Seismic anisotropy can help to extract azimuthal information for predicting crack alignment, but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development, which is achieved through determining the crack density from seismic or VSP data. In this research we study the dependence of seismic anisotropy on crack density, using synthetic rocks with controlled crack geometries. A set of four synthetic rocks containing different crack densities are used in laboratory measurements. The crack thickness is 0.06 2 / 32 mm and the crack diameter is 3 mm in all the cracked rocks, while the crack densities are 0.00, 0.0243, 0.0486 and 0.0729. P and S wave velocities are measured by an ultrasonic investigation system at 0.5 MHz while the rocks are saturated with water. The measurements show the impact of crack density on the P and S wave velocities. Our results are compared to the theoretical prediction of (Chapman, 2003) and (Hudson, 1981). The comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks, although Chapman's model fits the experimental results better. The measured anisotropy of the P and S wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density, as predicted by several theoretical models. The laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration.
S U M M A R YWe have carried out two seismic physical experiments to acquire wide-azimuth P-wave 3-D seismic data with a scaled down model (1:10 000) and scaled-up frequencies (10 000:1). Our aims are to verify the physical basis of using P-wave attributes for fracture detection, to understand the usage of these attributes and their merits, and to investigate the effects of acquisition geometry and structural variations on these attributes. The base model consists of a fractured layer sandwiched between two isotropic layers (Epoxylite). Inside the fractured layer there is a dome and a fault block for investigating the effects of structural variations. The two experiments were carried out using different acquisition geometries. The first experiment was conducted to maximize the data quality, with an offset-depth ratio of only 0.68 to the bottom of the fracture layer. For comparison, the second experiment was carried out to maximize the anisotropy effects, with the offset-depth ratio to the bottom of the fracture layer raised to 1.34.For each experiment, about 20 km 2 of wide-azimuth 3-D data were acquired with a P-wave source. The physical modelling confirms that the P-wave attributes (traveltime, amplitude and velocity) exhibit azimuthal variations diagnostic of fracture-induced anisotropy. For the first experiment with noise-free data, the amplitude from the top of the fracture layer yields the best results that agree with the physical model parameters and free of the acquisition footprint. The results from other attributes (traveltime, velocity, AVO gradient) are either contaminated by the structural imprint, or by the acquisition footprint due to the lack of offset coverage. For the second experiment, despite the interferences from multiples and other coherent noise, the traveltime attributes yield the best results; both the acquisition footprint and the structural imprint are reduced due to the increased offset coverage. However, the results from the amplitudes are affected by the noise and are less reliable. Analysis of the two experiments reveals that the offset-depth ratio to the target is a key parameter for the success of the P-wave techniques. Smaller offset-depth coverage may only be applicable to amplitude attributes with high quality data; whilst large offset coverage makes it possible to use traveltime attributes. A reliable estimation from traveltime attributes requires an offset-depth ratio of 1.0 or more.
On seismic migration sections, anomalous bright spots, called the string of beads response (SBR), are common features of carbonate karst reservoirs at the seismic scale in the Tarim Basin, China. To understand the SBR features of different karst caves, which is an important issue for local exploration, we conducted a physical modeling experiment. Within the physical model, we included various single caves with different scales, velocities, shapes, and fluids, as well as multiple caves in different spatial distributions. SBRs of all caves were extracted and summarized from the migration sections. First, we investigated effects of the cave scale, velocity, spatial distribution, shape, and fluids on SBRs. The relative amplitude of SBRs increased with the cave width ranging from 25 to 400 m and decreased with the cave velocity. The SBR split into two new SBRs when the cave height was larger than 100 m. Spatially distributed multiple caves resulted in some special SBRs, such as long SBRs, inclined SBRs, waved SBRs, and chaotic SBRs. The cave shape contribution to SBRs could be neglected in deep strata practically. The relative amplitude of SBRs of caves filled with gas and oil was stronger than those filled with water. Then, we established an interpretation chart of the corresponding relationship between six types of SBRs and their potential caves. Short SBRs were the responses of caves with a height of less than 60 m. Long SBRs corresponded to two kinds of cave units: (1) a single cave with a height between 70 and 100 m and (2) two caves (height less than 60 m) vertically distributed with a small distance (less than 60 m). Chop-shaped SBRs indicated caves with a width of more than 100 m. Inclined SBRs, waved SBRs, and chaotic SBRs corresponded to multiple caves spatially distributed in triangles, rhombuses, and clusters, respectively.
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