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When seismic isnít enough

Interview with Dr. Michal Ellen Ruder, Wintermoon Geotechnologies Inc on the use of gravity and magnetic methods for Oil and Gas exploration.

August 6, 2010

1. Under what conditions will gravity methods outdo seismic? Why?

Seismic data quality is not uniform in all geologic settings. Due to their elastic properties, some rock lithologies do not readily transmit acoustic energy.  As a result, seismic signal can be greatly attenuated, providing little or no seismic imaging below these acoustically-challenging horizons. If there is a significant lateral density variation below the problematic acoustic horizon, gravity can provide unique information about these rocks.  Gravity is not negatively impacted by rock elastic properties.

Another common imaging problem for seismic data is a near-vertical fault or interface (like a salt or shale diaper). Seismic signal is typically scattered by these interfaces. In contrast, both gravity and magnetics have their best imaging at vertical interfaces across which lateral density or magnetic susceptibility contrasts exist.

2. In what situations are gravity methods absolutely indispensable if you want results?

Gravity methods are vital for providing an independent corroboration of total volume and shape of salt when investigating sub-salt hydrocarbon potential. Seismic data quality can be significantly improved when we use gravity modeling results to improve and refine our seismic velocity model of the base of salt. This result is fed back into the seismic pre-stack depth migration processing to provide an improved seismic image of the sub-salt horizons’ geometries. Ultimately, this leads to more successful prospecting. We are presently using this approach in salt-prone basins offshore West Africa, South America, North America, and northern Europe.

3. Can you describe some cases where seismic and gravity methods complement one another?

Gravity is correctly characterized as a ‘low-resolution’ geophysical exploration tool. Without constraining information, a gravity model can be modified in an infinite number of ways in order to match the observed gravity signal. Seismic, well log, and other geologic data provide critical constraining information for gravity models, limiting the degrees of freedom of our modifications and enabling us to produce a meaningful, constrained earth model that is consistent with both gravity and seismic datasets.

4. What techniques have you personally developed or enhanced?

At Wintermoon Geotechnologies, we use a multi-faceted approach to interpretation. We work hard to identify and separate ‘regional’ and ‘residual’ signals within the observed gravity and magnetic datasets using our state-of-the-art Fourier-based 2D filtering routines. We have pioneered the construction of complex, geologically constrained 2D and 3D forward and inverse models. We construct models which incorporate geometries and physical properties derived from all available geologic and geophysical data, and we then invert the observed gravity and magnetic data to gain further insight into the geologic question at hand. Our 3D forward and inverse modeling approach has been successfully used in prospect evaluation projects in the Gulf of Mexico, offshore Brazil, and offshore West Africa.

5. Gravity/magnetic methods used to be used mainly to tell an explorer where to best place seismic. Nowadays you’re advocating its use as complementary and integrative to seismic as well. What has caused the shift?

Grav/mag data quality has improved significantly since the 1980’s. Not only can we acquire data more quickly now, but also our data resolution is much better and we can now resolve more subtle (lower amplitude and shorter spatial wavelength or high frequency) perturbations in both the gravity and the crustal magnetic fields. This improved resolution now allows us to imagine very small-scale and shallow lateral variations in density and magnetic susceptibility. These are often associated with the hydrocarbon play itself. Modern grav/mag surveys can now provide a set of observations that is more directly associated with hydrocarbon play character, in addition to the ‘traditional’ regional information for which we have long used potential field data.

6. What has been the hottest development in the past two years with respect to using grav/mag in oil and gas exploration?

We have two areas in which recent significant advances have been made. These include, in acquisition, airborne gravity gradiometry which provides a rapidly acquired, high-resolution image of local gravity anomalies, and in software development, improved 3D inversion algorithms for sophisticated gravity and magnetic earth models.