Magnetic and Gravity Data Acquisition in Deepwater: a new Frontier for the Potential Field Geophysics

Abstract

ABSTRACT The O&G industry is moving offshore into increasingly deep waters in order to explore for, develop and exploit hydrocarbon reservoirs in new frontier areas. Standard gravity and magneto-gradiometric acquisitions in such areas are usually performed with ship-borne and airborne techniques. This implies that, especially in ultra-deep waters, the measuring sensors are very far from the geological causative bodies, thus leading to a poor SNR. Eni E&P is committed to adopting and deploying novel proprietary approaches and tools in these challenging environments. As part of an ultra-deep water E&P initiative, Eni and Tecnomare have designed, developed and tested the "AUVG", a new Autonomous Underwater Vehicle capable of carrying gravimetric and magnetometric instruments onboard for the acquisition of potential field data close to the sea bottom in ultra-deep water environments. The vehicle, which is carried by a supply vessel and launched from it near the area of interest, is autonomously capable of navigating underwater for up to 20 hours, reaching the sea bottom and starting the data acquisition, performing pre-programmed trajectories, avoiding unexpected obstacles, storing the entire data set and emerging in a predetermined position. The vehicle can be located using a radio, strobe lights and a satellite localizer. By means of a Wi-Fi data connection, it is also possible to download data and re-program a new geophysical survey. Gravimetric and magnetometric data acquired by means of an AUV in deep water are characterized by a much higher SNR as well as a higher spatial resolution. These improvements have been experimentally highlighted by a recent test carried out in deep waters in the Ionian Sea. INTRODUCTION The measurement of gravity and magnetic field data, also generally named potential field data, is extensively used in exploration geophysics. The common goal of potential field methods is to improve the definition of subsurface geology. Gravity field data provide information about anomalous density within the Earth. The interpretation of the gravity anomaly and/or related quantities (such as its gradient) is useful to understand where the density contrasts are located. It is particularly useful in saline contexts, where the salt bottom is not always visible in seismic data. Magnetic field data are useful to highlight magnetization contrasts caused by basement uplifts and subsequently to identify oil and gas traps, if necessary. Although it is not possible to determine a single source for an observed potential field anomaly, their interpretation can be useful in obtaining a geological screening of the area; they can provide constraints in the seismic interpretation phase and can also be used as input data for joint inversion applications. It should be noted that, compared to the seismic acquisition, the potential field acquisition is considerably less expensive.