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Geophysical StratigraphyG342 Sedimentation and StratigraphyLecture 20: geophysical stratigraphy2 May 2006Assoc. Prof. A. Jay Kaufman Geophysical StratigraphyElectrical stratigraphyDue to the lack of surface outcrop in many areas geophysical methods of correlation havebeen developed. The oldest of these is sub-surface well logging, which provides a direct sampleof lithologies hidden beneath. In modern drilling a bentonite mud is pumped down through thedrill pipe to cool and lubricate the drill; as the mud is forced to the surface it brings rock chips upwith it that the site geologists describes and in some cases preserves.A device called a sonde is lowered into the hole and as it is raised it measures variouselectrical and physical properties of the surrounding lithologies, which can then be correlatedwith the assembled core. Two of the most common are the spontaneous potential (SP) andresistivity (R). A SP log measures the difference in electrical potential between electrodes at thesurface and the sonde, which is related to the movement of ions between fluids in the drilledformation and the borehole. In essence, this is the permeability of the rock. The R log measuresthe resistivity of the fluid in the surrounding rock to an applied electrical current, which is anindirect measure of the porosity of the lithology. Resistivity increases with decreasing porespace. 1A dipmeter is another geophysical tool that measures resistivity in four directions, thusproviding information about the attitude of the strata. This information helps in the recognition ofsub-surface folds and faults, and in paleoenvironmental interpretation.A gamma ray log measures the natural radioactivity of the strata, usually associated withthe decay of 40K in minerals of the surrounding rocks. This method is sensitive to rocks that arehigh in potassium, thus shale and arkose, as well as muddy and lithic sandstone give high gammaray values.Seismic stratigraphyLong used to determine the interior structure of the Earth, seismic stratigraphy hasbecome the most powerful tool in oil field stratigraphy. With this tool, it is currently possible to: 1) recognize the shape of stratigraphic sequences 2) interpret their depositional histories 3) recognize unconformities and reconstruct transgressional-regressional history of an area4) detect fluid contents of rock5) identify hydrocarbon accumulations In both two and three dimensional imagery one is now able to resolve areas of tens tohundreds of meters.2Peter Vail contributed mightily to the science of sequence stratigraphy, but in essencecreated the science of seismic stratigraphy. His enormous contribution came from a keyconceptual breakthrough – that seismic reflections approximate chronostratigraphic surfaces.Key Terms:reflector: A boundary that creates a seismic reflection (see impedance contrast)reflection: The acoustic waves created by sound bouncing off of a reflector.impedance: A physical rock property of sound propagating through rock.impedance contrast: A physical boundary within rocks producing a reflection. It does NOT haveto be a rock boundary, and can be a fluid contact, a diagenetic effect, or other. A positiveimpedance contrast produces a positive reflection, a negative impedance contrast theopposite.wavelet: The acoustic wave which, when convolved with impedance boundaries, produces areflectionimpedance = rock velocity * rock densitySeismic attribute:The “wiggles” of seismic reflections (traces) have a number of aspects that can bemeasured: amplitude, duration (in two-way travel time), area, etc. These attributes can bequantified and mapped. Those numerical and map results show tremendous potential to predictsubsurface characteristics of rocks.Reflection seismology is the science of making a big bang and listening for its echoes.Sound waves travel in all directions, but only those that travel almost directly downward. Onland, the returning sound wave is detected by a geophone.3Seismic profiles are not stratigraphic cross sections. The vertical scale is two way traveltime. If the seismic velocity is the same in all the underlying rocks then the travel time andthickness are linearly related. A seismic reflection is made by a change in seismic velocity,usually due to a density contrast, or acoustic impedence.Seismic sequences are analyzed by marking the unconformities in a seismic section anddetermining their degree of onlap or offlap and their ages. These can then be plotted as a series ofunconformity-bounded sequences with a time axis. The lateral extents of the units are shown inthe image, while the missing time is represented by the vertical lines. From this diagram, a curveof relative onlap and offlap can be constructed, which inform us about relative sea level changethrough time.The most widely applied use of seismic stratigraphy is in the interpretation ofunconformities. Each unconformity bounded package on a seismic profile is called a seismicsequence. After a number of passive margin sequences were analyzed it was noted that the ageand magnitude of unconformities were about equal, suggesting a global control, which wasinferred to be sea level. Hence sea level curves through all of the Phanerozoic have beenconstructed from seismic


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UMD GEOL 342 - Geophysical Stratigraphy

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