To locate underground natural gas deposits, AleAnna Resources chooses the most efficient and cutting-edge technologies. One of these is AVO analysis, which allows geophysicists to determine a rock’s characteristics, such as porosity, density, and fluid content.
WHERE WERE WE
As we said in our previous article, thanks to the elastic waves sent into the earth, it is possible to build a very accurate picture of the subsoil: vibroseis generate the waves, which pass through the ground and are then bounced back to the surface, where they are recorded by geophones.
Once we obtain this x-ray of earth’s interior, gathered data are filtered and optimised for interpretation.
One of the most cutting-edge method for studying this kind of information is AVO analysis.
AVO analysis is an extremely innovative technique that geophysicists use to determine, by starting from seismic data, a rock’s fluid content and its petrophysical characteristics, such as porosity, density, shear wave information, and – above all – the presence of hydrocarbons.
FIRST ISSUES WITH TERMINOLOGY: Amplitude vs Offset
Let’s start by clarifying the first technicalities, without being intimidated: it is a complex terminology, but – once explained – it will be instantly clear.
In Geophysics, “Amplitude Versus Offset (AVO)” or “Amplitude variation with Offset” is the general term for referring to the dependency of the seismic attribute called amplitude, with the distance between the source (vibroseis) and the receiver (geophone) called offset.
So the Offset is the link between vibroseis (i.e. the source), which generate the elastic waves, and every single geophone (i.e. the receiver). which record the data bounced back to the surface.
It is clear that the waves bounce off the geological strata in different ways: it depends on the angle of incidence. The basic physics behind AVO analysis is that the strength of a reflection does not only depend on the acoustic impedance, but also on the angle of incidence.
Hence, the principle of AVO is that the amplitude of a wave bounced back to the surface determines the distance between the source (i.e. vibroseis) and the receiver (i.e. geophone).
ANGLE OF INCIDENCE
Another technical term: don’t get discouraged! Sometimes, we are not familiar with the vocabulary, but we understand the concept. This is the case of the angle of incidence.
Let’s take a glass window, for instance. If you look straight through it, you can see perfectly well. But when you move to a wide angle, it suddenly becomes very reflective, up until you reach the critical angle (also known as limiting angle), that is the peak value beyond which the wave cannot pass through the surface and is completely reflected.
So, the principle of AVO is the relationship between reflection and angle of incidence, that is when the elastic wave – generated by vibroseis – cannot pass through rock strata and is then bounced back to the surface, where it is recorded by geophones.
Each wave has a different value: depending on a greater or lesser angle of incidence, each wave is bounced back to the surface near or further away from the source respectively.
The distance is deducible because the exact location of the source and receivers is always known.
MINIMISING RISKS, OPTIMISING RESULTS
Once completed the AVO analysis of a seismic project that shows positive outcomes, it becomes a new essential step for the research of natural gas, because it minimises risks and optimises results.
This is why AleAnna Resources uses the most advanced technologies available such as 3D seismic imaging and AVO analysis for its projects. This allows us to greatly improve our success margins and to minimise the environmental impact, by maintaining the highest performance and security standards, in compliance with current legislation.