Vertical Sampling Interval Research Articles

Seismological reference earth model in South China (SREM-SC): Crust and uppermost mantle

The South China Block is located on the eastern margin of the Eurasian Plate and the western margin of the Pacific Plate. The South China Block is currently in a tectonically compressed environment, while the Tibetan Plateau is moving eastward and the Philippine Sea Plate is moving westward from geodetic observations. The South China Block is an ideal place to revisit tectonic history from the Archean to Cenozoic, where its information could be well preserved in the crust. In this study, we aim to build the crustal and uppermost mantle component of the Seismological Reference Earth Model in South China (SREM-SC) to provide a background velocity model for geological interpretations and fine-scale velocity inversion. The S-wave velocity model comes from combining models inverted by ambient noise tomography and surface wave tomography. The P-wave velocity model is obtained from converted S-wave velocity and joint inversion tomography. The density model is inferred from an empirical relationship with P-wave velocity. The Moho depth is obtained by a weighted averaging scheme of previously published receiver function results. The P-wave and S-wave velocity models have a grid interval of 0.5° in both latitude and longitude, and with a vertical sampling interval of 5 km down to the 60 km depth. This work provides the 3-D crust and uppermost mantle structures and a representative reference model beneath South China.

Comparison of MetOp-A/-B GRAS radio occultation data processed by CDAAC and ROM

Both the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Data Analysis and Archival Center (CDAAC) and the European Radio Occultation Meteorology Satellite Application Facility (ROM) process the radio occultation data from the Global Navigation Satellite System Receiver for Atmospheric Sounding (GRAS) onboard the MetOp-A/-B satellites. CDAAC- and ROM-processed GRAS data from 2015 are compared here. A three-step sequential quality control (QC) procedure is applied to both CDAAC and ROM data. Another QC check on the vertical sampling interval of the bending angle is applied to ROM data, which has a varying vertical sampling interval that is much higher than the CDAAC’s constant 20 m vertical sampling interval and causes a significant number of outliers below 10 km. The mean of the fractional difference in bending angle between CDAAC and ROM is reduced from ± 0.4 to ± 0.2%, and the standard deviation of the fractional difference in bending angle between CDAAC and ROM is reduced from 3 to 2% after QC in the upper troposphere and lower stratosphere (UTLS). The mean differences in temperature and specific humidity derived by CDAAC and ROM are within ± 0.4 K and ± 0.02 g kg−1 in the UTLS, respectively. The mean fractional difference in refractivity retrievals between CDAAC and ROM is on the order of 0.05% in the UTLS, and 2% below and 0.4% above it. There is a significant latitudinal dependence of the mean fractional difference in refractivity retrievals in the lower troposphere (below 400 hPa). The mean fractional difference is the largest near 850 hPa at the equator (about 2.7%).

Testing the integrity of fine spatial resolution palaeoecological records: microcharcoal data from near-duplicate peat profiles from the North York Moors, UK

Microcharcoal data which cover the millennium preceding the Ulmus Decline from near-replicate peat profiles 30 cm apart are presented from North Gill, an intensively studied upland spring-head site in north-east England with an established detailed palaeoecological record. The Ulmus Decline at this site has been consistently dated to ca. 4700 14C years BP. The microcharcoal records from these two very closely adjacent points, which must have had virtually identical microcharcoal source areas, are assessed for their degrees of similarity and variability at 1 cm vertical sampling intervals, in an investigation which parallels a previous comparative pollen study of the two profiles. The major trends and frequency peaks and troughs of the twin microcharcoal curves were found to correspond well, suggesting that microcharcoal records from small peat mires within forested catchments have fine spatial integrity, are reproducible and can be interpreted securely as reliable records of fire history in their small source areas. Much variation occurred at the level of individual spectra within coeval zones which must record the same fire history and ecology however, producing differently shaped microcharcoal curves. While the record of major trends will be accurate, therefore, too precise an interpretation of microcharcoal data at this temporal scale may not be justified. Micro-spatial variations in frequencies of fungal spores associated with local burning are also presented and evaluated.

Monitoring the Maximum Depth of Drainage in Response to Pumping Using Borehole Ground Penetrating Radar

There are many methods available for nondestructive measurement of volumetric water content. However, no current method can monitor rapidly to great depths with high spatial resolution over large sample volumes and with minimal need for medium-specific calibration. Borehole ground penetrating radar (BPGR) may provide this capability. It is well established that BGPR can make rapid measurements to great depth. Like time domain reflectometry (TDR), BGPR infers the water content from dielectric permittivity measurements, which show a robust correlation with volumetric water content. The goal of this investigation was to determine whether BGPR water content measurements made with a vertical sampling interval that is smaller than the antenna length could be used to measure the water content profile with high spatial resolution. Repeat water content profiles measured with BGPR during a pumping test and under static conditions 1 yr later are presented. The results show that BGPR measurements are highly repeatable, allowing for differencing of profiles to determine the water content change profile. However, this high repeatability required calibration over a depth range below the water table due to instrument drift and operator inconsistencies. Although critical refractions obscure the water content profile near the ground surface, there is no evidence that refracted waves have deleterious effects on travel time profiles collected across the water table, allowing for determination of the maximum depth of drainage from the water content change profiles. There is good agreement between the patterns of maximum depth of drainage and water table depth during pumping and recovery. However, the maximum depth of drainage, referenced to the middle of the BGPR antennae, is consistently 50 cm deeper than the water table. The results demonstrate that the primary limitation on the achievable resolution of water content monitoring with BGPR is the user-selected measurement sample interval, which can be much smaller than the antenna length. However, the cause of the near constant downward offset of the BGPR measurements compared with the water table depth must be studied further to allow for direct use of BGPR to track water movement during pumping and recovery.

Monitoring the Maximum Depth of Drainage in Response to Pumping Using Borehole Ground Penetrating Radar

There are many methods available for nondestructive measurement of volumetric water content. However, no current method can monitor rapidly to great depths with high spatial resolution over large sample volumes and with minimal need for medium‐specific calibration. Borehole ground penetrating radar (BPGR) may provide this capability. It is well established that BGPR can make rapid measurements to great depth. Like time domain reflectometry (TDR), BGPR infers the water content from dielectric permittivity measurements, which show a robust correlation with volumetric water content. The goal of this investigation was to determine whether BGPR water content measurements made with a vertical sampling interval that is smaller than the antenna length could be used to measure the water content profile with high spatial resolution. Repeat water content profiles measured with BGPR during a pumping test and under static conditions 1 yr later are presented. The results show that BGPR measurements are highly repeatable, allowing for differencing of profiles to determine the water content change profile. However, this high repeatability required calibration over a depth range below the water table due to instrument drift and operator inconsistencies. Although critical refractions obscure the water content profile near the ground surface, there is no evidence that refracted waves have deleterious effects on travel time profiles collected across the water table, allowing for determination of the maximum depth of drainage from the water content change profiles. There is good agreement between the patterns of maximum depth of drainage and water table depth during pumping and recovery. However, the maximum depth of drainage, referenced to the middle of the BGPR antennae, is consistently 50 cm deeper than the water table. The results demonstrate that the primary limitation on the achievable resolution of water content monitoring with BGPR is the user‐selected measurement sample interval, which can be much smaller than the antenna length. However, the cause of the near constant downward offset of the BGPR measurements compared with the water table depth must be studied further to allow for direct use of BGPR to track water movement during pumping and recovery.

Monitoring the Maximum Depth of Drainage in Response to Pumping Using Borehole Ground Penetrating Radar

There are many methods available for nondestructive measurement of volumetric water content. However, no current method can monitor rapidly to great depths with high spatial resolution over large sample volumes and with minimal need for medium-specific calibration. Borehole ground penetrating radar (BPGR) may provide this capability. It is well established that BGPR can make rapid measurements to great depth. Like time domain reflectometry (TDR), BGPR infers the water content from dielectric permittivity measurements, which show a robust correlation with volumetric water content. The goal of this investigation was to determine whether BGPR water content measurements made with a vertical sampling interval that is smaller than the antenna length could be used to measure the water content profile with high spatial resolution. Repeat water content profiles measured with BGPR during a pumping test and under static conditions 1 yr later are presented. The results show that BGPR measurements are highly repeatable, allowing for differencing of profiles to determine the water content change profile. However, this high repeatability required calibration over a depth range below the water table due to instrument drift and operator inconsistencies. Although critical refractions obscure the water content profile near the ground surface, there is no evidence that refracted waves have deleterious effects on travel time profiles collected across the water table, allowing for determination of the maximum depth of drainage from the water content change profiles. There is good agreement between the patterns of maximum depth of drainage and water table depth during pumping and recovery. However, the maximum depth of drainage, referenced to the middle of the BGPR antennae, is consistently 50 cm deeper than the water table. The results demonstrate that the primary limitation on the achievable resolution of water content monitoring with BGPR is the user-selected measurement sample interval, which can be much smaller than the antenna length. However, the cause of the near constant downward offset of the BGPR measurements compared with the water table depth must be studied further to allow for direct use of BGPR to track water movement during pumping and recovery.

High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 3—Wavefield separation of reflections

Using crosswell data collected at a depth of about 3000 ft (900 m) in west Texas carbonates, one of the first well‐to‐well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution. Both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S‐wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high‐resolution reflection images. In this study, the most important elements for an effective wavefield separation were the time‐alignment of seismic arrivals prior to filter application and the implementation of wavefield‐separation filters in multiple domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) vertical sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sampling intervals, resulting in spatial aliasing and limiting the utility of the data for reflection processing. The wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.

The use of velocity and conductivity data for the quantification of heterogeneity: A comparison

Hydraulic conductivities of a portion of the Twin Lake aquifer are derived from grain size analyses performed at 3‐cm vertical sampling intervals in four boreholes, spaced l m apart. Correlation of conductivity logs between four boreholes leads to a geometrical model which corresponds closely to the spatial structure derived from the velocity field of previous tracer tests. Histogram and correlogram analyses of conductivities agree closely with those of velocities, and the two data sets appear equally useful for characterizing the aquifer and its spatial variability. Using Darcy's law, flow velocities and conductivities are combined to calculate a theoretical, hydraulic gradient. Comparison to gradients observed at the site reveals agreement on a large scale. However, differences exist in detail because observed piezometric heads fail to reflect small‐scale heterogeneity. Since these differences are significant for contaminant transport prediction, it is recommended for future studies to directly measure flow velocities from a tracer experiment, rather than to reply on head measurements and on Darcy's law for the calculation of transport parameters.

High Resolution Crosswell Imaging of a West Texas Carbonate Reservoir

Using crosswell data collected at a depth of about 3000 ft (900 m) in West Texas carbonates, one of the first well-to-well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution. Both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S-wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high-resolution reflection images. In this study, the most important elements for an effective wavefield separation were the time-alignment of seismic arrivals prior to filter application and the implementation of wavefield-separation filters in multiple domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) vertical sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sampling intervals, resulting in spatial aliasing and limiting the utility of the data for reflection processing. The wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.

A palaeomagnetic and mineral magnetic study of the loess sequence at Liujiapo, Xian, China

AbstractEmploying an average vertical sampling interval of 20 cm, oriented samples were collected from 26 m of the loess section at Liujiapo, near Xian, China in order to refine the existing magnetostratigraphic record. Palaeomagnetic results reveal two short reversed‐polarity subzones within a profile of predominantly normally magnetised sediments, an interpretation that is at variance with previous results from this section and from other studies of Chinese loess. Investigation of sediment mineral‐magnetic properties indicates a close correspondence between down‐section variations in intensity of magnetisation, susceptibility and saturation isothermal Journal of Quaternary Science remanence (SIRM), and lithology. The ratios IRM‐100mT/SIRM and SIRM/ susceptibility appear to be particularly sensitive indicators of the degree of soil development and hence of palaeoclimate.

Estimates of the coherence of ocean currents over vertical distances

Pairs of moored current meters at Site D have been used to collect simultaneous series of ocean current measurements over vertical separations of from 1 to 1500 m. Within the range of frequencies that can be resolved with the limited time series available, currents are not coherent across the seasonal thermocline. Otherwise currents are generally coherent over some range of lower frequencies, and incoherent at higher frequencies. It was found that the frequency at which the coherence drops below 0·5 varies inversely as the separation, δZ, between sensors according to the relation: f 0·5 δZ = 13 m/hr. At small separations, the maximum coherent frequency is apparently related to the Brunt-Väisälä frequency. More extensive measurements are needed to obtain a good description of the relation between coherence and vertical separation, but the results presented here may be adequate for defining vertical sampling intervals in the design of the next stage of array experiments.