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.