Understanding the information content in gravity gradiometry data through constrained inversions for salt bodies

Gravity gradiometry inversion can provide important knowledge about a salt body and assist in subsalt imaging. However, such inversions are faced with difficulties associated with the lack of response from the nil zone in which the salt density is nearly identical to that of the background sediments and weak signals from the deeper portion of the salt. It is well understood that such difficulties could be alleviated by incorporating prior information, such as the top of salt from seismic imaging and petrophysical data, into the inversions. How to effectively incorporate such prior information is still a challenge, and what level of increased knowledge such constrained inversions can provide remains to be understood. We have investigated and compared the additional knowledge provided by incorporating different forms of prior information, including a top-of-salt surface, and an expected density contrast model. These different types of information are incorporated through different strategies of constrained inversion, including an inversion with bound constraints on the density contrast, inversion after a reduction-to-binary process, and discrete-valued inversion. We apply these strategies first to synthetic gravity gradiometry data calculated from the SEG/EAGE salt body and evaluate the improvements to the recovered salt provided from successive imposition of increased prior information. We further apply the strategies to a set of marine gravity gradiometry data collected in the Gulf of Mexico and examine the additional knowledge gained from the imaging of the salt in the region. We show that much more valuable knowledge about the salt can be obtained with the right prior information imposed through an effective strategy, and demonstrate that such gravity gradiometry data contain information about the salt body at depths much greater than previously recognized.

Subsalt imaging in northern Germany using multiphysics (magnetotellurics, gravity, and seismic)

Imaging subsalt is still a challenging task in oil and gas exploration. We have used magnetotellurics (MT) to improve the integration of seismic and gravity data to image the Wedehof salt dome, located in the Northern German Basin. High-density natural field source broadband MT data were acquired and enhanced the definition of the top and overhanging salt structures in addition to imaging the salt dome root. Salt boundaries show strong resistivity contrasts with the surrounding sediments and thus represent a good target for electromagnetic measurements, especially for top salt and salt flanks imaging. With integrated 3D gravity modeling focusing on the salt dome’s flanks at intermediate depths, an improved model was achieved. The new model provided sound input to a follow-up seismic depth migration that led to an improved imaging of the subsalt target proven by subsequent exploration drilling. The integrated interpretation of MT, gravity, and seismic combines the strengths of the different physics, thus increasing imaging reliability and reducing exploration drilling risks. Using a conservative workflow that included a feasibility study with field noise evaluation and careful acquisition parameter testing prior to survey start, a broadband array data acquisition, and advanced processing, the survey area's severe cultural noise issues could be overcome.

Designing an exploration-scale OBN: Acquisition design for subsalt imaging and velocity determination

Direct wave arrivals are the most robust signals to determine velocity. They have been used for almost a century in hydrocarbon exploration. This is because the arrival time is explicitly available and provides a direct measurement of the average velocity of the subsurface raypath. To acquire these direct arrivals in a seismic experimental setting, it is necessary that the waves turn back to the surface after they start traveling into the earth. As is well known, it is possible to turn waves back up if they encounter faster propagation velocities than previously experienced. Using these simple concepts, we show how it is possible to design a seismic acquisition to measure subsalt velocities when the salt cover is very thick and potentially not homogeneous. Until now in marine seismic surveying, the physical limitations of the earth meant that the use of direct wave arrivals was restricted to relatively shallow depths of investigation. By combining the application of node technology with a well-established physical phenomena (i.e., refraction in the basement), it is possible to retrieve subsalt velocities from seismic surveys.

Seismic Subsalt Imaging with Prestack Data Enhancement Methods

The Levantine basin is a world class site to study the onsets of salt tectonics. A commercial data set will be processed in a conventional processing flow, as well as, several prestack data enhancement methods. These methods include partial Common Reflection Surface processing, Partial Time Migration and the application of a Differential Evolution CRS global search. The data set is masked by noise and multiples, which will be attenuated. The Kirchhoff migration from the conventional processing flow shows the best results and is compared to the commercial TGS time migration. The shallow subsalt has been imaged. The deeper sections did not provide enough seismic energy to gain any improvement from prestack data enhancement.