- Home
- A-Z Publications
- First Break
- Previous Issues
- Volume 6, Issue 1, 1988
First Break - Volume 6, Issue 1, 1988
Volume 6, Issue 1, 1988
-
-
High-resolution 3D reflection seismics on a tidal flat: acquisition, processing and interpretation
Authors J. Corsmit, W.H. Versteeg, J.H. Brouwer and K. HelbigThe academic teacher of exploration geophysics is faced with the quandary of how to provide hands-on experience of reflection seismic acquisition, by far the most important exploration method of our time. Gravity, magnetics and geoelectrics are well within the reach of even a small university department, and refraction seismics is probably done more often for research than for commercial purposes. Reflection seismics, however, requires expensive equipment and large crews. Even where reflection surveys are carried out in an academic context, the acquisition -and often the processing- is done by professionals; the consequences of even innocent mistakes are too dire to let the student learn by trial and error. This is true for any reflection seismic project, but a fortiori for three-dimensional (3D) surveys. As far as two-dimensional (2D) seismics is concerned, the didactical problem was solved at Utrecht University by working on tidal flats (Doornenbal & Helbig 1983). The ease with which high frequencies can be generated allows the survey parameters to be scaled down by at least a factor of 10. The resolution of the data thus acquired is nearly unparalleled on land and the small scale reduces the logistic parameters (sizes, weights, distances and cost) and the difficulties of supervision to a magnitude that can be handled easily. For nearly a decade all students of geophysics have acquired, processed and interpreted a meaningful amount of reflection seismic data. Fortuitously the results of this exercise provide significant data for a continuing sedimentology research project. In recent years the importanee of 3D seismics has steadily increased, so that the question arose as to how the specific features of the technique could best be introduced to the students. Moreover, the correct interpretation of the small-scale sedimentary features observed in 2D surveys on tidal flats often requires intersecting lines. The same flooding of the flats that helps us generate and transmit high frequencies makes the maintenance of positional markers difficult. Particularly after a storm it is almost impossible to relocate a line to an accuracy of 1 m. Therefore, data referring to one structure have to be acquired in a single field period. The survey described here was designed to test the possibility of expanding the mandatory field work to cover 3D seismics. It provided a starting point for the development of the necessary software and display technology. J. Corsmit and W.H. Versteeg (then students at Utrecht) were asked to develop the field technique, acquire a data set for test purposes, write the necessary expansions to the existing 2D processing package (Doornenbal & Helbig 1983) and carry out the preliminary interpretation. J. Brouwer and K. Helbig were involved in planning and supervision and are responsible for some of the display technology. The acquisition phase lasted two weeks. Three people were continuously involved, but a routine survey of this magnitude could, in principle, be carried out by two people (a total of about 120 man-hours). The data were originally processed on an HP-l000 minicomputer. Processing was time consuming since it was combined with programme development. To reprocess the data on our current Gould PN 6000 super-minicomputer takes about 8 h connect time (with known parameters). Interactive processing of a comparable new data set would require about 32 h. Several of the illustrations were prepared on the Gould PN 6000.
-
-
-
Ricker wavelets in their various guises
More LessThe so-called Ricker wavelets remain in vogue among many synthetic seismogram producers, though the physical basis for the theory of such wavelets was proved to be invalid many years ago. For most synthetic seismogram studies, however, the important thing is to be able to model the wavelet remaining after processing. The wavelets used are long-range approximations to the true Ricker wavelet and the advantages claimed for them are that they are easily computed and unambiguously specifiabie by one parameter only (a frequency or period). Unfortunately they not only differ from processed wavelets in many cases, but a variety of conventions regarding their specification has crept in over the years and now there is confusion over what any practitioner means by his particular 'Ricker wavelet'. This article reviews all the known forms (and some unknown ones) that Ricker wavelets can take and points out the possible confusions that can occur as well as their lack of accord with real or processed wavelets. A recently arrived form is the minimum-phase Ricker wavelet which is examined and shown to possess a time delay which depends on the sample interval used in computing it. A two-parameter Ricker-type wavelet is described that allows greater versatility. The main conclusions are that Ricker wavelets employed by others are to be treated with extreme suspicion, and that Ricker wavelets should never be used at all if one has any choice.
-
Volumes & issues
-
Volume 42 (2024)
-
Volume 41 (2023)
-
Volume 40 (2022)
-
Volume 39 (2021)
-
Volume 38 (2020)
-
Volume 37 (2019)
-
Volume 36 (2018)
-
Volume 35 (2017)
-
Volume 34 (2016)
-
Volume 33 (2015)
-
Volume 32 (2014)
-
Volume 31 (2013)
-
Volume 30 (2012)
-
Volume 29 (2011)
-
Volume 28 (2010)
-
Volume 27 (2009)
-
Volume 26 (2008)
-
Volume 25 (2007)
-
Volume 24 (2006)
-
Volume 23 (2005)
-
Volume 22 (2004)
-
Volume 21 (2003)
-
Volume 20 (2002)
-
Volume 19 (2001)
-
Volume 18 (2000)
-
Volume 17 (1999)
-
Volume 16 (1998)
-
Volume 15 (1997)
-
Volume 14 (1996)
-
Volume 13 (1995)
-
Volume 12 (1994)
-
Volume 11 (1993)
-
Volume 10 (1992)
-
Volume 9 (1991)
-
Volume 8 (1990)
-
Volume 7 (1989)
-
Volume 6 (1988)
-
Volume 5 (1987)
-
Volume 4 (1986)
-
Volume 3 (1985)
-
Volume 2 (1984)
-
Volume 1 (1983)