Exploration Geophysics
Attenuation of Coherent Noise
As a Research Scientist at the Amoco Production Company Tulsa Research Center, Dr. Legg developed new methods for attenuating coherent noise in marine seismic data. Most coherent noise is source-generated and includes backscatter and sideswipe from strongly reflecting seafloor and shallow subsurface features. Water bottom and other multiples in areas with strong impedance contrasts are also important to attenuate. Careful design of both the acquisition survey arrays, as well as the processing sequence and parameters, are needed to attenuate coherent noise in difficult areas.
Our experience has shown that many imaging problems considered to be penetration problems requiring greater source energy, in fact, are coherent noise problems that require more careful acquisition and processing design. In many cases, stronger source energy only creates greater coherent noise energy that obscures the subsurface primary target reflection horizons. Well-designed acquisition and processing are able to provide high quality subsurface images in difficult areas with smaller energy sources. Reduced source energy levels is increasingly desirable, to satisfy environmental concerns such as avoiding harassment of marine mammals or disturbing fisheries, and to permit acquisition in shallow water coastal areas. The exponential advances in computer and electronic technology allow more advanced digital hydrophone and geophone systems that are capable of broader bandwidth recording with greater signal-to-noise ratio in high-density arrays.
Pull-Apart Basins and Pop-Up Structures
Bends and step-overs in major strike-slip faults create important geologic structures where hydrocarbons may accumulate. Releasing bends and step-overs create transtensional zones often recognized as pull-apart basins where organic sediments and reservoir sands accumulate. Restraining bends and step-overs create transpressional uplifts called “pop-ups” with folds and reverse faults that may trap migrating hydrocarbons. Pull-apart basins filled with turbidites subsequently may be inverted into pop-ups as strike-slip continues along the major fault zones. The Neogene oblique-rifting of the Borderland created abundant transtensional basins where organic-rich sediments were cooked by the widespread volcanism.
Seismic reflection data produce high-resolution images of the subsurface structure necessary to map potential hydrocarbon and other mineral resources. Dr. Mark Legg has over 25 years experience in seismic data acquisition, processing, and interpretation.
Dr. Legg has conducted both land-based and marine seismic data acquisition surveys. His Ph.D. dissertation and post-graduate research were based on high-resolution seismic reflection studies of the geologic structure and tectonic evolution of the California Continental Borderland. This large mostly submarine region of basins and ridges, with high-relief, steep structural dips, and exhumed basement and volcanic seafloor outcrops, provides a real challenge to quality subsurface imaging. Meeting this challenge, Dr. Legg has gained exceptional skill to understand geologic processes and structure in complex tectonic regions.
Data acquired include:
- Single-channel and multichannel seismic profiles
- Very high-resolution sub-bottom profiles
- Side-scan sonar imagery
- Magnetometer and gravimeter profiles
- Seismic refraction data with sonobuoys and OBS
Commercial projects include high-resolution surveys for geologic hazards investigations, hydrocarbon exploration, and research and development of new technology for marine seismic reflection profiling. Dr. Legg gained experience in state-of-the-art seismic reflection data acquisition and processing as a Research Scientist at the Tulsa Research Center of the Amoco Production Company.
Processing seismic reflection data improves the subsurface image by increasing the signal-to-noise ratio of the primary reflection wave-fronts while attenuating both random and coherent noise energy. Signal enhancement by deconvolution and other filtering produces a record section with the widest possible bandwidth in a high-resolution subsurface image needed for structural and stratigraphic interpretation. Careful attention to deriving accurate velocity information provides better CMP stack and migrated records. Dr. Mark Legg has more than 25 years experience in processing seismic data ranging from single-channel high-resolution profiles for hazard surveys to multichannel hydrocarbon exploration surveys.
Skilled interpretation of seismic data requires both a thorough understanding of the seismic method and substantial expertise in geological processes and structures. Three decades of research in the geologic structure and tectonic evolution of the California Continental Borderland provide Dr. Legg with unique skill for seismic data interpretation in active tectonic regions. The complex tectonic evolution, ranging from Mesozoic subduction to Tertiary oblique-rifting and Quaternary transpression, created a fascinating deformation history that is recorded in the many sediment covered ridges and filled basins of the Borderland. Turbidite sedimentation from widespread submarine canyons and fan systems provide high-resolution stratigraphy that is readily imaged with seismic methods. Structural inversion of Miocene transtensional basins created important traps for large accumulations of hydrocarbons. The dominant strike-slip (“wrench”) tectonic environment formed narrow, elongate, basins and ridges that endured successive episodes of subsidence and uplift - called “Yo-Yo tectonics” - as adjacent blocks pass restraining and releasing bends along the irregular fault traces. Complex faulting tends to produce compartmented hydrocarbon reservoirs that require careful mapping in order to maximize hydrocarbon production from these strike-slip fault systems.