Monday, December 29, 2014

Sintetik Seismogram

Gambar sintetik seismogram (Badley, 1985)

Koefisien refleksi positif terjadi di interface medium litologi lunak menuju litologi yang lebih keras. Akan tetapi dalam kondisi riil, kontras kecepatan (velocity) lebih berpengaruh daripada kontras densitas. Trace seismik dihasilkan dari konvolusi koefisien refleksi (RC) dan wavelet. Pada gambar terlihat bahwa inteface batuserpih-batugamping membentuk RC yang positif dan signifikan, sehingga kontras impedansi yang dihasilkan juga signifikan.

Sunday, December 28, 2014

Kontras Impedansi Antara Dua Medium

Gambar 1. Tabel informasi energi yang terefleksi antara dua medium (Sherrif, 2002)

Pada tabel diatas terlihat jelas bahwa meskipun dua medium memiliki nilai densitas yang sama, tapi properti kecepatan akustik berbeda, akan menimbulkan kontras impedansi. Seperti pada interface sand-limestone atau limestone-sand, hal yang sama juga terjadi pada interface shale-limestone dan limestone-shale, kontras impedansi yang dihasilkan sangat mudah dikenali pada penampang seismik.

Tuesday, December 16, 2014

Poisson ratio

Although Lame's constants are convinient, other elastic constants are also used. The most common are Young Modulus (E), Poisson ratio , and the bulk modulus (k). The preceding theory assumes an isotropic medium. Rocks, especially sedimentary and metamorphic rocks, are frequently not isotropic. In discussing wave propagation we generally ignore such differences and treat sedimentary rocks as isotropic media.

Poisson ratio must have values between 0 and 0.5, since both lamda and mu are positive. Values range from 0.05 for very hard, rigid rocks to about 0.45 for soft, poorly consolidated material. Liquids have no resistance to shear and hence for them mu=0 and poisson ratio=0.5 . 

Tuesday, December 2, 2014

Flat Spots

As a rule of thumb, flat spots are likely to be found in porous sandstones or carbonates down to about 2.5 km. Below this depth the effect of gas on velocity is less marked and the chance of getting a good reflection from a gas contrast is reduced. Flat spots will always have positive reflection coefficients, appearing as a trough on seismic sections displayed with SEG normal polarity or a peak on reverse polarity sections. Although gas contatcs are usually horizontal in depth, they do not always appear horizontal in time due to the push-down effect of the lower velocity in the gas interval.

Flat spots are perhaps the best indication of gas, athough other diagnoistic acoustic-impedance changes between the cap rock and gas-bearing reservoir affect the amplitude and polarity of the top-reservoir reflection.

Amplitude anomalies fall into two groups :
1. Anomalies of very high amplitude, commonly termed bright spots
2. Anomalies of very low amplitude, commonly termed dim spots

Bright spots are usually associated with porous sand. In a typical sand-claystone sequence, the claystone-to-sand reflection is ordinarily positive and of medium strength for water or oil saturated sands of poor porosity. The reflection becomes weak or nonexistent with higher porosity.