Near Surface Modeling for Static Corrections
Throughout the class there are two persistent themes, the near-surface is complex and any single method does not work everywhere. The presentations start with the geomorphology of the near-surface which is examined, in terms of processes and environments. The physics of raypath propagation is explained. There are assumptions for the whole course: Corrections are not dynamic, physical ground conditions are isotropic and elastic modeling and time domain seismic data processing.
A single layer from surface to datum is the initial near-surface model. This is also known as field statics a simple solution supplied by acquisition field crews and often applied as a “first look”. The first augmentation of the basic model is from extra information obtained from uphole survey. The upholes are the only direct measurement of the near-surface depth and velocity layers. The multiple layer model creates improved static corrections over the initial model. Furthermore the uphole interpretation and multiple layer model can adequately define the regional time structure.
Refraction statics, of increasing complexity, does provide models which improve the short wavelength focusing of the seismic horizons. The course will describe the theory and assumptions of intercept-time, plus-minus and generalized linear inversion refraction techniques. Reduced travel time analysis is shown to infill the delay time distribution. The modeling culminates with an introduction to refraction tomography.
Upon completion of the course, participants will be able to:
- Judge the geomorphologic and environmental contributions to the complexity of the near-surface. Distinguish the rock physics and wave properties that influence seismic P-wave velocity and propagation.
- Ability to build single or multiple layer velocity models from interpretations of uphole survey data and additional traveltimes due to sand dunes.
- Construct delay times and calculate statics from reciprocal time refraction analysis.
- Differentiate the concepts of common receiver and common emergent point.
- Demonstrate the concepts of generalized linear inversion, ray tracing, and tomography.
- Physics of wave propagation
- Single layer velocity model
- Multiple layer modeling
- Arrival time of refracted arrivals
- Intercept-time refraction statics
- Delay time method and common refractor point
Geophysicists performing seismic data processing requiring knowledge to build near-surface models for static corrections. Interpreters of seismic time domain images who desire to understand the complexity of the near-surface model with respect to data quality.
Basic knowledge of structural geology and lithology. Basic knowledge of seismic acquisition methods and time domain seismic data processing.
About the Instructor
Ralph Bridle (1957) is a retired Geophysical Consultant formerly with Saudi Aramco in the Near Surface Modeling Unit of the Geophysical Data Processing Division in Dhahran, Saudi Arabia. He achieved his Mining BSc(hons) from Camborne School of Mines, Cornwall, England.
Ralph's first experience was in mining, at Shamva gold mine in what was Rhodesia. In 1980 he moved to geophysics and employed by Seismograph Service Ltd (SSL) on acquisition field crews in Pakistan, Bangladesh and Indonesia. Subsequently he transferred to seismic data processing at locations in Libya, Australia, Tunisia, Oman, Somalia and Nigeria. Most of his experience is in data acquisition and processing of seismic data in deserts.
From 1991 until retirement in 2017 Ralph gained more experience in Saudi Aramco’s Near Surface Modeling Unit. His specialty was modeling the near surface for static corrections, modeling multiple layers from uphole surveys, various refraction static methods, tomography and application of geostatistics. Ralph was an enthusiastic mentor and gave numerous in-house classes ranging from one hour to five days. His legacy is found in papers published by SEG and EAGE. In 2014 the SEG awarded him Active Membership for Life. Since 2017 he has been resident, with his wife, in the Philippines.