I want to know the different criteria for identifying the basement surface on seismic reflection sections, assuming no boreholes reached this surface. Please support with references if possible.
Chaotic and homogeneous reflections sometime characterised by crisscrossing multiples, smiles, and diffracted reflections. In some instances, you can find very high amplitude and disorganised reflections (sills) within these chaotic package. Where the basement is volcanic, you can see seaward dipping reflectors SDRs and downward V-shaped edifices that sharply truncates adjacent stratified reflections. Take a look at several works of Sverre Planke, Hansen and Cartwright, Smallwood and Maresh, Alves et al 2015 etc
Dr. Kamaldeen
Thank you for your answer. I'll take a look at these works.
Dear Ahmed: your question is very interesting because the geological basement is an important reservoir target for oil and gas. Please, let me share with you these general ideas:
FIRST OF ALL:
1- You need to remember that the geological basement such as metamorphic or igneous rocks, perhaps is different as the geophysical basement: not only the magnetic, and gravity, but also the seismic basement.
2- The geophysical basement images are related with the relative contrast of acoustic - elastic (seismic), density (grav) and magnetic susceptibility (mag) properties.
3- The strongest contrast, the better illumination.
4- The strongest geophysical relative contrast is the magnetic susceptibility of the rocks. It reachs two, three, or more, orders of magnitude.
5- The "seismic" basement has just a 5%, 10%, 15% of acoustic contrast. So, it is difficult to illuminate, and seismic data quality INSIDE the "seismic" basement has a low SNR.
6- Because of the absence of stratified, coherent reflectors, illumination of basement faults is more problematic than illumination interfaces within the sedimentary column.
7_ The seismic / acoustic / basement, you can imagine as a complex media, for example: raypath distortion, multiples, difractions.
8- The P-wave acoustic reflection coefficient at an interface separating two media (sediments and basement in this particular case) is known to vary with the density and velocity. But, a multiplicity of factors influence seismic reflection coefficients, e.g., mineral composition, matrix, comentation, porosity, fluid content, Depth of burial (pressure) …
SECOND ONE:
The answer of your question is different if you use:
1- seismic - paper section.
2- WS seismic interpretation.
If you are at 1-, maybe we are talking about an "old" data, without migration, maybe 2D sections. If is your scenario, maybe you can see a good bottom reflector, with difractions, chaotic reflections, and it is your boundary.
If you are at 2, you must apply "atributes" (complex seismic trace analysis) in order to study basement.
Due to the complexity of this non-layered target, it is important to enhance seismic data processing, reducing noise and multiple for improving the faults and fractural zone, such as using filters, stacking of middle and far offsets, Kirchhoff migration PSDM, CBM, attributes such as structural dip and azimuth, structural curvature, amplitude gradients, amplitude curvature. The criteria to try to identified the basement could be:
1- Attributes such as impedance inversion, could be more usefull.
2 Geometric attributes such as coherency, curvature, have been successful in delineating the basement in sedimentary basins.
Please, see the seismic section that I attached.
Best wishes sir, Mario
Dr. Mario
Thank you for your detailed answer. It is really perfect.
Not so simple. It also depends on your "basement definition" , the nature of the rocks involved (metasedimentary rock, crystalline basement, weathered basement, ect...) and the deformation.
In many case the top often fits with a strong amplitude reflections and erosional surface above transparent/chaotic facies (or well layered if you are dealing with a basement shear zone).
In both cases, I suggest you to test your interpretation with potential field (gravity/magnetic) to be sure.
Dear Ahmed Abdelmaksoud,
You have gotten excellent responses from previously responses from before responder…
In plus … if you have the possibility to collect information about subbasement lithology directly if they have outcrop presence, because every basement has outcrops…
Perhaps the next articles about an alternative interpretation of gravity should be good for the better result:
https://www.researchgate.net/publication/327201504_Gravity_a_paradym_shift_in_reasoning
https://www.researchgate.net/publication/277364319_Tenyekkel_igazolhato_a_gravitacio_valos_oka
(in Hungarian… I have a translation made by me but without correction, linguistically)
Regards,
Laszlo
Thank you Dear László for your answer and for sharing your work and experience.
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