Summary
In real situation, it always exists strong reflection interfaces in shallow which are attributed to some low velocity layers. The shallow low-speed layers like coal seams can form strong reflection interfaces with surrounding rocks(Dong et al, 2006). With the strong reflection and interference effects between several thin beds, the resolution of seismic reflection also is limited (Sheriff, 1997). Because of the shielding effect of interfaces, seismic wave energy hardly s through coal seams to deeper layers. So a physical model firstly has been manufactured to certify this phenomenon and then a numerical modeling is used to strengthen the conclusion.
Introduction
In order to study effect coal seams with low density and low speed on the underlying beds, we firstly designed and manufactured a three dimension physical model of coal seams according to the geologic structure and parameters of the stratum of one basin south china.
There are several faults, fractures and collapse columns in this physical model, but this paper just focuses on the shielding effect of coal seams on seismic imaging underlying layers which can cause the of the reflection events beneath the coal beds.
The design and manufacture of the physical model
According to the actual structure, we narrow down the actual geological area of 5×4 square kilometers to the size of 100×80 square centimeters using the scale of 5000 to 1 which means that each centimeter of the physical model stands for 5 meters in the field. The physical model is made the different mixing ratio of silicone rubber, epoxy resin and curing agent. Figure 1 shows the vertical profile of the physical model obtained by laser measurement. There are six surrounding beds named T1 to T6 and four coal beds called C1 to C4 which the thickness of these coalbeds ranges from 0 to 25 meters.
