Trace-By-Trace Normal Porosity

In this second approach, I obtain the normal porosities on a trace-by-trace basis. The normal trend is determined locally at each surface position by fitting polynomials to the near-surface and deep-sediment baseline porosities which are assumed to be free of gas and hydrate. The fitting is done using an iterative least-squares approach and second order polynomials.

Figure shows the anomalous baseline porosities overlain by the polynomial-fitted normal porosity curves. As before, the first panel represents the porosities in an area without a prominent BSR. Both porosity lines are in good agreement. Since the second order polynomial can adequately reproduce the baseline porosity profile, this polynomial is an appropriate functional form of the porosity-depth profile of the presumably brine-saturated sediments. The middle section shows the beginning of a small hydrate anomaly and a clearly identifiable hydrate anomaly is visible in the third panel, as well as a considerable gas-related anomaly of opposite polarity.

 

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Figure 8 Trace-by-trace normal porosities (dashed lines) overlying the anomalous baseline porosities (solid line).

After having determined the reference normal porosity trend at each surface position, I again calculate the actual magnitude of the gas and hydrate anomalies by subtracting normal and anomalous baseline porosities. The resulting residuals are shown in Figure . Compared to the averaging technique, both the hydrate and the gas-related anomalies are clearly enhanced with respect to small local anomalies caused by curve misfits.

 

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Figure 9 Residual obtained by subtracting the trace-by-trace normal porosities from the anomalous baseline porosities.

In order to examine the residuals caused by the hydrate and the gas in more detail, I again plot a histogram of the porosity differences (Figure ). The residual histogram shows the residuals for the part of the line without BSR (sold line) compared to the one with BSR (dashed line). The hydrate and gas anomalies are clearly identifiable and separable from other misfits. The relatively high positive residuals in the region without BSR are generated close to the seafloor by fitting a stiff polynomial to the uppermost sediments (see Figure ). These misfits are treated as artifacts and are thus not included into the calculations.

 

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Figure 10 Histogram of the porosity residuals obtained using the trace-by-trace porosity trend. The solid line represents the residuals between 0 and 25 km (without BSR). The dashed line represents the residuals between 25 and 52 km.