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.

Sunday, November 30, 2014

Geophysical Methods for Geothermal Exploration

Gravity

Gravimetry studies changes in density to characterize subsurface properties. This method is well applied when identifying dense subsurface anomalies including granite bodies , which are vital to locate in the geothermal exploration projects. Subsurface fault lines are also identifiable with gravitational methods. These faults are often identified as prime drilling locations as their densities are much less than surrounding materials.
Changes in groundwater levels may also be measured and identified with gravitational methods. This recharge element is imperative in creating productive geothermal systems. Pore density and subsequent overall density are affected by fluid flow and therefore change the gravitatonal field.



Magnetotellurics

Magnetotellurics (MT) measurements allow detection of resistivity anomalies associated with productive geothermal structures.
Geological materials are generally poor electrical conductors and have a high resistivity. Hydrothermal fluids in the pores and fractures of the earth , however, increase the conductivity of the subsurface materials. This change in conductivity is used to map the subsurface geology and estimate the subsurface material composition. Resistivity measurments are made using a series of probes distributed tens to hundreds of meters apart, to detect the electrical response of the earth.
Since flowing geothermal waters can be detected as zones of low resistance, it is possible to map geothermal resources using such a technique.



reference : http://en.wikipedia.org/wiki/Geothermal_exploration

Thursday, November 27, 2014

Velocity Anomalies Beneath Reefs

  Carbonate reefs are important reservoir rocks in many part of the world. Reefs are associated with rapid lateral facies and associated velocity changes, and are so complex that it its difficult to generalize about their detailed attributes. Reef limestone may have higher velocity than the adjacent facies (e.g., back- and forereef shales) ; or they may have a lower velocity where surrounded, for example, by denser limestones and dolomites. Both situations will give rise to velocity anomalies on a time section. Where the reef has a higher interval velocity than the adjacent strata, a velocity pull-up will develop. If the reef is draped by younger sediments, the pull up could suggest that the overall positive feature is a deep-rooted structure (i.e, prereef in age) and may lead to a pessimistically false geological interpretation. Alternatively, where the reef limestone has lower interval velocity than the surrounding sediments, the velocity anomaly results in a push-down, and the reef base could appear as a low on seismic time section. This is identical to the shale push-down and the distortion on the time section is open to misinterpretation.

Thursday, November 20, 2014

Static Corrections

Static corrections are probably the most critical processing step for land data. They are especially important in areas of rough terrain and where the near-surface velocity is highly variable-either laterally or horizontally. This near surface velocity affects reflection continuity, resolution, the accuracy of velocity analysis, and structrual form.

Usually before land data is analyzed for NMO velocities and stacked, corrections have to be made to account for anomalous effects caused by significant elevation or near-surface velocity changes caused by the water table, by alluival and by weathering.

Poor static corrections result in poor reflector continuity and poor velocity control. Accurate static corrections can make the difference betwen an outstanding section and one that is not usable. Static corrections are a processing problem over which the interpreter has little control, other than to request reprocessing.

However, there are some pitfalls in static corrections that can, in some circumstances, produce good-looking but structurally incorrect sections, and as a rule of thumb, beware of reflections that bear a relationship to the surface shape.

Static corrections for marine data involve correcting for the depth of source and hydrophones below the sea-level datum (theoretical mean sea level). However, if incorrect they can cause considerable problems for ties of well to seismic and in tying data from surveys with different static corrections


Reference : Badley, E. Michael. Practical Seismic Interpretation.