Dense Spatial Sampling Research Articles

Quantitative diffuse optical tomography for small animals using an ultrafast gated image intensifier

The quantitative accuracy of fluorescence and bioluminescence imaging of small animals can be improved by knowledge of the in situ optical properties of each animal. Obtaining in situ optical property maps is challenging, however, due to short propagation distances, requirements for high dynamic range, and the need for dense spatial, temporal, and spectral sampling. Using an ultrafast gated image intensifier and a pulsed laser source, we have developed a small animal diffuse optical tomography system with multiple synthetic modulation frequencies up to >1 GHz. We show that amplitude and phase measurements with useful contrast-to-noise ratios can be obtained for modulation frequencies over the range of approximately 250 to 1250 MHz. Experiments with tissue simulating phantoms demonstrate the feasibility of reconstructing the absorption and scattering optical properties in a small animal imaging system.

How is the European Lithosphere Imaged by Magnetotellurics?

I review recent investigations on the electrical conductivity of the lithosphere and asthenosphere in Europe. The principal method in the reviewed studies is the magnetotelluric method, but in many cases other electromagnetic methods (e.g., magnetovariational profilings and geomagnetic depth soundings) have provided additional information on subsurface conductivity or have been the primary method. The review shows that the magnetotelluric method has been used, and is being used, in all kinds of environments and for many different processes shaping the crust and lithosphere. The crust is very heterogeneous, both with respect to the scale of conductive/resistive features and interpretations: research targets vary from Archaean palaeostructures to ongoing processes. The European database of the depth to the lithosphere-asthenosphere boundary (LAB) in Europe is updated, and a new map showing lateral variations of the depth of LAB is provided. The compilation shows that (1) the Phanerozoic European lithosphere, with considerable variations (45–150 km), is much thinner than the Precambrian European lithosphere, (2) the Trans-European Suture Zone is a major electrical border in Europe separating electrically (as well as geophysically and geologically in general) two quite different settings, (3) the thinnest lithosphere is found under the extensional Pannonian Basin (45–90 km), (4) in most of the East European Craton there are no indications of a high conductivity zone in upper mantle. In many regions there is no information at all on upper mantle conductivity, which calls for pan-European projects to operate arrays of simultaneously recording instruments with long recording periods (2–8 months) and dense spatial sampling (20–50 km).

Analysis of shallow gas and fluid migration within the Plio-Pleistocene sedimentary succession of the SW Barents Sea continental margin using 3D seismic data

Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the 2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas accumulations and the sedimentary environments in which they occur. The presence of gas is inferred from seismic reflection segments with anomalously high amplitude and reversed phase, compared with the seafloor reflection, so-called bright spots. Fluid migration is inferred from vertical zones of acoustic masking and acoustic pipes. The 3D seismic volume allows a spatial analysis of amplitude anomalies inferred to reflect the presence of gas and fluids. At several locations, seismic attribute maps reveal detailed images of flat spots, inferred to represent gas–water interfaces. The data indicate a focused fluid migration system, where sub-vertical faults and zones of highly fractured sediments are conduits for the migration of gas-bearing fluids in Plio-Pleistocene sediments. Gas is interpreted to appear in high-porosity fan-shaped sediment lobes, channel and delta deposits, glacigenic debris flows and sediment blocks, probably sealed by low-permeability, clayey till and/or (glacio)marine sediments. Gas and fluid flow are here attributed mainly to rapid Plio-Pleistocene sedimentation that loaded large amounts of sedimentary material over lower-density, fine-grained Eocene oozes. This probably caused pore-fluid dewatering of the high-fluid content oozes through a network of polygonal faults. The study area is suggested to have experienced cycles of fluid expulsion and hydrocarbon migration associated with glacial–interglacial cycles.

Coaxial coil towed EMI sensor array for UXO detection and characterization

A new broadband electromagnetic induction (EMI) array sensor, GEM-5, for detecting and characterizing Unexploded Ordnance (UXO) has been developed in order to provide high production rates for EMI surveys. The sensor consists of a single rectangular loop transmitter around a linear array of seven pairs of coaxial receiver coils, with each coil in a pair located at the same vertical distance above and below the loop transmitter. The coil pairs are wired in an inverted series so that the signal consists of the difference between the voltage induced in the upper and lower coils. This particular configuration provides a high degree of primary field cancellation, dense spatial sampling rate due to simultaneous and continuous operation of all sensors, suppression of motion-induced and environmental noise, and strong source fields at typical UXO burial depths providing deep detection range. Our prototype tests indicate that the array yields a lower static and motion-induced noise over the critical low frequencies than that of existing sensors, and in particular, the signal-to-noise ratio at 90 Hz is 32 dB higher. Environmental noise can be largely removed from the difference measurements. The field test results from UXO test sites show that the prototype sensor has smoother background and appears to detect more seeded targets than the GEM-3 concentric sensor, however some of that gain can be attributed to higher power transmitter electronics.

Multichannel near‐array corrections for short‐period teleseismic data

The increased availability of data from broadband and short‐period seismometer arrays has stimulated the development of multichannel high‐resolution imaging algorithms for scattered teleseismic waves. One of the main factors limiting the resolution of teleseismic images is the interference from unwanted phases at higher frequencies (0.3–1.5 Hz). These unwanted phases are generally associated to scattering at near‐surface features. We describe initial results from a multichannel inversion‐based method designed to suppress scattered waves from surface topography. Such an approach is possible because the dense spatial sampling of short‐period arrays allows us to sample these phases well enough to characterize them. Using a subset of the data, we first estimate a model for the distribution of near‐surface scatterers. Next, we predict scattered waves in the entire data set using this scatterer distribution model. Both the inversion and prediction steps are based on an integral representation of the scattered‐wave field which takes into account the near‐field character of body‐to‐surface wave scattering. Finally, we subtract the predicted scattered waves from the entire data set. Data from the LARSE93 experiment are used to illustrate and validate the present algorithm.

Identification of crustal and upper mantle heterogeneity by modelling of controlled-source seismic data

High-frequency controlled-source seismic sections with dense spatial sampling show the existence of heterogeneity at different depth levels of the continental crust and upper mantle. Our sources of information are the Peaceful Nuclear Explosion (PNE) seismic data sets recorded to large offsets in the former Soviet Union supplemented by recordings from the North American Early Rise deep seismic experiment and normal-incidence reflection seismic sections collected in northwest Europe. Heterogeneity in the crust and upper mantle can be uniquely identified in reversed high-frequency (2–10 Hz) PNE seismic sections collected with dense spatial sampling (nominal receiver spacing of 10–15 km) out to 4000 km offset. We document pronounced seismic scattering from three heterogeneous zones: The lower crust from ∼20 km to ∼40 km depth, an ∼80 km thick low-velocity zone below ∼100 km depth, and the ∼320–460 km depth interval around the top of the mantle transition zone. We calculate the full seismic wavefield in heterogeneous crust–mantle models with a two-dimensional finite-difference algorithm. We represent the heterogeneous layers by random fluctuations of the elastic parameters and Q-values. The spatial (horizontal and vertical) correlation lengths and the standard deviation of the scattering media are constrained by comparison of observed and calculated seismic sections. The lower crustal heterogeneity causes a coda to the upper mantle arrivals at all recorded frequencies. This coda is a prominent feature for whispering-gallery phases (teleseismic Pn), which travel as multiply reflected refractions below the Moho to more than 3000 km offset from the PNE sources. The heterogeneous mantle low-velocity zone causes a scattered coda trailing the first arrivals in the ∼800–1400 km offset range. The best fit to the observations along profile Kraton in Siberia is obtained by an 80 km thick heterogeneous low-velocity zone below 100 km depth, represented by fluctuations described by a von Karman distribution function with a Hurst number of 0.5 and spatial correlation lengths of 5–10 km (horizontally) and 3–5 km (vertically). Scattered arrivals, which trail the reflection from the ‘410’ discontinuity and a reflection from a shallower depth of ∼320 km, constrain heterogeneity around the top of the transition zone. This heterogeneity is modelled by fluctuations in the 320–460 km depth interval with correlation lengths on the order of 20–40 km by 5–10 km. Scattering from the two shallower heterogeneous zones improves the description of the phases from around the transition zone.

High resolution of streamer seismic data: obtaining optimal results

Andrew Long, PGS Marine Geophysical, defines the requirements for successful high resolution 3D marine seismic surveys and how these can be met. High resolution of seismic data results when high frequency geological features in close proximity can be precisely focused without any contamination from artefacts or noise (i.e. high signal-to-noise ratio). In general, recoverable frequency bandwidth is an important factor for vertical resolution, and dense 3D spatial sampling is an important factor for spatial resolution. Vertical and horizontal resolution is linked by the success of high frequency focusing and imaging. Without pursuing a theoretical treatise, the following section discusses the geophysical principles that are relevant for high-resolution seismic imaging. High resolution of 3D seismic data is achieved by the application of rigorous geophysical principles. In particular, the systematic pursuit of 1) uniform target illumination, 2) dense spatial sampling of the reflected wavefield, and 3) careful processing and imaging, will collectively deliver an optimum result. Note that the final step can never be successfully completed without the platform of proper target illumination and wavefield sampling. Several 3D data examples are used to demonstrate how flexible survey design and implementation can robustly achieve very high resolution target imaging for all target depths and challenges.

An advanced slip model for the Umbria-Marche earthquake sequence: coseismic displacements observed by SAR interferometry and model inversion

SUMMARY We report an improved slip model for the 1997 September‐October Umbria-Marche earthquake sequence in central Italy. The model is based on coseismic displacements estimated by differential synthetic aperture radar (DInSAR) interferometry using an advanced inverse modelling approach that enables the characteristics of the SAR data, especially their dense spatial sampling, to be fully exploited. The paper first looks at DInSAR analysis using the simplest configuration, that is, a single-image pair that temporally covers the earthquake sequence. Yet, given that this DInSAR configuration is often used for coseismic studies and the limited availability of SAR data, we duly consider the limitations of the DInSAR results and discuss the rigorous DInSAR procedure carried out, highlighting key errors associated with such observations. We then focus on the inversion of the estimated DInSAR displacements and on our advanced heterogeneous slip model for this earthquake sequence’s main shocks, emphasizing its most significant features.

The LSDARC method of seismic refraction analysis: principles, practical considerations and advantages

ABSTRACTThe LSDARC (least‐squares difference and reciprocal combination) method of seismic refraction analysis is an adaptation of the reciprocal method that optimizes resolution of refractor depths and lateral variations in refractor wavespeed, and provides direct estimates of errors. Both time depths and differences between time depths are calculated separately to allow determination of more complete and accurate adjusted time depths, and therefore optimal estimates of refractor depth. The most efficient way of estimating refractor wavespeed is not to use the ‘velocity function’, but to subtract the time‐depth terms from the measured times to project the sources and receivers on to the refractor surface. A least‐squares procedure is then used to calculate corrections to the times for each shot gather and apply them to make all times appear to be produced by a single shot placed on the refractor surface at one end of the profile. Refractor wavespeeds are estimated from these corrected times using variable distance windows that can be adjusted to get a preferred trade‐off between variance in wavespeed and window length. An alternative and simpler method of wavespeed determination involves using first‐break times to prepare slowness profiles in opposite directions, with a model for refraction across a dipping interface, to compute true refractor wavespeeds. However, although the alternative method will generally resolve significant variations in wavespeed, the estimated errors are larger than when time depths are used.The methodology is valuable for engineering‐scale refraction surveys in which economic considerations prevent dense spatial sampling of the ground. The main features of the method are illustrated using short refraction profiles undertaken during student field schools.

Marine self-potential gradient exploration of the continental margin

The self-potential (SP) method for mineral exploration is seldom used on land, primarily because of electrode noise problems and nonunique interpretations. Marine measurements of the horizontal gradient of the SP field, on the other hand, are relatively simple to make with an array of electrodes towed behind a ship. With low ship speeds of 5 to 10 km/hour, dense spatial sampling (∼1 m) can be obtained with resolution of better than 1 μV/m. In this paper we report on gradient SP data recorded on the continental shelf of South Australia by a horizontal array of towed electrodes approximately 20 m above the seafloor. Ocean waves and swells with periods of 5 to 15 s yielded large amplitude signals ±20 μV/m, but subseafloor mineralization produced SP gradient anomalies of ±50 μV/m and widths of 2 km or more in a number of parallel traverses. Integrating the observed SP gradients along each line delineated SP anomalies of amplitude up to −100 mV. Self-potential and magnetic anomaly data show limited spatial correlation and have different wavelengths, suggesting that SP sources are probably nonferrous minerals, such as graphite, and are deeper than the magnetic sources. The source of the SP signal is probably reduction-oxidation (redox) potential ([Formula: see text]) variations across a conducting body below the seafloor. We approximate the source as being two dimensional and find the most probable locations of line sources by an image reconstruction method. Numerical finite-element modeling of more realistic source regions suggests shallow, easterly dipping (∼15°) conductors of 1 Ω.m in the uppermost 2 km.

Infilling of sparse 3D data for 3D focusing operator estimation

ABSTRACTSeismic migration can be formulated in terms of two consecutive downward extrapolation steps: refocusing the receivers and refocusing the sources. Applying only the first focusing step with an estimate of the focusing operators results in a common focal point (CFP) gather for each depth point at a reflecting boundary. The CFP gathers, in combination with the estimates of the focusing operators, can be used in an iterative procedure to obtain the correct operators. However, current 3D seismic data acquisition geometries do not contain the dense spatial sampling required for calculation of full 3D CFP gathers. We report on the construction of full 3D CFP gathers using a non‐full 3D acquisition geometry. The proposed method uses a reflector‐orientated data infill procedure based on the azimuthal redundancy of the reflection data. The results on 3D numerical data in this paper show that full 3D CFP gathers, which are kinematically and dynamically correct for the target event, can be obtained. These gathers can be used for iterative updating of the 3D focusing operators.

Energy and Soil-Moisture Effects on Nonlinear Deformation Associated with Near-Surface Seismic Reflection Sources

The ability to collect seismic-reflection data in the upper [Formula: see text] of the subsurface (ultrashallow seismic reflection: USR) is important because it offers an alternative to ground-penetrating radar in regions exhibiting high attenuation of electromagnetic signal. Previous work has demonstrated that the critical controllable factors in obtaining USR data are dense spatial sampling and high [Formula: see text] frequency content. The frequency content is strongly controlled by the characteristics of the sources used to generate the seismic energy. Qualitative experiments have indicated that the higher-energy near-surface seismic reflection projectile sources lack some of the high-frequency energy exhibited by smaller-energy sources. In addition, soil moisture content has been shown to be an important variable in the frequency content of data even if the same source is used. In both cases, the reduction in the frequency content of the data has been hypothesized to correlate with greater amounts of near-source, nonlinear deformation. A corollary of this hypothesis states that source energy and soil-moisture affect the amount of nonlinear deformation generated by near-surface seismic energy sources. As an initial test of this corollary, experiments have been conducted in a controlled laboratory setting to quantitatively determine the relationship between source energy, soil-moisture conditions, and the amount of near-source, nonlinear deformation. Projectile sources with different energy (ammunition) were fired into a sand pit having varied moisture content, and the resulting voids were filled with plaster. The volume of the casts (representing the most substantial region of nonlinear deformation) was then measured. Results indicate that source energy and soil moisture have a direct correlation with the amount of near-source, nonlinear deformation.

Recording seismic reflections using rigidly interconnected geophones

Ultrashallow seismic reflection surveys require dense spatial sampling during data acquisition, which increases their cost. In previous efforts to find ways to reduce these costs, we connected geophones rigidly to pieces of channel iron attached to a farm implement. This method allowed us to plant the geophones in the ground quickly and automatically. The rigidly interconnected geophones used in these earlier studies detected first‐arrival energy along with minor interfering seismic modes, but they did not detect seismic reflections. To examine further the feasibility of developing rigid geophone emplacement systems to detect seismic reflections, we experimented with four pieces of channel iron, each 2.7 m long and 10 cm wide. Each segment was equipped with 18 geophones rigidly attached to the channel iron at 15‐cm intervals, and the spikes attached to all 18 geophones were pushed into the ground simultaneously. The geophones detected both refracted and reflected energy; however, no significant signal distortion or interference attributable to the rigid coupling of the geophones to the channel iron was observed in the data. The interfering seismic modes mentioned from the previous experiments were not detected, nor was any P‐wave propagation noted within the channel iron. These results show promise for automating and reducing the cost of ultrashallow seismic reflection and refraction surveys.

Preliminary results from at-sea tests of the Autonomous Buoyed Environmental Sensor System (ABES)

ABES is an autonomous oceanographic measurement system which provides relatively dense spatial and temporal sampling of the highly variable shallow-water ocean environment. Compared to more traditional sampling methods, ABES is low cost; albeit providing slightly lower measurement accuracy and resolution. ABES is not intended to replace existing high-resolution oceanographic instrumentation, but fill the niche for a low-cost, lower-resolution system that would provide higher spatial sampling. The instrument is a sensor string connected to a cylindrical electronics canister. The prototype sensor string contains one electromagnetic current meter, three tilt sensors, three magnetoresistive compasses, and ten conductivity, temperature, and depth sensors. These sensors are distributed along a 40-m cable. A system controller, GPS receiver, and spread-spectrum radio are housed in the 1-m-long electronics canister. Together, these components weigh 10 kg in air and occupy a volume of 5 l. Data collected during three at-sea tests in 1998 and compared to traditional CTD measurements suggest that ABES can effectively define the temporal and spatial characteristics of the shallow-water environment. [Work supported by ONR 321-SS and 322-PO.]

First Results from the Parkes Multibeam High-velocity Cloud Survey

AbstractWith the onset of the HI Parkes All-Sky Survey (HIPASS), a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HI sky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately.

Exploring the Shetland–Faeroes Basin using wide-angle seismic technology

Densely sampled wide-angle seismic profiles using ocean-bottom seismometers have been used to obtain detailed velocity models of the Shetland–Faeroes Basin in both two and three-dimensions. We demonstrate that the wide-angle (refraction) technique, which employs dense spatial sampling to long offsets, may be successful in areas such as the Atlantic margin, where deep water and/or volcanic horizons mitigate against successful profiling using conventional methods. We employ a series of ray-trace and tomographic inversion procedures on travel time data from profiles across the basin axis to give detailed in situ velocity models and accompanying resolution and uncertainty estimates. Comparison of the in situ velocity of the Mid-Faeroe Ridge with deterministic lithology–velocity data from nearby boreholes strongly implies that Cretaceous–Tertiary shale and sandstone sequences extend to depths of 4.5 km. The presence of pre-Cretaceous sedimentary lithologies at 4.5–8.0 km depth is supported by in situ velocities of 4.0–4.5 km s −1 which are best explained by well-consolidated sediments and are incompatible with likely constituents of the basement. The southeastern flank of the Mid-Faeroe Ridges is marked by a prominent fault marking a lateral velocity variation of about 0.5 kms −1 . A rapid increase in velocity at 8 km depth suggests transition into the basement, whose velocity is similar to high-pressure measurements of basalts from the Faeroe Islands. Data from an areal grid of shots and receivers centred over the Mid-Faeroe Ridge have been inverted to give a fully three-dimensional velocity model of a small volume to depths of 10+ km. Variations in the depth of isovelocity surfaces from this model correspond closely with the geometry of the Ridge, and the position of the flanking normal fault and its adjacent basin to the southeast.

A rotating parallel hole collimator for high resolution imaging of medium energy radionuclides

A rotating parallel hole collimator has been designed and built for high resolution gamma camera imaging of medium energy radionuclides. This cast lead collimator has 2.0 mm thick septa and was designed for imaging 50-100 mCi I-131 radiolabeled monoclonal antibodies being administered intratumorally for brain tumor therapy. Projection data from different collimator rotation positions are summed to create a composite projection dataset with more dense spatial sampling than projection data at a single position. Measured single photon emission computed tomography (SPECT) resolution with I-131 was 5.5-7.5 mm full-width at half-maximum (FWHM) for a 13 cm radius of rotation compared with 9.9-13.9 mm with a conventional medium energy collimator. SPECT resolution for the rotating collimator was only slightly worse with I-131 than with Tc-99m, indicating good attenuation by the septa. Sensitivity was 56 counts/sec/mCi with a tight region of interest (ROI) around the projection of an I-131 line source 12 cm from the collimator surface and 64 counts/sec/mCi with a tight ROI around the projection of an 11 cm diameter I-131 disk source 10 cm from the collimator surface. SPECT studies of 3.7 cm and 6.2 cm diameter shell-core tumor phantoms filled with I-131 were made. The lower activity core and higher activity shell were well resolved in a SPECT study using the rotating hole collimator. SPECT shell to core activity concentration ratios were more accurate with the rotating collimator than with a medium energy collimator. These results show the potential of this rotating collimator for high resolution human tumor studies with I-131 tracers.

Large deformation mechanical testing of biological membranes using speckle interferometry in transmission II: Finite element modeling

In holography and speckle interferometry the measurement range is generally limited by the greatest number of fringes that can be resolved in a single image. As a result these techniques have been generally confined to small displacement measurement applications. In the case of out-of-plane measurements one can overcome this limitation by simply adding incremental measurements at individual detector pixels. In the case of in-plane measurements, however, summing incremental measurements is not a straightforward procedure since the interference pattern moves laterally across the detector as the material deforms. We describe a modeling technique based on finite elements which solves this problem. In combination with a full field method such as holography or speckle interferometry, it provides a very sensitive measurement technique with dense spatial sampling and large dynamic range. Experimental results of speckle interferometry operating in transmission to measure in-plane displacements of biological membranes are presented, where total material displacements are of the order of millimeters. The results also demonstrate how the finite strain tensor is calculated analytically from the data at any point on the material.

Turbulent flow structure at concordant and discordant open-channel confluences

Models of flow at river-channel confluences that consist of two concordant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structure in the streamwise and vertical directions at both concordant and discordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influence on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The spatial distribution of normal stress varies with bed geometry as bed discordance generates a distortion of the mixing layer between the confluent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a decrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resulting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.

The Gaussian scale-space paradigm and the multiscale local jet

A representation of local image structure is proposed which takes into account both the image's spatial structure at a given location, as well as its "deep structure", that is, its local behaviour as a function of scale or resolution (scale-space). This is of interest for several low-level image tasks. The proposed basis of scale-space, for example, enables a precise local study of interactions of neighbouring image intensities in the course of the blurring process. It also provides an extrapolation scheme for local image data, obtained at a given spatial location and resolution, to a finite scale-space neighbourhood. This is especially useful for the determination of sampling rates and for interpolation algorithms in a multilocal context. Another, particularly straightforward application is image enhancement or deblurring, which is an instance of data extrapolation in the high-resolution direction. A potentially interesting feature of the proposed local image parametrisation is that it captures a trade-off between spatial and scale extrapolations from a given interior point that do not exceed a given tolerance. This trade-off suggests the possibility of a fairly coarse scale sampling at the expense of a dense spatial sampling (large relative spatial overlap of scale-space kernels). The central concept developed in this paper is an equivalence class called the multiscale Zocal jet, which is a hierarchical, local characterisation of the image in a full scale-space neighbourhood. For this local jet, a basis of fundamental polynomials is constructed that captures the scale-space paradigm at the local level up to any given order.