velocity model resulted in sharper images from the sedimentary section. INTRODUCTION It has been known for some time that seismic waves in the earth can travel with different velocities depending on the direction of travel. In most localities this difference in seismic velocity with direction is small. However, in some areas the difference is large enough to cause distorted seismic images and inaccurate depth predictions. The collection of both a checkshot and multi-level walkaway VSP at the Pluto Discovery in the Mississippi Canyon area of the Gulf of Mexico provided an opportunity to assess and model the velocity anisotropy present in the area. Pre-well depth estimates based on a 3D pre-stack depth migration were found to give horizon depths that were five percent greater than those actually encountered in the well. A checkshot could easily correct this problem; however, a broader assessment and interpretation of the velocity anisotropy were desired. A walkaway VSP acquired in the sidetrack well provided the data necessary to measure the vertical versus horizontal velocity in the area. A transversely isotropic velocity model was chosen whose parameters (described below) were taken from Thomsen (1986), and Tsvankin and Thomsen (1994). The Pluto Discovery is in the Mississippi Canyon area of the Gulf of Mexico, approximately 150 miles southeast of New Orleans. The discovery lies in about 2,800 feet of water. The partners in the drilling of the well were British Petroleum, Chevron, and BHP Petroleum. The first well was drilled to 22,389 feet. A checkshot survey was recorded at approximately 250 foot intervals throughout the length of the original well. A sidetrack well was oriented northward from the original hole to investigate additional targets. A walkaway VSP survey was recorded at two different levels in the sidetrack well. An isotropic earth model is easy to imagine. The velocity of the seismic wave at any point within the earth section depends not on its direction, but rather on its spatial location. This holds true even for a vertically gradient earth. For a TI earth, the velocity is considered constant within a horizontal plane, but that velocity is different from the vertical velocity. A TI earth can arise in three main ways. First, a truly anisotropic rock has velocities that differ parallel and perpendicular to bedding planes. A shale could be seen to have this characteristic. Secondly, a series of isotropic rock layers can give rise to an anisotropic earth. 1998 SEG Expanded Abstracts TI velocity parameters at Pluto, GOM Lastly, stress fields within the earth may cause velocities to vary with direction. It is the first two of these mechanisms, anisotropic rocks and/or layered isotropic rocks, that are thought to give rise to the anisotropy found in the Gulf of Mexico. The horizontal velocity is generally greater than the vertical velocity, and theoretically must be if the TI anisotropy arises from layers of isotropic rocks. The parameters necessary for completely characterizing a TI earth include the interval stacking velocity, delta, and eta. The delta parameter is the percentage difference between the squares of the interval stacking velocity and interval checkshot velocity. The delta parameter has been thought of as the vertical anisotropy and corrects the seismic stacking velocity to the checkshot velocity. Eta is a measure of the difference between the squares of the horizontal and stacking velocities. VELOCITY COMPARISONS There are several different relevant velocity functions useful in seismic exploration. Checkshot velocities provide the definitive tie between seismic and well data. Stacking and migration velocities are used in processing seismic data and are normally picked to provide the best seismic image. Thus they are not necessarily tied to the petrophysical parameters of the geologic section. Figure 1 shows that there is a distinct difference between these three velocity functions of checkshot, stacking, and migration for the Pluto area. The stacking velocity function is the average of the velocity functions from the velocity control locations near the well. The migration velocity function is the one used in the prestack depth migration of the seismic data set. Obviously if either the stacking or migration velocity functions were to be used for time-depth conversions, the estimated depths would be too deep. The pre-stack depth migration for Pluto proved to have depth estimates that were 5% too deep at TD.

Integrated use of the Earth model in subsalt seismic prestack imaging

Seismic imaging and the development of accurate earth models enhance geologic understanding and reduce risk in the challenging deepwater areas of the Gulf of Mexico. The standard seismic imaging practice is complicated by extensive sheets of allochthonous salt, which hinder clear imaging of subsalt reflectors due to nonuniform illumination and overprinting by multiples and converted waves. This talk highlights several work practices that we perform to help constrain and validate the subsurface model with respect to the data, and thereby reduce risk within a consistent interpretation. They include complex 3D structural representation and visualization, 3D ray tracing that links the model to the data, visual diagnosis of prestack traces, and integrated displays of earth model, seismic data and inferred earth properties. Rapid iterations and modifications are achieved through software developed on a common computing platform.

Time-domain solution for horizontal coaxial dipoles on a half-space

The practice of transforming frequency‐domain results into the time domain is fairly common in electromagnetics. For certain classes of problems, it is possible to obtain a direct solution in the time domain. A summary of these solutions is given in Hohmann and Ward (1986). Presented here is another problem which can be solved directly in the time domain—the magnetic field of horizontal coaxial dipoles on the surface of a homogeneous half‐space. Solutions are presented for both an impulse transmitter current and a step turnon in the transmitter current. The solution in the time domain is obtained by taking the inverse Laplace transform of the product of the frequency‐domain solution and the Laplace‐domain representation of the current waveform.

First SEG Volunteer Day a big success

On Saturday, 21 September, before the SEG convention began in earnest, the first SEG Volunteer Day was held at the Houston Food Bank. About 32 volunteers (Figure 1) participated in sorting and packing donated food for distribution across southeast Texas. Many of the volunteers were from the University of Houston GeoSociety. The Houston Food Bank is the largest food bank in the United States (by pounds) distributing more than 80 million pounds in the past year to more than 600 partner agencies (soup kitchens, food pantries, senior centers, and other agencies) in 18 southeast Texas counties. The Houston Food Bank generates food for more than 137,000 people per week. Most of the food goes to the working poor—those persons who have jobs but still struggle to put enough food on the table for their families. Only 6% of the people served are homeless; the other 94% are working people who struggle to pay bills and provide adequate food on the table.

Spectral analysis of airborne electromagnetics; discussion and reply

Bartel and Becker present original and academically challenging ideas; however, to state that INPUT® is basically the same as GEOTEM® is only correct with respect to the transmitter‐to‐receiver geometry and the transmitter waveform (McMullan et al., 1989).

Crone pulse electromagnetic response of a conducting thin horizontal sheet; theory and applications; discussion

Rai uses a simple formula for the step response of a conducting, horizontal thin sheet in the time domain and applies it to the Crone pulse electromagnetic (PEM) system. He also uses this formulation to interpret some field results. The idea of an infinite, horizontal, conductive thin sheet is valid in some cases for both ground and airborne EM systems. However, I disagree with some of the derivations of the thin-sheet equation as presented in the subject paper. The applicability of the study is not questioned; but the interpretation of the field example may be different.