What Do P-Wave Velocities Tell Us About the Critical Zone?

Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

Tectonic setting of kimberlites in the Vilyui-Markhinsky fault zone according to modern data

The lateral heterogeneity of the Vilyui-Markha fault zone was determined, the central and western subzones were identified. The high-grade diamondiferous Mir and Nakyn kimberlite fields are confined to the central subzone. The low-grade diamondiferous Syuldyukar kimberlite field is confined to the western subzone of the VilyuiMarkha zone. The analysis of the fault network density in the research area was carried out. It was found that the fault network density increases within the subzones, which characterizes them as increased permeability areas favorable for kimberlite melts uprising. This fact can be another tectonic criterion for setting up diamond prospecting operations.

Poisson impedance for lithology characterization

Tyumen formation is a formation with very considerably lateral heterogeneity. It influences on process of exploration oil and gas traps. No one can deny that seismic survey is a main method for prediction lateral heterogeneity. Nowadays, geoscientists can model different useful seismic attributes, apart from structural maps. Seismic inversion is most effective tool for transformation amplitude seismic data into elastic properties. However, sometimes result of seismic inversion is very hard to understand and analyze. In this work, authors will show additional methods for better understanding results of seismic inversion using Impedance Poisson. Area of research is located in the Western Siberia.

A study of the lateral heterogeneity with the ellipticity of Rayleigh waves derived from microtremors

Summary We examine the potential of frequency-dependent Rayleigh wave ellipticity, derived from microtremors, for the investigation of heterogeneous subsurface structure. Based on numerical simulation, we analyze the effects of interference waves in microtremors, primarily the various propagation directions of the Rayleigh waves, linear polarization waves, and white noise, on the ellipticity frequency-dependent estimation of the Rayleigh waves. A data processing scheme to separate the Rayleigh waves from the interference waves is proposed and verified by synthetic data. We performed a field experiment in the mountainous areas of Southwest China to show that the ellipticity frequency-dependency of Rayleigh waves in the period range of 0.05 to 5 s can be estimated from the microtremor records with the proposed data processing scheme. In addition, the method is feasible for investigating lateral heterogeneity within the top several hundred meters in the mountain regions. The study also reveals that the features of the ellipticity anomaly of a local heterogeneity are related to the propagation directions of the Rayleigh waves, and to reduce the ambiguity of the anomaly, the propagation direction of the waves picked for the ellipticity estimation should be consistent with (along or opposite to) that of the survey line. Then, to eliminate the effects of the phase differences due to the propagation direction, or time, the ellipticity for each location should be estimated by a single event rather than multiple events from the derived Rayleigh wave arrivals.