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The AAPG/Datapages Combined Publications Database

Ohio Geological Society

Abstract

OGS-AAPG

Ohio Geological Society: Into the New Millennium: The Changing Face of Exploration in the Knox Play, Sixth Annual Fall Symposium, October 20, 1999

Pages 72 - 80

S/N RATIO AND BANDWIDTH CONSIDERATIONS WHEN UTILIZING SEISMIC DATA IN EXPLORING FOR SUBTLE TRAPS - EXAMPLES FROM THE KNOX PLAY

Edward R. Tegland, Exploration Development, Inc., 10970 S. Pikes Peak Dr., Parker, CO 80138
Patrick H. Bygott, Exploration Development, Inc., 10970 S. Pikes Peak Dr., Parker, CO 80138

ABSTRACT

The concepts of bandwidth and SIGNAL TO NOISE (S/N) ratio are highly interrelated and for the most part are controlled by the acquisition technique employed and the near surface geology over which the seismic data collection is carried out. External forces such as wind noise, cultural/ traffic noise, and instrumental problems may also play a part in defining the final quality of information. These latter items can be countered in part by diligent efforts of the seismic acquisition contractor. The question, "What constitutes noise?" is briefly reviewed.

What is S/N ratio? Generally, this is defined as the ratio of peak signal amplitude to RMS noise level in the general time window of interest. In areas where there is significant amplitude variation between different geologic events one normally defines this by the behavior of the stronger reflectors. The seismic interpreter should remember that at best the normal seismic display could reveal a 20 dB (10 to 1) amplitude ratio. That is to say, if two reflected events have amplitude levels that are more widely separated than this, only the larger event will be observed unless extreme display measures are taken. By the same measure, if noise amplitudes exceed 10 times the amplitude of the desired signal one has little chance of interpreting the data.

What is bandwidth? Bandwidth is the difference between the highest and lowest measurable frequency present in the data. More important to the seismic interpreter is the "useable bandwidth". This is the frequency range low to high over which coherent interpretable data can be detected. In other words the useable bandwidth is that which has a S/N ratio that allows the signal to be seen and tracked in space. The relationship between time, Previous HitdipTop rate, and useable bandwidth will also be reviewed.

Model data created from an Ohio Knox synthetic Seismogram will be used to illustrate what this means to the person observing seismic data in traditional display format. This model data is also used to illustrate the role of processes such as stacking. Real Beekmantown data examples will be used to illustrate the more advanced processes.

Presented in this discussion will be some basic concepts that are aimed at maximizing the "useable bandwidth" from the data processing point of view. The pros and cons of various steps that influence the useable bandwidth will be examined with samples of actual Knox seismic data. Also discussed will be some broader concepts in the interpretation of the seismic data, i.e. escaping from the "one line, one zone syndrome".

The interpreter/end user should be aware that there is no "silver bullet" that will slay all the dry hole vampires. Therefore, the up and downside aspects of various techniques must be understood. Various noise attenuation techniques are in everyday use and the relative merits of each are discussed. Where possible real data examples will be used to illustrate key points. Obviously, one could move a few hundred feet away and be in another problem world. Therefore, each project should be viewed as somewhat unique until proven otherwise.

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