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Salt Imaging

Salt bodies present an interest in the oil and gas industry because of their association to specific structures that allow the accumulation of economic reserve of oil. The uplift or formation of such bodies lead to deformations in the subsurface that are typically faulting, anticlines, syncline and graben [1].  Salt domes are typically mound like formations that form as intrusion in overlying layers[2].They occur as a result of uplift of evaporites that were buried millions of years ago. As the sediment accumulate in the basin, the increase in pressure causes the salt to migrate upward [1]

Problems in Seismic imaging

It is often difficult to acquire good quality images of Salt bodies. Several physical features related to them hinder the success of seismic methods for their resolution. Their complexity in geophysics arises from the steepness of their flank which are dipping vertically, the surrounding strata, the very high acoustic impedance and velocity difference that occur at the interface  between the Salt formation and the   surrounding sedimentary strata.[3] All of these features that interact oddly with the seismic energy are sources of errors within the data that can lead to misinterpretation of the seismic image.

On a more precise level the seismic issues that are related to salt bodies have to do with:

             The complexity of the ray paths of seismic energy near the Salt,

Seismic velocity anisotropy,

Wave mode conversions,

Reflected refractions.

Acquisition limitations

Complex ray paths

The complex geometry of salt bodies causes irregularities in the travel paths of seismic waves. The propagation of sound waves through the salt formation experiences changes in direction that makes the collection of data unclear. Standard migration of seismic waves is based on the assumption that the waves do not change path on their way down and their way up. In the case of Salt, the direction of propagation changes on both ways between the surface and the reflector. The complexity in data arrival related to double bounces and the directions of propagation in the Salt create feature on the seismic that display a false base within the salt image [3]     

Seismic velocity anisotropy

The chemical composition of the salt body is very diverse. Different evaporite minerals can be found within the diapir. Usually, the model for depth imaging of salt is built upon the assumption that the formation is purely halite. This model takes into account a seismic velocity of 4500 m/s through the body. However, the  presence of other evaporites and their distribution within the diapir can mislead the interpretation of its thickness and the detection of features that are due to the location of layers of evaporites. Indeed, the speed of the seismic wave and  the time arrival help to determine the thickness of beds. In case the velocity model is not well chosen, the image may present misleading characteristics. [3]

Wave mode conversions

Conversion from P wave to S wave occurs at reflector interfaces. Ignoring that phenomenon can lead to interpretation errors. Compressional waves are usually the only types of waves to be considered for seismic imaging. However, if wave mode conversion is ignored, the resulting features may be considered as noise in the data and mislead the interpreter.[3] Since, salt diapirs are found offshores, in marine environment, We may not expect longitudinal waves. They may not travel through water but they are produced when seismic energy interact with the diapir.  [3]

reflected refractions at salt wall

The steep walls of the salt diapir can cause  reflections of down going refracted waves  along  horizontal strata interfaces that are next.  This can produce erroneous interpretation of the seismic image. The seismic interaction on the horizontal reflector  will cause the refraction to travel upward as head wave. The consequence would be  a double bounce reflection signal at the receivers. Some confusions may  arise as some raypaths will cause unclear traces in the revesrse time migration.[3]

References

1-https://www.britannica.com/science/salt-domes

2-https://geology.com/stories/13/salt-domes/

3-Seismic imaging in and around salt bodies Ian F. Jones and Ian Davison