Basalts represent a major exploration problem in many basins potentially attractive for oil exploration. Basalt layers cannot be regarded as homogeneous bodies, in fact, they show extremely variable physical properties. This is related to the emplacement mechanisms, where successive lava flows normally do not follow the same paths and are episodic. Sedimentation, weathering or erosion may occur between one extrusive event and another depending of the subaerial or subaqueous environments. From the geophysical point of view, this produces heterogeneous bodies characterized by vertically and laterally varying velocities with irregular (i.e. rugose) interfaces.

How does the above influence seismic imaging? A number of problems arise! Interbed multiples as well as larger period multiples can be significant and should be removed from the data to visualize the weak sub-basalt reflections. Velocity analysis is often difficult both in time and depth domains for the basalt and sub-basalt sections because of the poor S/N ratio. Related to this problem is the fact that basalt thickness and base basalt topography are generally difficult to estimate and that diffractions from the rugose base of basalts interfere destructively with primary reflections. Another source of complication relies in repeated conversion modes between P and S phases (and vice versa) that may occur during the downgoing or upgoing ray path. These problems can be overcome in many cases by adopting a correct approach for both data acquisition and seismic data analysis.

LONG OFFSET RECORDING GEOMETRIES

Data Acquisition requires essentially that seismic data be recorded with long-offset geometries and possibly low-frequency energy sources. Critical and post-critical wide-angle reflections as well as refractions become prominent at the large offsets and carry vital sub-basalt information, which are exploited during processing. Furthermore, P-S mode conversions, if present, are expected to show maximum energy at intermediate to large offsets.

FEASIBILITY STUDIES

3D ray-tracing of primary P-waves as well as converted P-S modes are used in conjunction with Finite Difference acoustic or elastic modeling to gain information on the type of seismic phases expected, their interaction and their potential for the sub-basalt imaging problem. The potential of long offset seismic phases for imaging can be evaluated using pre-stack depth migration imaging algorithms developed to handle the long-offset propagated reflected energy.

Synthetic data showing the contribution of long offsets for imaging layers beneath high-impedance acoustic barriers.

SEISMIC IMAGING IN DEPTH

While acceptable imaging can be carried out also in time domain, pre-stack depth migration remains the candidate procedure for correctly handling long-offset sub-basalt reflections due to laterally varying velocity fields encountered by seismic phases traveling for long distances. Correct imaging requires that the velocity field be evaluated correctly.

Accurate 3D Vint/Depth model building is therefore a key aspect for any pre-stack depth migration procedure. Geosystem provides a wide range of solutions which are generally handled directly in the depth domain by using a combination of tomographic tools such as turning-ray tomography and refraction-reflection tomography, with grid-based and layer-based model parameterizations or a combination of both (see Tomography).

The appropriate choice of the migration algorithm is extremely important for performing a correct imaging of weak sub-basalt reflections and, depending on the cases, Kirchhoff PSDM with different method of travel-time calculation (i.e. minimum travel-time, maximum amplitude or multivalued travel-times) or Gaussian beam pre-stack depth migration can be chosen.

Sub-basalt dipping layers from time migration (left) and Wide Offset PSDM (right).

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