Seismik Inversi

Seismik inversi adalah proses pemodelan geofisika yang dilakukan untuk memprediksi informasi sifat fisis bumi berdasarkan informasi rekaman seismik yang diperoleh.

Upaya inversi merupakan kebalikan (inverse) dari upaya pengambilan data seismik (forward modeling).

Sebagaimana yang kita ketahui forward modeling adalah operasi konvolusi antara wavelet sumber dengan kontras impedansi akustik bumi (koefisien refleksi).

Proses inversi merupakan proses 'pembagian' rekaman seismik terhadap wavelet sumber yang diprediksi.

Berdasarkan gambar diatas kita melihat bahwa secara bebas dapat dikatakan bahwa impedansi akustik (hasil inversi) merepresentasikan sifat fisis 'internal' batuan sedangkan rekaman seismik merepresentasikan 'batas batuan'. Sehingga hasil inversi dapat digunakan untuk menginterpretasi perubahan fasies dalam suatu horizon geologi. (Sebenarnya bagi ahli geofisika, sifat fisis internal pun dapat 'dilihat' berdasarlam karakter amplitudo atau frekuensi rekaman seismiknya, anda setuju?).

Pemilihan 'wavelet yang diprediksi' pada proses inversi merupakan prosedur yang sangat penting, anda harus yakin betul bahwa sifat 'wavelet yang diprediksi' mencerminkan horizon yang menjadi target anda. Caranya ? diantaranya dengan mengekstrak wavelet pada horizon yang menjadi target inversi. Inipun tidak ada jaminan...karena sifat wavelet yang tergantung terhadap fasa dan attenuasi.

Gambar data real dan 'hasil inversi' diatas adalah courtesy Ashley Francis, Earthworks Environment & Resources Ltd. - U.K


Referensi : http://ensiklopediseismik.blogspot.com/2007/06/seismik-inversi.html

Sedimentology : Contour Currents

Density differences in surface ocean water caused by temperature or salinity variations create vertical circulation of water masses in the ocean commonly referred to as thermohaline circulation. Circulation is initiated primarily at high latitudes as cold surface waters sink toward the bottom, forming deep-water masses that flow along the ocean floor as bottom currents. The path of these bottom currents is influenced by the position of oceanic ridges and rises and other topographic features such as narrow passages through fracture zones. Owing to density stratification of ocean water, bottom currents adjacent to continental margins tend to flow parallel to depth contours or isobaths and thus are often called contour currents.

The movement of these currents is also affected by the Coriolis force, which likewise tends to deflect them (left in the Southern Hemisphere and right in the Northern Hemisphere) into paths parallel to depth contours; thus, they are sometimes also called geostrophic contour currents.

In the modem ocean, Antarctic bottom water runs down the continental slope, circulates eastward around the Antarctic continent possibly several times, and then flows northward into the Atlantic, Indian, and Pacific oceans (Stow, 1994). In the North Atlantic, deep bottom water flows south out of the Norwegian Greenland seas, Labrador Sea, and other parts of the North Atlantic. Interaction of these deep-water masses creates a highly complex ocean circulation system.

Because contour currents are best developed in areas of steep topography where the bottom topography extends through the greatest thickness of stratified water column (Kennett, 1982), they are particularly important on the continental slope and rise. Photographs of the deep seafloor have revealed current ripples in some areas and suspended sediment clouds and seafloor erosional features in others, both of which suggest that some contour currents can achieve velocities on or near the seafloor great enough to erode the seabed and transport sediment. Evidence is now available (e.g., Hollister and Nowell, 1991) which suggests that the speed of these bottom currents may be accelerated in some parts of the ocean to velocities on the order of 40 em/ s, perhaps because of the superimposed influence of large-scale, wind-driven circulation at the ocean's surface. That is, eddy kinetic energy may be transmitted from the surface of the ocean to the deep seafloor. Intensification may also occur where the Coriolis Force causes deep flows to bank up against the continental slope on the westem margins of ocean basins, where it is unable to move upslope against gravity and thus becomes restricted and intensified (Stow, 1994). Where bottom currents are intensified, resulting motions near the seafloor are so energetic that they have been referred to as "abyssal storms" or "benthic storms" (Hollister and Nowell, 1991), particularly because huge amounts of fine sediments are stirred up and transported by these energetic pulses. Contour currents are believed to have had a particularly important role in shaping and modifying continental rises, such as those off the eastern coast of North America.


Refference : Boggs Jr, Sam. 2006. Principles Of Sedimentology And Stratigraphy 4th Edition. Pearson Education, Inc. New Jersey.

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