REGIONAL SEISMIC LINES REPROCESSED USING POST-STACK PROCESSING TECHNIQUES: NATIONAL PETROLEUM RESERVE--ALASKA

by Miller, J.J.¹, Agena, W.F. ¹, Lee, M.W. ¹, Zihlman, F.N. ¹, Grow, J.A. ¹, Taylor, D.J. ¹, Killgore, Michele¹, and Oliver, H.L. ¹

Open-File Report 00-286

2000

This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

¹U.S. Geological Survey, MS 939, Box 25046, Denver Federal Center, Denver, CO 80225-0046

Regional seismic lines reprocessed using post-stack processing techniques:

National Petroleum Reserve - Alaska

ABSTRACT

This CD-ROM contains stacked, migrated, 2-Dimensional seismic reflection data and associated support information for 22 regional seismic lines (3,470 line-miles) recorded in the National Petroleum Reserve – Alaska (NPRA) from 1974 through 1981. Together, these lines constitute about one-quarter of the seismic data collected as part of the Federal Government’s program to evaluate the petroleum potential of the Reserve. The regional lines, which form a grid covering the entire NPRA, were created by combining various individual lines recorded in different years using different recording parameters. These data were reprocessed by the USGS using modern, post-stack processing techniques, to create a data set suitable for interpretation on interactive seismic interpretation computer workstations. Reprocessing was done in support of ongoing petroleum resource studies by the USGS Energy Program.

The CD-ROM contains the following files: 1) 22 files containing the digital seismic data in standard, SEG-Y format; 2) 1 file containing navigation data for the 22 lines in standard SEG-P1 format; 3) 22 small scale graphic images of each seismic line in Adobe Acrobat^® PDF format; 4) a graphic image of the location map, generated from the navigation file, with hyperlinks to the graphic images of the seismic lines; 5) an ASCII text file with cross-reference information for relating the sequential trace numbers on each regional line to the line number and shotpoint number of the original component lines; and 6) an explanation of the processing used to create the final seismic sections (this document).

The SEG-Y format seismic files and SEG-P1 format navigation file contain all the information necessary for loading the data onto a seismic interpretation workstation.

INTRODUCTION

The USGS reprocessed twenty two 2-D regional seismic transects across the National Petroleum Reserve – Alaska (NPRA) using current seismic data processing techniques. These lines include the 20 original Regional Compressed Sections (RCS) (Ikelman, 1986) plus 2 additional lines, totaling 3,470 line-miles. Although the complete NPRA seismic data set totals nearly 15,000 miles (Gryc, 1985, page C27) and is the largest public-domain onshore seismic data base in the United States, the 22 regional seismic lines presented here provide a useful summary of the NPRA data for understanding the overall geologic framework of the Brooks Range foreland basin and for assisting in research studies concerning the oil and gas potential of the region.

The original RCS lines were created by combining selected seismic lines into long regional lines that cover the whole of NPRA in a nominal 20 x 20-mile grid. Twelve (12) lines are oriented generally north-south and ten (10) lines are oriented generally east-west (Figure 1, Table 1). The seismic lines which composed each of the original RCS lines were the stacked, unmigrated, record sections that were processed by Geophysical Service, Incorporated (GSI) immediately after the data were collected (1974 through 1981.)

Although the processing of the original RCS lines was done well, we encountered the following limitations when attempting to interpret the data:

1. The data were not migrated. The probable reason for not migrating the data is that at the time of the original processing, migration was a time-consuming and expensive process.
2. Except for a few lines, the magnetic tapes containing the original RCS lines could not be located and therefore we were unable to perform the migration process or to load these data into a modern seismic interpretation workstation for on-screen interpretation. We had digital versions of most of the stacked, unmigrated line segments which made up the RCS lines, but these had been processed by widely varying means and needed additional processing before being combined (discussed below.)
3. The individual seismic lines which made up a single RCS line were recorded in different years using different recording parameters, the most significant parameter being different distances between geophones on the surface ("group intervals".) These different group intervals meant that the horizontal resolution along each RCS line could change at the line segment boundaries. Within a single RCS line, the distance between seismic traces could be 165 ft. for one line segment, 110 ft. for another segment and 55 ft. for a third segment. To compensate for this difference in trace spacing, the original line segments were displayed on mylar film at different horizontal scales and then physically spliced together by hand. The result was an RCS line having a constant horizontal scale which was sufficient for visual inspection of the data. However, in order to digitally process a seismic line through migration, the actual distance between traces must be constant.
4. The data were processed using a "floating" datum that was a smoothed approximation of the surface elevations. The datum values used at line intersections were not always the same and therefore, the reflections did not tie in two-way travel time.

REPROCESSING

Because of the limitations described in the previous section, and in order to properly prepare the data set for migration and for interpretation on a computer workstation, we performed the processing steps described below. Two of processing algorithms used were developed in-house and are noted below. All other processing was performed using the industry-standard ProMAX^® seismic data processing system developed by Landmark Graphics^®.

1. We reconstructed the RCS lines from the individual stacked, unmigrated seismic lines for which we had digital data (table 2). This "post stack" processing included amplitude scaling, frequency filtering, and noise burst editing, in order to create versions of the data that were relatively consistent in character from line to line.
2. For those line segments for which we had no digital stacked data (table 2), we started with the raw field data and completely reprocessed each line through the steps described in #1, immediately above. The processing performed prior to stacking included editing of noisy data, amplitude scaling, spiking deconvolution, datum statics, Normal Moveout (NMO) corrections using velocities determined by interactive velocity analysis, and residual static corrections calculated by a surface consistent residual statics routine.
3. For all the lines, we adjusted the datum to a common horizontal sea level datum.
4. We converted the distance between traces to a constant of 110 ft., either by interpolation or decimation. A constant trace-to-trace interval is necessary in order to perform migration using the algorithms commonly available in commercial seismic data processing systems.
5. We migrated the data using smoothed stacking velocities adjusted to the sea level datum. For the line segments that were processed using post-stack methods only, we used the stacking velocities that were printed at the top of each segment’s mylar display. For those line-segments that were processed beginning with the field tapes, we used the stacking velocities that we determined (refer to step #2, above.)
6. After migration we applied post stack predictive deconvolution (using the 2^nd zero crossing of the autocorrelation function as the predictive distance) in order to increase the temporal resolution of the data. The software used for this process was developed in-house.
7. After predictive deconvolution, we applied a signature deconvolution process (sometimes referred to as wavelet deconvolution) to compensate for changes in the source waveform between lines and to further increase the temporal resolution of the data. The software used was developed in-house and was a version of Gray’s (1979) Variable Norm deconvolution method, adapted for post-stack data.

This processing produced a data set that is well-suited for analysis on a seismic interpretation workstation. Reflections on the seismic lines tie in both travel-time and waveform at all line intersections and the distance between individual traces on all seismic lines is a constant 110 ft.

Table 1. List of seismic lines contained on this CD-ROM with total mileage, geographic orientation, SEG-Y format file name, and SEG-Y format file size of each line. Click on any line name in the column labeled “Line” to display a graphic image in PDF-format of the stacked, migrated record sections.  The links in the column labeled “SEG-Y files” are to the SEG-Y format files for each line. The files are very large files and require special processing software. These files can be downloaded by:

* Macintosh: hold down the option key and click on the link with your mouse.
* Windows: right click on the link with your mouse.

Table 2: List of the component lines that make up the regional lines. Asterisks (*) indicate those lines which were completely reprocessed from the field tapes. All other line segments were reprocessed from the stacked, unmigrated data previously processed by industry.

NAVIGATION CONVENTION

Interactive seismic interpretation software requires a unique index number to relate each seismic trace in a line to a surface position on a map. Further, migration algorithms assume that for each seismic line, the distance between consecutive traces is at some regular, constant interval and as described above, we reprocessed the data to meet this requirement, making the distance between any two seismic traces a constant of 110 ft. For many 2-D seismic surveys, the unique index number used is the line’s shotpoint number, but because the regional lines were created by combining different line segments, the original shotpoint numbers for any given regional line are not necessarily unique. For example, the first line segment of a regional line might have shotpoint numbers 10 through 50 and another line segment of the same regional line might have shotpoints 30 though 80. In order to have a unique number for each trace in a regional line, we renumbered the traces starting at 1 for the first trace in the line and then increased the number by one, sequentially through the last trace in the line. We stored this unique number in the header of each seismic trace in the word reserved for the Common Depth Point (CDP) number, and used this unique number as the index number for the navigation data.

For each line segment, Latitude and Longitude coordinates for the surface position of each shotpoint were available. For each regional line, we combined the navigation data for the various line segments together and changed the shotpoint numbers to the associated, unique, CDP (index) number. The file NPRANAV.SEG contains location coordinates for all of the regional lines contained on this CD-ROM. The location map (figure 1) was generated directly from this file. Users loading these data into their own interpretation workstation should consider the unique CDP number to be the "shotpoint" number.

There is no consistent formula to relate the original shotpoint number of each line segment to the CDP number of the regional line. Depending on the line and the interpolation needed, there could be 16, 12, 8, 4, or 2 CDP’s (traces) between shotpoints. However, file CDP_SP.TXT contains a table giving a cross-referenced list relating regional line number, to original line number, to original shotpoint number, and to CDP number for each regional line.

DESCRIPTION OF THE DATA FILES CONTAINED ON THE CD-ROM

The following information is provided on this CD-ROM:

Seismic Data Files (Files R_1.sgy to R_20.seg; R_1ALT.sgy; R_13ALT.sgy): These are the processed seismic data in SEG-Y format. The SEG-Y seismic data format standard was developed by the Society of Exploration Geophysicists (SEG) in 1975 and has been in common use throughout the geophysical industry since that time. A complete description of the SEG-Y format is given in Barry and others (1975). All of the seismic data files in this publication are disc-images of the SEG-Y data format described therein. That document assumes 9-track, magnetic tape recording media; naturally, the inter-record and inter-block gaps described for magnetic tape media will not exist on the disc-image. The disc-image is therefore a continuous file composed of a 3200-byte EBCDIC header, a 400-byte binary header, and data traces, each composed of a 240-byte trace-header followed by a fixed number of seismic data samples. The data samples are type 2: fixed point integer (2 bytes (16-bits) per sample). The SEG-Y format provides a large number of trace header-words for the purpose of recording information related to each trace. Because the data on the CD-ROM are stacked, processed data, only the following two header words have valid information:

1) CDP number: bytes 21-24, the standard trace header word reserved for the Common Depth Point (CDP) number. This CDP number begins at 1 for the first trace of each line and increases by 1 sequentially to the end of each line. These are the reference numbers used in the navigation file to define the location of each trace.

2) Shotpoint Number: bytes 17-20, the standard trace header word reserved for the energy source point number. These shotpoint numbers are those from the original seismic lines which make up each regional line. As mentioned previously, file CDP_SP.TXT contains a table giving a cross-referenced list relating regional line number, to original line number, to original shotpoint number, and to CDP number for each regional line.

Navigation File (NPRANAV.SEG): This is the location data in SEG-P1 format, the standard format for latitude, longitude, x, and y coordinates, and surface elevation, as defined by the Society of Exploration Geophysicists. Navigation information is given for the first and last trace of each line and traces within each line at an interval of 25 traces.

Digitial Map Image (MAP.PDF): This file is a digital image in Adobe Acrobat^® PDF format, containing a map view of the trace locations, generated directly from file NPRANAV.SEG. The label for each line number is also a hyperlink that allows the user to view the associated digital image of the seismic line (see below).

Cross Reference File (CDP_SP.TXT): A table giving a cross-referenced list relating regional line number, to original line number, to original shotpoint number, and to CDP number for each regional line. A printout of the first few lines in this file are given below:

Digital Images of Seismic Displays (Files r1mt.pdf through r20mt.pdf, r1altmt.pdf, r13altmt.pdf): Small scale displays of each line, generated from a GEOGRAPHIX/SEISVISION^® work station at a horizontal scale of 1:250,000 for distance and 1.5 inches/second for two-way travel time ("PRN" files). Although the vertical exaggeration of these displays is approximately 6:1 (assuming an average velocity of 10,000 ft/sec), the longest line (R-15) is about 78 inches long. The PRN files from SEISVISION were converted to compressed Adobe Acrobat^® PDF files using the "Distiller" function in Adobe Illustrator^®. The PDF files can be opened, viewed, and printed using the free downloadable version of Adobe Acrobat^® Reader 4.0.

Notes on display annotation: There are two sets of numbers at the top of each display, one labeled Shot indicating Shotpoint number, the other CMP which is an acronym for Common Mid Point and is equivalent to the term CDP used elsewhere in this document. On the displays, these numbers are the same because as described above, we made the Shotpoint number equal to the CDP number in order to have a unique index number for each trace within a line.

Display Software (Files PLOTSEIS.EXE and PLOTSEIS.TXT): An IBM P/C compatible seismic display program, PLOTSEIS.EXE, is included on this disc for the purpose of displaying the SEG-Y data files. These programs are also published separately by Zihlman (1996). Users should follow the installation/operating instructions contained in the documentation file PLOTSEIS.TXT. To run PLOTSEIS directly from the CD-ROM, it may be necessary to specifically type the path name without a backslash character. For example, if the drive letter is Z, the default path will appear as "Z:\", but you should type "Z:" only. After doing this a menu of files will appear on the screen. You can then select a ".sgy" file and display the traces on the screen.

No display software is provided for any other systems.

DISCLAIMER

This Compact Disc (CD) publication was prepared by an agency of the United States Government. Neither the United States Government nor any agency thereof nor any of their employees makes any warranty, expressed or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.

Although all data and software published on this CD-ROM have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and (or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data, software, or related materials.

SYSTEM REQUIREMENTS

This disc will operate on any hardware platform capable of reading the ISO 9660 standard. The seismic data files are disc images of the SEG-Y format (Barry and others, 1975). The display programs and other software packages can access the data files directly from the CD-ROM. Technical information is contained in files in ASCII text format, MS-WORD97 format and Internet format. Graphics files are contained in Adobe Acrobat^® PDF format.

Minimum hardware/software requirements are as follows:

All platforms: CD-ROM drive with ISO 9660 software driver, software capable of reading SEG-Y format data files, ASCII text files, and PDF-format graphics files. Technical information can also be accessed by software capable of reading MS-WORD97 format or Internet format files.

IBM-compatible P/C: To use the display software: 640K main memory, hard (fixed) disk, monochrome monitor, EGA/VGA/SVGA graphics, MS or PC-DOS 3.1 or higher. See the next section for details of the IBM P/C-compatible seismic display software provided on this CD-ROM.

FIGURES

Figure 1. Digital image in Adobe Acrobat^® PDF format, containing a map view of the trace locations, generated directly from file NPRANAV.SEG. The label for each line number is also a hyperlink that allows the user to view the associated digital image of the seismic line in Adobe Acrobat^® PDF format.

TABLES

Table 1. List of seismic lines contained on this CD-ROM with total mileage, geographic orientation, SEG-Y format file name, and SEG-Y format file size of each line. Click on any line name in the column labeled “Line” to display a graphic image in PDF-format of the stacked, migrated record sections.  The links in the column labeled “SEG-Y files” are to the SEG-Y format files for each line. The files are very large files and require special processing software. These files can be downloaded by:

Macintosh: hold down the option key and click on the link with your mouse.
Windows: right click on the link with your mouse.

Table 2: List of the component lines that make up the regional lines. Asterisks (*) indicate those lines which were completely reprocessed from the field tapes. All other line segments were reprocessed from the stacked, unmigrated data previously processed by industry.

REFERENCES CITED

Barry, K.M., Cavers, D.A., and Kneale, C.W., 1975, Recommended standards for digital tape formats, in Digital Tape Standards: Society of Exploration Geophysicists ["Recommended standards for digital tape formats"] reprinted from Geophysics, v. 32, p. 1073-1084; v. 37, p. 36-44; v. 40, p. 344-352, p. 22-30.

Gray, W. C., 1979, Variable norm deconvolution, Ph.D thesis, Stanford University, Stanford, California. 101p.

Gryc, G., 1985, The National Petroleum Reserve in Alaska, Earth-Science Considerations, U.S. Geological Survey Professional Paper 1240-C, 94p.

Ikelman, J.A. (editor), 1986, Catalog of Geological and Geophysical Data for the National Petroleum Reserve in Alaska – Key to Geophysical Records Documentation No. 16 (Revised Edition), National Geophysical Data Center, Boulder, Colorado, p. 24-25.

Zihlman, F.N., 1996, PLOTSEIS, A DOS display program for SEG-Y formatted seismic data: U.S. Geological Survey Open-File Report 95-516, 7p., 1 - 3/1/2" disk.