PRUTZMAN, JOHN M., Bell Geospace, Inc., Houston,TX

Abstract: Full Tensor Gradient Imaging Below Salt and Basalt

Abstract

The US Navy developed a stealth technology for real-time mapping of bathymetry allowing their trident nuclear submarines to navigate without sonar. Recently this technology has been successfully applied in the oil and gas industry to mapping subsurface geology that is hard to image seismically.

The commercial version of this technology is known as the full tensor gradient (FTG) system. Both salt and basalt typically have such high impedance contrast and provide such strong ray path distortion that the deeper reflecting seismic energy is difficult to collect and focus. Therefore, subsalt and sub-basalt structure is hard to image and hard to interpret with seismic data. Sensitivity tests using models show that full tensor gradient data can image the subsalt and sub-basalt structure at a resolution suitable for prospect level mapping. Even when the seismic data is not interpretable, the full tensor data provides useful information since the gradiometer is responding directly to density contrasts rather than impedance contrasts. The full tensor gradient measurements yield five independent constraints resulting in final interpreted earth models that are much more accurate. This is demonstrated with several case studies.

Case studies demonstrate that subsalt structure below 3,500 meters can be imaged, sub-basalt structure can be mapped, and base and edges of salt and basalt predicted accurately.

Introduction

During 1994 and 1995 the US Navy collected full tensor gradient (FTG) data in the Gulf of Mexico as part of an effort to prove the applicability of this technology to the oil and gas industry. The effort was successful and in spring 1998 the first gradiometer was released to the oil industry followed earlier this year by a second gradiometer. FTG data has now been collected in both the Gulf of Mexico and the North Sea.

Full Tensor Gradient Data

The gradiometer developed by the US Navy is a moving platform, full tensor gradient machine. That is, it measures the full tensor of the Earth's gravity field while the instrument is moving. The gradient data answers the question how each vector component of the gravity field varies in each of the three directions, which yields a nine-component tensor as illustrated below. Although there are nine tensor components only five are independent. However, a sixth component,Tzz, is always used because it is the gradient most easily related directly to subsurface geology.

Sensitivity Models

Prior to collecting FTG data, a sensitivity test is usually run based on plausible alternative geological models in order to determine if the gradient differences are large enough to detect with the gradiometer. As an example, consider the 2D salt model below that has a salt wing on its left flank (Fig. 1). The gravity and gradient responses are plotted above each model. The vertical gradient shows a maximum 5 Eötvös change. If the salt wing is replaced with sedi-.ments, then the model response changes as shown below (Fig. 1). The vertical gradient is about 5 W Eötvös suggesting about a half Eötvös difference exists between the salt wing and sediment wing. If the salt wing creates an over pressure zone beneath, then the gradient response increases.The results below are indicative of a small body just below the level of imaging at that depth assuming no overpressure. A typical limit of resolution is considered to be 1-2 Eötvös depending on survey conditions.

Similar modeling with basalt indicates the potential to map the size, shape, and thickness of basalt even when sediments and basalt are stacked. Below are model results suggesting the effectiveness of FTG data to delineate structure below basalt (Fig. 2). Notice that Tzz is responding to the sub-basalt structure.

Case Studies

As an example of the imaging capabilities of FTG data, a salt and subsalt mapping project is shown. Top of salt starts at -2,200 meters (-7,000 feet) subsea with base of salt about 1,000 meters (3,300 feet) deeper. Below the salt are several horizons of interest. The initial seismic interpretation from a 3-D survey is shown below in Figure 3, along with the gradient optimized interpretation (Fig. 4).

The gradient optimized interpretation shows a prospective anticline that the seismic data failed to image.The gradient optimized interpretations of the deeper horizons are consistent with this picture.

A case study West of Shetlands indicates FTG data can also be useful in mapping basalt and sub-basalt structure. Seismic data shows the difficulty of imaging in the presence of strong impedance contrasts due to refraction and defocusing effects. Since FTG data only responds to density contrasts, it is not defocused or refracted allowing imaging of the basalt and sub-basalt structure.
 
 

Figure 1. 2D FTG model of salt wall with sediment wing (above) and salt wall with salt wing (below).

Figure 2. 2D FTG model of succession below single layer of basalt.

Figure 3. Interpretation of structure below salt, from 3-D seismic survey, (subsea values in feet).

Figure 4. Gradient optimized interpretations of data from area in Figure 3, showing closed structure (values in subsea feet).

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England