AAPG EXPLORER GEOPHYSICALCORNER The Geophysical Corner is a regular column in the EXPLORER, edited by Satinder Chopra, chief geophysicist for Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer. This month’s column deals with a basic question: What is seismic interpretation? What Is Seismic Interpretation? By ALISTAIR R. BROWN Seismic Interpretation is the extraction <strong>of</strong> subsurface geologic information from seismic data. On that definition we all are agreed. However, if we seek a more penetrating explanation, we find practitioners get tongue-tied and talk around the subject in a variety <strong>of</strong> ways. In this article I attempt to give a longer, more descriptive definition that will apply to every interpretation project involving reflection seismic data. The danger in seismic interpretation is in thinking that everything we see is geology! * * * Reflection seismic data comprise: u Continuity <strong>of</strong> reflections indicating geologic structure. u Variability <strong>of</strong> reflections indicating stratigraphy, fluids and reservoir fabric. u The seismic wavelet. u Noise <strong>of</strong> various kinds and data defects. Seismic interpretation is the thoughtful procedure <strong>of</strong> separating these effects. The seismic wavelet starts as the pulse <strong>of</strong> seismic energy, which, generated by the energy source, travels down through the earth, is reflected and travels back up to the surface receivers carrying the geological information with it. This recorded wavelet is minimum phase <strong>of</strong> some frequency bandwidth, and during data processing it is converted (we hope) into a zero-phase wavelet, making interpretation easier and more accurate. The interpreter is not directly interested in the wavelet itself but rather in the geological information that it carries. Thus, understanding the wavelet and distinguishing its characteristics from details <strong>of</strong> the geology is one <strong>of</strong> the critical tasks <strong>of</strong> today’s interpreter. * * * Noise is ever-present in seismic data. It may be random noise, it may be multiple reflections, it may be refracted energy, it may be other energy <strong>of</strong> unknown source. The data may suffer defects because <strong>of</strong>: 3 Irregular data acquisition showing as footprint. 3 Obstacles to the data acquisition crew. 3 Equipment difficulties in the field. 3 Processing problems. The interpreter must know enough about the acquisition and processing to recognize these undesirable features, and thus to not confuse them with the geology he/she seeks. * * * Seismic energy is reflected from interfaces where the acoustic properties <strong>of</strong> the rocks change. These interfaces follow sedimentary boundaries created at the time <strong>of</strong> deposition <strong>of</strong> the sediments. Following the continuity <strong>of</strong> these reflections then defines for us the structure imposed on these boundaries by the tectonic forces <strong>of</strong> geologic history. Following this continuity and making structure maps is thus the most basic, and most traditional, activity <strong>of</strong> seismic interpretation. To aid in this endeavor the seismic The horizon track on Lines 57 and 60 defining the structure, and the Horizon Slice sliced through the data volume 40ms below. (From Interpretation <strong>of</strong> Three-Dimensional Seismic Data, AAPG Memoir 42, SEG Investigations in Geophysics No. 9, Seventh Edition, 2011.) BROWN interpreter can manipulate the data and the display in various ways. The time-honored approach to prepare the data for structural interpretation is to apply AGC (Automatic Gain Control) in the late stages <strong>of</strong> data processing. This reduces amplitude variability (where most <strong>of</strong> the statigraphic information lies), and hence increases visible data continuity. The interpreter also may compress the display color bar to optically saturate and thus to render invisible more <strong>of</strong> the amplitude variations. Other techniques include the use <strong>of</strong> Instantaneous Phase (which completely destroys amplitude information) and Structurally Oriented Filtering. All these are good ideas – provided the interpreter realizes that they are directed at structural interpretation only, and that the requirements <strong>of</strong> later, more advanced types <strong>of</strong> seismic interpretation are quite different. * * * Once the structure has been established, the interpreter turns his Policy Watch from page 58 u As a guide to the present and future role <strong>of</strong> the federal government in STEM education, the National Science and Technology Council released a December 2011 report, “The Federal Science, Technology, Engineering and Mathematics Education Portfolio.” The report found that in fiscal year 2010 the federal government spent over $3 billion on STEM initiatives – less than 1 percent <strong>of</strong> all education funding in the United States. Much <strong>of</strong> the federal STEM funding targeted economically disadvantaged and minority groups. Federal STEM programs supported: minority-serving colleges and universities, improving 60 MAY 2013 WWW.AAPG.ORG The danger in seismic interpretation is in thinking that everything we see is geology! attention to stratigraphic interpretation and the detection <strong>of</strong> hydrocarbon fluids. Overwhelming important here is seismic amplitude – and the amplitude may be presented to the interpreter or extracted from the data in various ways. The data loaded to the workstation must be True Amplitude and Zero Phase, and the interpreter must satisfy himself that the data used are such. Understanding the wavelet is complicated and very important (part <strong>of</strong> the fundamental separation <strong>of</strong> effects) but outside the scope <strong>of</strong> this article. In order to increase the visibility <strong>of</strong> stratigraphic variations the interpreter will remove the structure – and the best way to do this is to make a Horizon Slice. The concept behind the Horizon Slice is the reconstitution <strong>of</strong> a depositional surface at a key point in geologic history. The structure used for the reconstitution is most commonly defined at the level <strong>of</strong> the objective. However, it is <strong>of</strong>ten better to define the structure at one level (conformable with the objective) and to use this to remove the structure at the curricula and teacher effectiveness for K-12, and increasing students’ knowledge and interest in STEM. Finally, yet importantly, many nonpr<strong>of</strong>it groups and volunteers work to encourage students to choose STEM careers – and support their education. AAPG Foundation provides grants to many STEM education initiatives, and the Youth Education Activities committee coordinates projects and volunteers. A few example projects are: u Continuing education courses and course scholarships for middle school and high school earth science teachers. u The <strong>American</strong> Geoscience Institute Earth Science Week. u The E.F. Reid Scouting Program, which supports Boy Scouts, Girl Scouts and other youth organizations, and objective level. This very effectively separates structure into step one and stratigraphy into step two. This procedure is illustrated in the accompanying figure. The horizon tracked on the two vertical sections follows a reflection with good structural continuity and little, if any, stratigraphic variability. The horizon track is then displaced downwards by 40 ms (a simple horizon computation on the workstation) to intersect the prominent red blob visible below it, and the amplitude is then extracted along the displaced track. The resulting Horizon Slice, on the right <strong>of</strong> the figure, shows a very clear channel (the spatial pattern <strong>of</strong> the red blob) with interesting amplitude variations along it. * * * When the seismic interpreter extends his analysis even further and enters the field <strong>of</strong> reservoir evaluation, the data requirements are even more stringent, but the Horizon Slice concept is still effective in removing the effects <strong>of</strong> structure. Some form <strong>of</strong> Inversion may be used here, and this process converts interface information (amplitude) into interval information (acoustic Impedance). The more advanced forms <strong>of</strong> inversion seek to remove the wavelet, and this is therefore part <strong>of</strong> the fundamental idea <strong>of</strong> separating effects. However, the challenge here is to exactly understand the wavelet that has to be removed. This is difficult, and many inversions suffer and projects fail because <strong>of</strong> this issue. So seismic interpretation is the thoughtful separation (with workstation assistance) <strong>of</strong> the various effects that the subsurface and the seismic acquisition process have mixed together! (Editor’s note: AAPG member Alistair Brown is a consulting reservoir geophysicist residing in Allen, Texas. He was the first joint AAPG-SEG Distinguished Lecturer, is the author <strong>of</strong> AAPG Memoir 42, “Interpretation <strong>of</strong> Three-Dimensional Seismic Data,” recipient <strong>of</strong> an AAPG Distinguished Service Award and a former editor <strong>of</strong> the EXPLORER’s Geophysical Corner.) helped develop the Boy Scout Geology Merit Badge. u Maps in Schools and Rocks in Your Head Programs, which <strong>of</strong>fer teacher training and classroom materials. u The Teacher <strong>of</strong> the Year award, which spotlights quality geoscience education. Members can learn about these activities, contribute to the AAPG Foundation and join the Youth Education Activities committee by visiting AAPG’s website. The GEO-DC blog will alert AAPG members on the progress <strong>of</strong> legislation reauthorizing the <strong>American</strong> COMPETES Act. Readers can subscribe to the blog on the GEO-DC web page, www.aapg.org/geoDC/. EXPLORER EXPLORER
AAPG EXPLORER WWW.AAPG.ORG MAY 2013 61