In complex structure area, seismic reflection usually gives poor quality image because of high difraction. To infer such complex structures, high resolution tomographic images can be used for reducing the ambiguities of a reflectivity image

Waveform Tomography processing can generate accurate depth velocity, indicated by strong correlation between the image of a velocity distribution and the image of its reflectivity. Depth velocity, with higher resolution, becomes an invaluable seismic asset and interpretation tool.

strong correlation between velocitygenerated by waveform tomography (top), reflectivity image (middle), and true velocity solution (bottom) of marmousi complex structure model. (Waviv Tech) 

There is no single method powerful enough to make a proper conclusion, therefore we should use combinational approach to do interpretation.

Combinational Interpretation Approach using perturbation, velocity, reflectivity, and play concept analysis.

Play Concept

Some artefacts usually occurred in seismic image due to errors of processing. The use of geological constraints, such as lithologies of area and the formation genesis, have significant role to differentiate artefacts from the real data. Therefore, some potential hidrocarbon traps can be determined confidently. 

Perturbation Analysis

The idea behind perturbation analysis is that a sharp velocity discontinuity means the border of different rock. It is done by comparing the final result to the initial model. From this comparison, we can see the velocity discontinuities (strong reflections). Perturbation analysis is used to determine the main reflectors. Perturbation analysis mainly correspond to the ‘low frequencies’ inversion of the image

Comparing initial model (top) to final inversion model (middle) can show the sharp velocity discontinuities (bottom).

Velocity Analysis

Velocity section can be used as layer continuity analysis. First, we can use absolute velocity of waveform tomography. With the idea that a same layer has nearly same effective pressure (if there is only weak anomaly) we can approximate the continuity of a layer. This tool is useful for predicting layer continuity especially in the border of a fault. Second, we can use velocity gradient – the rate of change of velocity – to infer the similarity of layer properties. Assuming there is a nearly constant overburden pressure increment, a layer with same material will have same velocity gradient. Continuity analysis mainly correspond to ‘middle frequencies’ inversion of the image.

Velocity analysis shows continue layers of Marmousi model. Viewing in absolut (top), gradient (middle), or edge-contour (bottom) of velocity can give insight of layer continuity.

Reflectivity Analysis

With the guidance from perturbation and continuity analysis, it is easier to draw layer horizons from reflection image. Note that if we use only reflectivity analysis, some ambiguities may occurred especially in complex structure area. Since reflection image contain ‘high frequencies’, the border of layer can be determined more accurately.

CASE : Complex structure imaging from ’surveillance’ seismic data. The data quality is poor due to “weight drop”-type acquisition on muddy surface.