Models

The two 2-D synthetic datasets were computed assuming an earth model that includes

• A 1.8 km thick overburden.
• A 0.2 km cap rock layer (shale).
• A 0.2 km target zone with three different lithologies for comparison (cemented brine sands, cemented oil sands, and tuff).

Figure 1 shows the P-wave velocity for both models.

 

model
Figure 1 P-wave velocity models used to generate the synthetic data. Model 1 (top): overburden with flat layers, model 2 (bottom): overburden with velocity anomalies.

In model 1, the overburden contains two flat layers with constant elastic properties on each layer, whereas model 2 includes a zone of complex velocity anomalies in the overburden. The rock properties for the model were taken from real well logs of a North Sea field. Typical values for different lithologies at this field are listed in Table 1.

$$\rho$$ 1|c|Lithology 1|c|V_p (km/s) 1|c|V_s (km/s)

Shale 2.4 0.95 2.25

Cemented brine sands 3.1 1.55 2.15

Uncemented brine sands 2.6 1.3 2.1

Cemented oil sands 2.9 1.6 2.05

Uncemented oil sands 2.35 1.33 2

Volcanic ash (tuff) 2.75 1.23 2.2

Limestone 4 2 2.4

Table 1. Typical rock properties for different lithologies at the North Sea.

Average values for overburden properties in the field are V_p=2.2 km/s, V_s=0.75 km/s, $\rho=2.15$ (g/cm³).

In model 1, overburden properties above the flat interface were taken to be the average values indicated above; overburden properties below the interface were the average values with a $10 \%$ increase. For model 2, we introduced lateral velocity anomalies by including a smoothed sinusoidal interface between the two layers.