Land Seismic Techniques for High-Quality Data

Oilfield Review Summer 2010: 22, no. 2. Copyright © 2010 Schlumberger. For help in preparation of this article, thanks to Abdulmohsin Al-Dulaijan, South Rub Al-Khali Co. Ltd, South Rub Al-Khali, Saudi Arabia; Jarrah Al-Genai, Kuwait Oil Company, Kuwait City, Kuwait; Danny Burns, Beach Petroleum, Adelaide, South Australia, Australia; Daniele Colombo, Saudi Aramco, Dhahran, Saudi Arabia; Dennis Cooke, Santos, Perth, Western Australia, Australia; Tim Dean, Tom Heesom, Anthony McGlue, John Quigley, Paul Taylor and Richard Whitebread, Gatwick, England; Benjamin Jeffryes, Cambridge, England; Leif Larsen, Perth, South Australia; Marco Mantovani, Milan, Italy; Denis Sweeney, Dubai, UAE; Randall Taylor, Origin Energy Limited, Brisbane, Queensland, Australia; and Pieter van Mastrigt, Brunei Shell Petroleum Company, Brunei. dBX, Desert Explorer, DX-80, MD Sweep, Q-Land, Q-Technology and UniQ are marks of Schlumberger. Since the first land seismic surveys, performed in the 1920s, the quality of data has improved dramatically through the use of better survey design practices, acquisition techniques and data processing. Despite these advances, seismic surveys still may deliver disappointing results in several challenging parts of the world—including some with significant onshore reserves. Often, the cause of inadequate imaging of deep structures lies in the presence of geologic complexity near the surface. Variations in topography and in the velocity of near-surface layers create distortions in the seismic signal. Strongly refractive layers near the surface can prevent deep penetration of seismic energy, as can intervals of anomalously low velocity. Abrupt lateral changes in near-surface properties warp raypaths and weaken the effectiveness of traditional processing methods. Provinces with rough topography, surface dunes, permafrost and buried soft layers are notorious for the obstacles they present to exploration; companies prospecting in these regions have learned to temper their expectations of seismic surveys. Recent advances in land seismic data acquisition and processing are enhancing imaging in these problem areas. Advanced sources, optimized acquisition practices and improved processing algorithms are producing seismic data rich in usable content, resulting in superior structural imaging and successful inversion for reservoir properties without compromising acquisition efficiency. Sophisticated evaluation of signals— particularly those usually considered purely noise—is enabling subsurface imaging in areas that previously yielded poor results. With additional information, such as gravity, electromagnetic or remote-sensing data, uncertainty in near-surface properties can be minimized, thereby improving imaging of deep targets. This article describes developments in land seismic data acquisition and processing to help oil and gas companies obtain effective results in geographic areas that have proved difficult to image in the past. Among the acquisition innovations are high-performance explosive and vibrator sources, a new way of activating vibrators that extends the bandwidth of seismic energy imparted to the earth and techniques for deploying multiple vibrators to acquire data more efficiently. New processing methods involve characterizing near-surface properties using additional information from surface waves and other geophysical surveys. Because these advances rely on the enabling technologies of point-receiver acquisition and continuous recording available with the most up-to-date land seismic systems, a brief review of these systems is in order.