Deep Thinking: 4C Proves Value on Seafloor

Marine 4C seismic technology was developed to assist hydrocarbon exploration and development – but 4C data have important marine engineering applications that have not been exploited.

The data discussed here illustrate how 4C data can be used to define geomechanical properties of a seafloor where engineers need to install production facilities.

Emphasis is placed here on determining bulk moduli and shear moduli of seafloor sediment. Bulk modulus, K, for a homogeneous medium is given by the equation:

K = [(VP)2 – (4/3)(VS)2]

Shear modulus, μ, for the same homogeneous material is defined by:

μ = (VS)2.

In these expressions, VP and VS are, respectively, P-wave and S-wave velocities in seafloor sediment, and is the bulk density of a sediment sample.

Figure 1 presents shallow data windows of compressional (P-P) and converted-shear (P-SV) profiles across an area of 4C/3D data acquisition. Data analysis will be confined to the layer extending from the seafloor (labeled WB) to horizon H4 shown on the profiles.

Procedures used by the seismic data processor caused the water bottom interface WB to not be imaged on the P-SV profile.

The profile crosses a gas-invaded zone centered on crossline coordinate 200. P-P horizons H1 through H4 are interpreted to be depth-equivalent surface to P-SV horizons H1 through H4.

For simplicity, the bulk density term in the two equations above is assumed to have a constant value of 1.8 gm/cm3 across the data analysis space.

Figure 2 displays seismic-derived VP velocities and calculated bulk moduli across the shallowest seafloor layer (WB to H4), and seismic-derived VS velocities and shear moduli values calculated for the layer are shown on figure 3.

Each elastic constant is shown as a 3-D surface and also in plan view. The position of the example profile (figure 1) is marked across each 3-D surface and illustrates the relationship between the gas-invaded zone seen on the P-P image and a normal fault that extends across much of the image area in the vicinity of crossline coordinate 200.

These figures show there is a one-to-one relationship between VP and bulk modulus, and between VS and shear modulus, for these high-porosity, near-seafloor, unconsolidated sediments.

Referring to equation 2, it is no surprise that VS and μ have a one-to-one correlation. The one-to-one relationship between VP and K is caused by the fact VP is much larger than VS within this shallowest seafloor layer.

In areas having hard seafloor sediment and for deeper layers where the VP/VS ratio has values appropriate for consolidated rocks, the VS term of equation 1 will be significant, and there will not be such a close correlation between K and VP.

The multicomponent seismic data application illustrated by this example can be done more rigorously by implementing a data-point by data-point inversion to create thin VP and VS layers that provide greater detail about zones of mechanical weakness.

The intent of this example is only to document that even simple velocity analyses of 4C data allow weak and strong areas to be recognized across a seafloor.

Of the two elastic moduli that are considered, shear modulus is particularly important for understanding where seafloor slumping is likely to occur.

Without 4C data, it is difficult to estimate shear moduli across large seafloor areas and to identify areas where seafloor slumping may be expected.

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