Analyze full-tensor gravity data with intuitive imaging techniques

Full-tensor gravity gradiometry (FTG) has proven to be an effective tool for both the oil and gas and mineral exploration industries since it was introduced in the late 1990s. The multicomponent gravity surveying technology provides a rich source of information that lends itself to imaging and delineating key geological structures. New techniques involving invariant analysis have been developed that facilitate quick and efficient imaging to extract detailed geological information.

Recent developments in imaging techniques now allow the explorationist to gain even more insight into exploration areas using the full tensor. Method and theory
Invariant imaging techniques use the full tensor and produce image representations independent of the observer’s axis of choice. These can be used to identify dominant regional density trends as well as detail of the 3-D shape of targets, such as the geometry of fault block, igneous intrusives, or salt body structure.

The invariant method also facilitates the combination of the horizontal components to image geological contacts or lineaments.

The geological strikes calculated using this equation identify linear features in the gravity field by combining neighboring consistent strike directions. In addition, these strike angles can be used in more traditional geological representations such as rose diagrams.

More recently, the concept of tensor rotation was introduced in a paper presented at the SAGA 2009 11th Biennial Technical Meeting in Swaziland. FTG data is typically rendered in a fixed-coordinate frame such as East-North-Down (END). This is largely an arbitrary choice used to conform to mapping conventions. However, some geological scenarios are better served using alternative coordinate frames and, by using full-tensor data, it is possible to realign the tensor responses to investigate their anomalous character.

This realignment of the tensor components is performed in near real time, allowing the rotations to be evaluated visually and the optimum rotation selected for a particular target feature’s orientation. Once the coordinate transform is selected, the full set of tensor components is calculated by further processing.

Example: Vinton Dome, Louisiana
Airborne FTG data were acquired over the Vinton salt dome in Louisiana in 2008 as part of a test program for the newly acquired BT67 aircraft by Bell Geospace. The data was processed and subsequently analyzed using the invariant techniques.

The band pass-filtered Tzz illustrates the overall shape and structure of the high-density cap rock near the center of the dome.

The horizontal components provide further information on the structure: Txx and Tyy identify the north-south and east-west edges of the cap rock, respectively; their negative sum equates to Tzz. Txz and Tyz locate the central axes of the salt body. The Txy component shows the typical pattern of two positive and two negative anomalies, which helps constrain the geometric extent of the cap rock.

The data is further enhanced by band-pass filtering all components between 820 and 8,200 ft (250 and 2,500 m) spatial wavelengths. The advantage of the filtering process better captures the signal associated with the salt body at large, hosting the high-density cap rock. Band-passing the Tzz component data suggests that the cap rock body is not of uniform shape. Computing the rotation invariant, I2, from the band pass-filtered FTG component data lends support to the view that the cap rock feature is a complexly shaped entity.

The irregularity is further investigated by analyzing its corresponding response with the horizontal component data. Equations are used to extract the anomalous responses associated with the edges of the cap rock and salt feature. These horizontal invariant lineaments identify the complex shape of the cap rock (inner circle) and also map the edge of the salt body itself (outer circle). The strike directions also can be located. The results support the horizontal invariant lineaments and facilitate further geological analysis.

Surmising the impact of rotating the band pass-filtered Txz component through 90º, subtle trends initially not readily visible become apparent upon rotating through 60º. The predominant northwest-trending linear pattern indicates an underlying geological trend that may have influenced salt movement in the area.

FTG data provide information relevant to unraveling complexity not only associated with shape, size, and thickness of target geology but also the target’s geological setting. Intuitive imaging techniques employed highlight detailed information relating to the cap rock and underlying salt distribution at Vinton in Louisiana and predict the presence of underlying geological trends perhaps influential in the salt body’s development.

Acknowledgement
This paper is based on an abstract of a presentation from the 72nd EAGE Conference & Exhibition and has been reprinted with permission from the authors.