Friday, February 12, 2010

What value VSP?

[Note: A version of this blog entry will appear in World Oil (March, 2010)]

From a geoscience point of view, there are two worlds in hydrocarbon exploration. First is the depth realm of geology as revealed by drilling and wireline logs. Second is the reflection time world of geology as seen by seismic data. The connection between time and depth is, predictably, a time-depth curve derived from sonic log or vertical seismic profile (VSP).

The sonic device is a long metal cylinder lowered into the well on wireline like any other logging tool. Once in place at some depth in the well, it is slowly pulled up. As the tool crawls up the well, it is generating data by emitting at one end high-frequency pulses and listening for pulse arrivals at the other end. The operating frequency for most sonic logs is ten to fifteen thousand Hertz, far beyond the 10-100 Hz range of surface seismic data. When the source acts, sound waves move through the drilling fluid and interact with rock along the well bore wall that typically has a much greater sound speed. This sets up a thing called a head wave that runs along the bore hole wall, as opposed to a reflection that would only travel in the fluid and bounce off the wall. Anyway, since the exact distance between the source and receiver is known, along with the fluid sound speed and hole diameter, the wave speed in the rock formation can be found. The sonic log actually outputs the delay time divided by the source-receiver distance in units of microseconds per foot, and the formation velocity in ft/s can be found using (1 000 000)/sonic. The quantity measured by the sonic log is termed the interval velocity because it represents the local formation P-wave speed averaged along a short interval of the well bore equal to the source-receiver spacing of 1-2 m. A digital sonic log yields an interval velocity reading every 0.3 m (0.5 ft) in depth.

The process of converting a sonic log to a time-depth curve involves integration. In effect the sonic log is a model of the earth consisting of thin layers. Each layer is the same thickness (0.3 m) and we know the velocity, so the vertical two-way travel time through each layer can be calculated. Starting from the top, we sum up these layer times to find the reflection time to all depths in the well. The result is a time-depth curve, but one with many potential sources of error.

First, a sonic log never reaches the earth surface. Tool specifications require the bore hole diameter to be less than about 50 cm (20 in), meaning that onshore sonic logs in petroleum exploration and production wells rarely get within 100 m of the surface. In seismic terms, this is the weathered zone and likely to contain unpredictable low-velocity rock. The sonic contains no information about this interval, so it is necessary to somehow figure out the reflection time from ground surface to the top of the sonic log and add this to the time-depth curve.

Sonic logs are sensitive to washouts and other hole problems, and it is not easy to get accurate sonic velocity in slow formations (velocity less then sound speed in mud). We also have the problem of frequency mismatch between sonic and 3D surface seismic data. Note only is the sonic seeing a tiny volume of rock compared surface seismic waves, it is also well-established that seismic velocity in fluid saturated porous rock varies with frequency. This leads to the notoriously difficult upscaling problem that involves modifying observed sonic readings to better match the long wavelength velocities seen by surface seismic data. Not many people agree on how to do this.

To address the missing near surface and other time-depth problems, a check shot survey can be run in conjunction with sonic logging. In a check shot survey receivers are located sparsely down the well, usually at casing points and key geologic boundaries. The measured quantity is just the first arrival time.

Compare all of this to a vertical seismic profile recorded using a source at the surface and many receiver locations down the well. The receivers record full traces for interpretation and receiver spacing is determined by spatial aliasing considerations, usually something like 3 m (10 ft). This gives actual traveltimes from the surface to points in the earth. The VSP considered here is often called a zero offset VSP, meaning that only a single source position is used and that it is as close to the wellhead as possible. There are also multioffset and multiazimuth VSPs which use many source locations. These are much more expensive and sometimes useful for local, high resolution imaging. However, a zero offset VSP is sufficient for event identification and other standard uses.

A zero offset VSP is the best and most direct method of associating a 3D seismic event with a geologic horizon since it has about the same frequency range (20-200 Hz) as surface data, the wavefield actually passes through the same near surface, and it is not sensitive to hole problems. And there are many important side-benefits. In standard 3D shooting we use various acquisition techniques to isolate P waves, but the earth is actually elastic and all types of shear and mode-converted waves are bouncing around down there confusing our interpretation. The VSP is unique in its ability to distinguish upgoing from downgoing waves, S from P and mode-converted waves, and primary reflections from multiples. The last point is critically important. The entire machinery of seismic imaging is based on primary events that have reflected only one time. Over the last half-century an arsenal methods have been developed to detect and remove multiples in surface seismic data. Even so, our abilities are limited by the nature of the data. VSP data is fundamentally different and allows direct observation of these multiples (and non-P waves).

You don't need a hundred VSPs in a project area, but you are at a competitive disadvantage without at least one.

Fig 1. Finite difference modeling was used on the velocity model to create the VSP and shot record data. The model, VSP, and shot are arranged to show the connecting role that VSP plays in relating velocity model in depth and the shot record in time.  Can you identify a multiple?

Monday, February 8, 2010

Turhan Taner

I was saddened to hear today that Dr. Turhan Taner passed away over the weekend. He lived here in Houston and had been suffering from a long illness. My wife Dolores was a great friend of his and, through her, I had the chance visit him several times. He did first class work in many aspects of applied geophysics, most recently in seismic attributes. His 1960s papers on travel time curves in layered media are still the foundation for understanding this key subject.

As you can see from the SEG Honors and Awards list, Tury won SEG Honory Membership (1978), Best Paper Presentation at the Annual Meeting (1978), and the prestigious Maurice Ewing Medal (1993). In addition to being an excellent scientist, he was a kind and generous man. Many benefited from association with him, and he will be missed by all. A good biography of Tury can be found here.

***** Text of Tury's Obituary *******

Dr. M. Turhan “Tury” Taner passed away Saturday, February 6, 2010, in Houston, Texas at the age of 82. Dr. Taner will be laid to rest next to his parents in Istanbul, Turkey. A memorial service will be held after his family returns from Turkey at Emerson Unitarian Church, 1900 Bering Dr. on Sunday, March 21st at 3:00 pm.

Turhan was born in Akhisar, Turkey to Izzet and Kadriye Taner. He received a Diplome Engineer in 1950 from the Technical University of Istanbul and came to the United States in 1953 to the University of Minnesota for a postgraduate program in engineering. He co‐founded Scientific Computers in 1959, and in 1964 co‐founded Seiscom Delta, a geophysical service company, where he served as chairman, director of research, and later, senior VP for technology. In 1980 he started Seismic Research Corporation (SRC), and in 1998 SRC merged with Petrosoft and Discovery Bay to create Rock Solid Images. Widely known within the geophysical community, Tury was the recipient of numerous accolades including the SEG’s highest award, the Maurice Ewing Medal in 1993 and The EAGE’s highest recognition, the Desiderius Erasmus Award for lifetime contribution in 2004. Tury was a pioneer, teacher, scholar, great practitioner and a household name in geophysics. During his career he authored or co‐authored several groundbreaking papers on geophysical methods and contributed to the development of many technologies still in use today. In addition to his passion for creating and developing geophysical algorithms such as semblance and multitudes of other seismic attributes, he loved music, food and wine, traveling, art, and soccer, but most of all, he loved his friends and family.

Tury is survived by his loving family, including his son Jeffrey Taner and wife Andrea; daughter Jane Harris and her husband Christopher; son John Taner and his wife Julie; sister Turcan Sozeri; niece Selen Ozel and her husband Haluk; grandchildren Adam Harris, Emily Taner, Lilly Taner, Daniel Taner, Jack Taner; great‐niece Beren Ozel, and great‐nephew Devin Ozel. In lieu of flowers, the family requests that donations in honor of Dr. M. Turhan Taner be made to the National Parkinson Foundation, 1501 NW 9th Avenue, Bob Hope Road, Miami Florida 33136‐1494.

Saturday, February 6, 2010


Last sunday at a gathering of Spanish expats in Houston (my wife is from Barcelona), geologist Daniel Minisini chatted with me and wanted to know if I would do a short video interview. He has a blog site, miniGeology, that archives brief (< 5 min) interviews with geoscientists. Check it out sometime. Unscripted, unbriefed, and unedited, the interview format yields a short spontaneous discussion. I've added my interview video (complete with interruption by the cleaning staff) to my Links sidebar.

It is an honor to be an early part of this creative and interesting video archive project. Thank you Daniel.

Monday, February 1, 2010

Millennium Mark

Just noticed today that the total number of visitors to this blog has passed 1000. Thanks to all who have taken the time to drop in. The visitor level has accelerated lately due to the World Oil column (What's New in Exploration) that appeared in January 2010 and footnoted the blog address. Another column is on the way for February. My original plan was to contribute 3-4 columns for 2010, but I'm honored to report that the Editor and Management of World Oil have asked me to serve as Contributing Editor. I have agreed to take on the monthly column for the next 12 months.

On the subject of writing, I was again honored to hear last week that ExxonMobil will order over a hundred copies of my book, Elements of 3D Seismology, as a reference for staff geoscientists in the technology group. This is good motivation for work toward the 3rd edition.

On an unrelated (?) note, I am in the third year of teaching the graduate class Geophysical Data Processing at U Houston. The class size numbers are: 2008 (18), 2009 (25), and 2010 (46). It is the largest graduate class in the Department of Earth and Atmospheric Sciences at U. Houston. Anybody see a trend here? This is one of the required core courses for MS Geophysics students, but that only accounts for 30. It is optional for all the rest. Interesting....

Class photo for University of Houston class GEO7341 Geophysical Data Processing for spring 2010 (head count = 46).