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SUMMARY Severely crooked 2-D reflection profiles generate a swath distribution area of common midpoint (CMP) traces. These traces are scattered in all directions and no longer correspond to CMP gathers directly below the acquisition line, effectively posing a pseudo-3-D problem. Standard seismic processing techniques neglect the 3-D character of crooked seismic transects and often yield suboptimal images. The out-of-plane energy ignored in 2-D stacking techniques causes amplitude smearing effects due to anomalous reflection traveltimes. The imaging of dipping structures in geologically complex areas, such as in Larder Lake, are particularly affected because of complex energy scattering. These out-of-plane reflected wavefield components degrade energy focusing and signal alignment, worsening as the obliqueness of the profile with respect to local geology increases. To mitigate the contribution of scattered out-of-plane energy, we implement azimuthal binning. Pseudo-processing lines, parallel to the maximum geological dip, are generated by using locally optimum bin orientations. We use a priori geological information to devise optimum bin orientations in accordance to the local strike. We find that the optimized binning method significantly outperforms the standard approach in terms of in-phase amplitude stacking and reflection continuity. This is because the standard approach is based on a bin geometry always perpendicular to the seismic line traverse, ignoring 3-D effects, whereas in the optimum binning approach each individual bin may have a different orientation along the line. Moreover, we enhance the final reflectivity image by performing seismic migration with a velocity model that incorporates near-surface velocity estimations from full-waveform inversion. We demonstrate that the combination of these two innovations results in a significant impact on the imaging quality of structures at depth. In addition to imaging structures at depth, we also illustrate the significance of their continuation towards the near surface. This is especially important in the Larder Lake area where tectonic activity may have emplaced ore deposits in the first few kilometres of the subsurface.

Bipolaris sorokiniana (Sacc.) Shoemaker is a major foliar and root-infecting pathogen of cool-season forage and turf grasses and small grains in the southeastern United States (2). In North America, B. sorokiniana has been reported from bermudagrass (Cynodon dactylon [L.] Pers.) once in California in 1961 (1), and rarely from other warm-season grasses in the southeastern United States. In May, July, September, and October 2002, B. sorokiniana sporulation was observed on leaves of common bermudagrass exhibiting necrotic lesions and dieback in waste application fields on three commercial swine farms in Chickasaw, Lowndes, and Webster counties, MS. Leaves were collected (50 per farm per month), surface-disinfested, plated on water agar, and observed for fungal sporulation on leaf surfaces after 7 to 10 days (3,4). The pathogen was detected on 1 to 3 farms each month in leaves that were infected with numerous other dematiaceous hyphomycetes (3,4). Three randomly selected single-spore isolates of B. sorokiniana from each of bermudagrass and annual ryegrass (Lolium multiflorum Lam.), collected at the Webster County farm, were compared for select features of morphology and pathogenicity on bermudagrass. Isolates differed significantly in growth rates, amount of sporulation, and spore sizes on cornmeal agar, but differences were not consistently related to hosts of origin. In plants inoculated by atomizing equal quantities of spores (2.8 × 104/ml) onto foliage, isolates of B. sorokiniana from bermudagrass and ryegrass both caused significantly (P = 0.05) more severe foliar necrosis after 10 days than B. cynodontis (5 pots of seeded plants per treatment in each of two experiments). B. sorokiniana was reisolated from disinfested, symptomatic bermudagrass leaf tissue following inoculations. To our knowledge, this is the first report of B. sorokiniana on bermudagrass in North America outside of California (1) and indicates that this pathogen is highly virulent on bermudagrass in the southeastern United States (3,4). Of potentially greater importance is the fact that one of the most common and widespread forage and turf grass species in the southeastern United States can serve as an alternate host for maintenance and increase of inoculum of B. sorokiniana during summer months. References: (1) R. M. Endo. Plant Dis. Rep. 45:869, 1961. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. ARS, USDA, 2003. (3) R. G. Pratt. Agron. J. 92:512, 2000. (4) R. G. Pratt. Plant Dis. 85:1206, 2001.

Johnsongrass (Sorghum halepense (L.) Pers.), broadleaf signalgrass (Brachiaria platyphylla (L.) Beauv.), and yellow foxtail (Setaria glauca L.) are common volunteer grasses in bermudagrass (Cynodon dactylon (L.) Pers.) pastures in the southeastern United States. Johnsongrass and broadleaf signalgrass are potential forages whereas yellow foxtail is a noxious weed. In 1999 and subsequent years, necrosis and dieback of leaves, stems, and roots, stunting, and plant death were observed on all three species in bermudagrass pastures in north Mississippi (3). Symptoms on johnsongrass and yellow foxtail were most severe where bermudagrass exhibited severe symptoms of infection caused by dematiaceous hyphomycetes (2,3); symptoms on broadleaf signalgrass often occurred independently. Symptomatic leaf tissues from 15 to 33 plants of each species and stem and root tissues from 4 to 14 plants of johnsongrass and yellow foxtail were surface disinfested, plated on water agar, and examined for sporulation after 5 to 10 days (2,3). Pathogens were identified by specific morphological features of spores and sporulation as on bermudagrass (3), and axenic cultures were established by spore transfers to cornmeal agar. Bipolaris cynodontis (Marig.) Shoemaker, Curvularia lunata (Wakk.) Boedijn, C. geniculata (Tracy & Earle) Boedijn, and Exserohilum rostratum (Drechs.) Leonard & Suggs were isolated from symptomatic leaves of all three grasses and frequently also observed on stems and roots. B. stenospila (Drechs.) Shoemaker was observed only on broadleaf signalgrass (19 of 33 plants) and B. spicifera (Banier) Subr. on johnsongrass and yellow foxtail. Species most frequent on leaves (58 to 100%) were B. spicifera, C. lunata, and E. rostratum on johnsongrass and yellow foxtail and B. cynodontis, B. stenospila, and E. rostratum on broadleaf signalgrass. The three grasses were grown from seed in potting mix in the greenhouse (one plant per 375-cm3 container), and five replicates 31 to 60 days old were inoculated with a mixture of three isolates of each pathogen observed on them in two experiments. Conidia produced from infested wheat and oat grain were atomized onto foliage (1.2 to 4 × 104 conidia per ml, 20 ml per plant) as described (2). All pathogens incited similar necrotic lesions and streaks on the three grasses after 12 to 15 days, and B. stenospila also caused extensive golden yellow chlorosis on broadleaf signalgrass. All pathogens caused significant (P = 0.05) necrosis (means = 5 to 35% of foliage necrotic based on visual estimates, controls = 1 to 3%), and all were reisolated and grown in pure culture by spore transfers to cornmeal agar from surface-disinfested, symptomatic leaf tissue of each grass. When bermudagrass grown from seed was inoculated at similar spore concentrations, isolates of E. rostratum, B. cynodontis, and B. spicifera from two or all three grasses caused symptoms as severe as did isolates from bermudagrass. Results document new North American or worldwide records of occurrence and pathogenicity for B. cynodontis, C. geniculata, and C. lunata on all three grasses, B. stenospila and E. rostratum on broadleaf signalgrass, and B. spicifera on johnsongrass and yellow foxtail (1). These volunteer grasses, bermudagrass, and the six fungi all appear to represent large, interacting complexes of multiple hosts and potentially cross-infecting pathogens. Reference: (1) D. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. USDA, ARS, 2005. (2) R. Pratt, Agron. J. 92:512, 2000. (3) R. Pratt. Phytopathology 95:1183, 2005.

Fungal diseases of annual ryegrass (Lolium multiflorum Lam.), a cool-season species grown for forage and turf in the southeastern United States, and bermudagrass (Cynodon dactylon (L.) Pers.), a warm-season perennial, usually are considered distinct. In May 2002 and 2004, symptoms of leaf and stem necrosis were observed simultaneously in forage bermudagrass and overseeded annual ryegrass on a swine waste application site in Mississippi. Sporulation by nine species of Bipolaris, Curvularia, Drechslera, and Exserohilum was observed on symptomatic leaves of ryegrass that were surface disinfested and plated on agar each year, and axenic cultures were established by spore transfers. These isolations represent four new worldwide and two new North American records of occurrence of fungal pathogens on ryegrass, and three new records for the southeastern United States and Mississippi. Bipolaris cynodontis, a common bermudagrass pathogen, and Drechslera dictyoides were observed most frequently on ryegrass during both years. In excised leaves of ryegrass inoculated with infested agar discs, most isolates of B. cynodontis from ryegrass and bermudagrass were equally virulent and caused necrosis equal to or greater than that caused by D. dictyoides, an established ryegrass pathogen. Isolates of B. cynodontis from both hosts also caused similar symptoms in foliage of ryegrass and bermudagrass following spore inoculations. Pathogenicity to ryegrass of five other species of dematiaceous hyphomycetes that represented new worldwide or North American records of occurrence also was demonstrated by foliar inoculations. All pathogens were reisolated from symptomatic tissues and grown in axenic culture for fulfillment of Koch's postulates. Results demonstrate that most of the same species of dematiaceous hyphomycetes infect both forage ryegrass and bermudagrass in Mississippi and indicate that B. cynodontis, in particular, may be a common and virulent pathogen of ryegrass in the southeastern United States.

We successfully apply the acoustic Laplace–Fourier waveform tomography method to delineate P-wave velocity structures of the mega-splay fault system in the central part of the seismogenic Nankai subduction zone offshore Japan, using densely sampled wide-angle ocean bottom seismograph (OBS) data originally acquired in 2004. Our success is due to new and carefully designed data preconditioning and inversion strategies to mitigate (i) the well-known non-linearity of waveform inversion, (ii) the challenges arising from crustal-scale survey designs (e.g. undersampling of the OBSs), and (iii) modelling errors due to the use of the acoustic assumption.We identify a sixfold set of key components that together lead to the success of the high-resolution waveform tomography image: (i) Availability of low-frequency components (starting at 2.25 Hz) reducing the non-linearity, and access to large offset data (up to 55 km) increasing the depth of illumination and the recovery of low wavenumber components. (ii) A highly accurate traveltime tomography result (with an rms error of approximately 60 ms) that further mitigates the non-linearity. (iii) A hierarchical inversion approach in which phase spectra are inverted first to reduce artefacts from the acoustic assumption, and amplitude information is only incorporated in the final stages. (iv) A Laplace–Fourier domain approach that facilitates a multiscale approach to mitigate non-linearity by restricting the inversion to the low frequency components and early arrivals first, and sequentially including higher frequencies and later arrivals. (v) A pre-conditioning strategy for eliminating undesirable high wavenumber components from the the gradient. (vi) A strategy for source estimation that reduce the influence of the instrumental design.In the OBS case study used for illustration purposes, Laplace–Fourier waveform tomography retrieves velocity anomalies as small as 700 m (horizontally) and 350 m (vertically) above the top of the Philippine Sea Plate. The resulting velocity structures include low-velocity zones and thrust structures which have not been previously identified clearly. The velocity models are validated by scrutiny of synthetic and observed waveforms, by evaluating the coherency of source estimates, and by comparison with 3-D pre-stack migrated (PreSDM) images. Chequerboard tests and point-scatter tests demonstrate both the reliability and the limitations of the acoustic implementation.

— The structural amplitude effect, associated with focusing and defocusing due to the reflector curvature, importantly contributes to reflection seismic amplitudes. This paper develops a conciliatory approach for estimating the structural amplitude effect and the attributes of amplitude variation versus offset (AVO). The AVO attributes are extracted from raw amplitudes, in which the structural effect is taken into account explicitly based on a structural model reconstructed from travel-time inversion. One of the goals is to conduct the AVO analysis not just locally (per CDP) but also horizontally to see the global variation along the reflection. The lateral variations of AVO attributes are decomposed by the Chebyshev expansion. The method is demonstrated with an example of weak shallow gas-water contact appearing on a 2-D seismic profile of a site survey in the North Sea.

Conventional ultrasonography reconstruction techniques, such as B-mode, are based on a simple wave propagation model derived from a high frequency approximation. Therefore, to minimize model mismatch, the central frequency of the input pulse is typically chosen between 3 and 15 megahertz. Despite the increase in theoretical resolution, operating at higher frequencies comes at the cost of lower signal-to-noise ratio. This ultimately degrades the image contrast and overall quality at higher imaging depths. To address this issue, we investigate a reflection imaging technique, known as reverse time migration, which uses a more accurate propagation model for reconstruction. We present preliminary simulation results as well as physical phantom image reconstructions obtained using data acquired with a breast imaging ultrasound tomography prototype. The original reconstructions are filtered to remove low-wavenumber artifacts that arise due to the inclusion of the direct arrivals. We demonstrate the advantage of using an accurate sound speed model in the reverse time migration process. We also explain how the increase in computational complexity can be mitigated using a frequency domain approach and a parallel computing platform.

SUMMARY A comprehensive validation of 2-D, frequency-domain, acoustic wave-equation tomography was undertaken in a ‘blind test’, using third-party, realistic, elastic wave-equation data. The synthetic 2-D, wide-angle seismic data were provided prior to a recent workshop on the methods of controlled source seismology; the true model was not revealed to the authors until after the presentation of our waveform tomography results. The original model was specified on a detailed grid with variable P-wave velocity, S-wave velocity, density and viscoelastic Q-factor structure, designed to simulate a section of continental crust 250 km long and 40 km deep. Synthetic vertical and horizontal component data were available for 51 shot locations (spaced every 5 km), recorded at 2779 receivers (spaced every 90 m), evenly spread along the surface of the model. The data contained energy from 0.2 to 15 Hz. Waveform tomography, a combination of traveltime tomography and 2-D waveform inversion of the early arrivals of the seismic waveforms, was used to recover crustal P-velocity structure from the vertical component data, using data from 51 sources, 1390 receivers and frequencies between 0.8 and 7.0 Hz. The waveform tomography result contained apparent structure at wavelength-scale resolution that was not evident on the traveltime tomography result. The predicted (acoustic) waveforms in the final result matched the original elastic data to a high degree of accuracy. During the workshop, the exact model was revealed; over much of the model the waveform tomography results provided a good correspondence with the true model, from large- to intermediate-(wavelength) scales, with a resolution limit on the order of 1 km. A significant, near-surface low-velocity zone, invisible to traveltime methods, was correctly recovered; the results also provided a high-resolution image of the complex structure of the entire crust, and the depth and nature of the crust–mantle transition. Some inaccuracies were observed near the edges of the images due to the limited ray coverage inherent to the footprint of the survey geometry. Several aspects of the waveform tomography strategy were critical to the success of the acoustic method with realistic, synthetic, viscoelastic data: (i) the accuracy of the starting model from traveltime tomography, (ii) implementation in the frequency domain, (iii) the use of complex-valued frequencies to effect time damping of the data residuals, (iv) the selection of a suitable subset of data and data frequencies, (v) progressive inversion of low- to high-frequency components of the data, (vi) initial, pre-inversion matching of the amplitudes between observed and modelled data, and (vii) sufficient preconditioning of both the data and the update images. Combined, these strategies were effectively equivalent to a multiscale approach that mitigated the non-linearity of the seismic inverse problem. During the inversion we carried out repeated forward modelling to ensure our modelled waveforms matched the observed data as closely as possible in both frequency and time domains. The synthetic data set used in this paper provides a benchmark for future testing of modelling, inversion, and imaging algorithms for wide-angle lithospheric imaging.

SUMMARY We provide a series of numerical experiments designed to test waveform tomography under (i) a reduction in the number of input data frequency components (‘efficient’ waveform tomography), (ii) sparse spatial subsampling of the input data and (iii) an increase in the minimum data frequency used. These results extend the waveform tomography results of a companion paper, using the same third-party, 2-D, wide-angle, synthetic viscoelastic seismic data, computed in a crustal geology model 250 km long and 40 km deep, with heterogeneous P-velocity, S-velocity, density and Q-factor structure. Accurate velocity models were obtained using efficient waveform tomography and only four carefully selected frequency components of the input data: 0.8, 1.7, 3.6 and 7.0 Hz. This strategy avoids the spectral redundancy present in ‘full’ waveform tomography, and yields results that are comparable with those in the companion paper for an 88 per cent decrease in total computational cost. Because we use acoustic waveform tomography, the results further justify the use of the acoustic wave equation in calculating P-wave velocity models from viscoelastic data. The effect of using sparse survey geometries with efficient waveform tomography were investigated for both increased receiver spacing, and increased source spacing. Sampling theory formally requires spatial sampling at maximum interval of one half-wavelength (2.5 km at 0.8 Hz): For data with receivers every 0.9 km (conforming to this criterion), artefacts in the tomographic images were still minimal when the source spacing was as large as 7.6 km (three times the theoretical maximum). Larger source spacings led to an unacceptable degradation of the results. When increasing the starting frequency, image quality was progressively degraded. Acceptable image quality within the central portion of the model was nevertheless achieved using starting frequencies up to 3.0 Hz. At 3.0 Hz the maximum theoretical sample interval is reduced to 0.67 km due to the decreased wavelengths; the available sources were spaced every 5.0 km (more than seven times the theoretical maximum), and receivers were spaced every 0.9 km (1.3 times the theoretical maximum). Higher starting frequencies than 3.0 Hz again led to unacceptable degradation of the results.

Sclerotinia trifoliorum and Sclerotium rolfsii are pathogens for which similar mechanisms of parasitism have been proposed. This suggested that resistance to these pathogens may be related in a common host plant. This study was undertaken to determine whether selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also increases resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Resistance in excised leaf tissues was evaluated according to the rate of necrosis induced by Sclerotium rolfsii following inoculation with mycelium. Resistance to Sclerotium rolfsii in whole plants was evaluated according to their survival following crown inoculations. Three alfalfa populations previously selected from cv. Delta for quantitative resistance to Sclerotinia trifoliorum exhibited enhanced resistance to Sclerotium rolfsii, in comparison with Delta or with susceptible populations, in excised leaf tissues. When whole plants of Delta and two of these populations, Sclerotinia trifoliorum resistant (STR) and Mississippi Sclerotinia resistant (MSR), were inoculated with Sclerotium rolfsii at 3 to 8 weeks of age, significant (P = 0.01) differences in survival were attributed to plant age at inoculation and alfalfa populations. Survival of both MSR and STR was significantly (P = 0.05) greater than for Delta; the best differential results were obtained by inoculating plants 5 to 7 weeks old. To evaluate relationships of resistance to Sclerotinia trifoliorum and Sclerotium rolfsii over a broader genetic background, additional populations were selected for resistance to Sclerotinia trifoliorum from four other alfalfa cultivars by leaf-inoculation techniques, and this resistance was confirmed by whole-plant inoculations. In excised leaf tissues, all four of these populations also expressed enhanced resistance to Sclerotium rolfsii in comparison with either parent cultivars or populations of comparable size selected at random. These results establish that selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also confers enhanced resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Results suggest, therefore, that genes for quantitative resistance to Sclerotinia trifoliorum and Sclerotium rolfsii in alfalfa are likely to be synonymous, overlapping, or closely linked.

USA, environmental stresses include saturated soils in the winter and hot, drought conditions in the summer. Summers in the southeastern USA produce a harsh environment Though environmental and other factors can reduce for survival of white clover (Trifolium repens L.) stolons. Long periods of drought and hot temperatures are interspersed with rain showers white clover growth and stands at any time of the year, that create ideal conditions for fungal pathogenesis. Previous studies complete death of the stand usually occurs in the sumindicated that Macrophomina phaseolina (Tassi) Goidanich may be mer (Gibson and Hollowell, 1966). an important pathogen that limits survival of white clover stolons in Fungal disease pressure plays an integral role in the the summer. The objective of this study was to determine the range survival of white clover stolons in the summer. The in response of 20 white clover cultivars, germplasms, and breeding normal summer environment of the southeastern USA and naturalized populations for resistance to M. phaseolina using a is ideal for fungal pathogenesis. Fungicides have been leaf tissue assay. Discs were cut from leaves excised from 50 plants shown to improve white clover yield and persistence of each entry and inoculated with an agar plug cut from the margin (James et al., 1980; Pederson et al., 1991). Fungicide of a M. phaseolina colony. Leaf discs were scored according to the rate of necrosis induced by the pathogen. The experiment was conducted as treatment during the summer months improves white a randomized complete block with four replicates and was repeated clover stolon density, growing point density, and stolon with 50 additional plants from each entry. Differences in responses length (Pederson and Pratt, 1995). Germplasm sources of entries to inoculation with M. phaseolina were observed in each differ in their response to fungicide treatment; Brown run of the experiment. Brown Loam Syn. 2 germplasm and North Loam Syn. 2 germplasm showed less response to fungiGA population had the least disease and the greatest number of plants cide treatment than ‘Regal’ and ‘Louisiana S-1’ white selected as resistant to M. phaseolina. Large-leaf plants selected for clover (Pederson and Pratt, 1995). These data suggest resistance gave highly consistent responses when retested, with 35% that this germplasm may have some resistance or tolerof the plants having no leaf necrosis following inoculation with M. phaseolina. The leaf tissue assay was not as reliable for selecting ance to summer fungal diseases. Naturalized populaconsistent resistant phenotypes among small-leaf white clover entries, tions that have survived in pastures for a number of as 37% of the plants selected as resistant were rated as susceptible years have also been suggested as sources of resistance upon retesting. Resistance to M. phaseolina was observed in adapted or tolerance to summer stolon diseases (Cope, 1978). white clover germplasm, and development of new cultivars with this Numerous fungi are frequently isolated from white resistance should improve white clover summer survival. clover stolons including Colletotrichum spp., Curvularia trifolii (Kauff.) Boedijn, Fusarium spp., Mycoleptodiscus terrestris (Gerdemann) Ostazeski, M. phaseolina, tolonsare the means by which individual white clo- Rhizoctonia spp., Sclerotium rolfsii Sacc., and others ver genotypes persist in the field. The seedling tap- (Halpin et al., 1963; Latch and Skipp, 1987; McGlohon, root system only survives for the first year or two of 1959; Pederson and Pratt, 1995). The relative imporgrowth. When the taproot, primary stem, and basal por- tance of individual fungal species for stolon survival has tion of the stolons die, plant survival is entirely depen- rarely been addressed. Some fungi appear to play no dent on new stolon growth from the terminal ends. Indi- active role in stolon necrosis, arriving only as secondary vidual stolons from the same plant are genetically pathogens when death and decay have already begun. identical, but may no longer be connected. New growth Other fungi may be quite pathogenic. Following a previand branching continues at the stolon tips while the ous study of white clover stolon growth with and without older basal portion of the stolon dies. The genotype will fungicide treatment (Pederson and Pratt, 1995), Pratt survive as long as new stolon growth at the tip exceeds (1998, unpublished data) evaluated the pathogenicity of the progression of necrosis from the basal end.

Previous studies established that excised leaf tissues of alfalfa can be inoculated with Sclerotinia trifoliorum to select for heritable resistance, but the original procedures were not practical for use in large-scale screening programs. In this study, simplified leaf inoculation procedures for more rapid screening for resistance, based on direct application of leaf tissues to colonies of the pathogen on agar media, were evaluated. Cotyledons, unifoliate leaves, and leaflets of trifoliolate leaves of plants 7 to 21 days old from three relatively susceptible cultivars and one resistant germ plasm were applied, with and without wounding, directly to colony margins of S. trifoliorum on cornmeal agar, V8 juice agar, and water agar. Leaves were scored according to the rate and extent of development of necrosis. Significant differences between alfalfa populations were expressed in unifoliate leaves and trifoliolate leaflets but not in cotyledons. Disease severity in the resistant germ plasm (Mississippi Sclerotinia-Resistant [MSR]) was less than in the three cultivars on all agar media. Wounding of leaf tissues increased disease severity and greatly reduced the incidence of symptomless leaves, which are indicated to be escapes, but wounding generally did not prevent expression of resistance in MSR. Results indicate that initial screening for resistance to S. trifoliorum in alfalfa seedlings may be accomplished by applying wounded unifoliate leaves and leaflets of trifoliolate leaves directly to colonies on cornmeal or V8 juice agars.

Effective breeding for economically important resistance to Sclerotinia trifoliorum Eriks in alfalfa (Medicago sativa L.) requires that the plant breeder have an understanding of any genetic basis for variability in measurements of resistance and the likelihood that selection for resistance to one isolate confers resistance to other isolates. Fifty alfalfa plants were repeatedly evaluated for their resistance to five isolates of S. trifoliorum. Tips of excised stems were inoculated with a S. trifoliorum isolate and stem necrosis was measured after 14 d. The inoculation of five stems from each of 50 plants with a single isolate was repeated five times. For the length of stem necrosis, the interaction of pathogen isolate and alfalfa plant was not significant but differences among alfalfa plants and among isolates were significant. The plants that, on average, developed larger necrotic regions did not have more or less variation in their replicated measurements of necrosis than those plants which averaged much smaller necrotic regions. The genotype of the alfalfa plant had a greater effect on the variability in replicate measurements of disease than the genotype of pathogen isolate. Resistance to different isolates were significantly correlated (r = 0.48-0.60) and selection of some plants for resistance to one isolate appeared to improve resistance to other isolates.

The viscoelastic wave equation is an integro‐differential equation that requires special methods when using time‐domain numerical finite‐difference methods. In the frequency domain, the integral terms are easily represented by complex valued elastic media properties. There are further significant advantages to using the frequency domain if the forward or the inverse problem requires modeling or inverting a large number of prestack source gathers. Numerical modeling is expensive for seismic data because of the large number of wavelenghths typically separating sources from receivers, which results in a need for a large number of grid points. A major obstacle to using frequency‐domain methods is the consequent storage requirements. To reduce these, we maximize the accuracy and simultaneously minimize the spatial extent of the numerical operators. We achieve this by extending earlier published methods introduced for the viscoacoustic case to the viscoelastic case. This requires the formulation of two new numerical operators: a differencing operator in a rotated coordinate frame and a lumped mass term. The new operators are combined with ordinary second‐ order, finite‐difference operators in an optimal manner to minimize numerical errors without increasing the size of the numerical operator. For a fixed number of grid points, the resulting second‐ order differencing scheme is no more expensive than an ordinary second‐order differencing scheme, but a numerical dispersion analysis shows that the number of grid points required per smallest wavelength is reduced from approximately 15 to approximately 4. The new scheme is also capable of handling embedded fluid layers without instability. We demonstrate that no further improvement in performance can be achieved using higher order spatial operators because of the associated computational overheads associated with the larger differencing operators. The new viscoelastic modeling scheme is used to study a crosshole data set in which the exact nature of the seismic coda is unclear. The results of the modeling study indicate this coda is likely related to the generation of mode‐converted shear waves within the complicated, finely layered sediments at the site.

We apply Laplace-Fourier 2D and 2.5D waveform tomography to a challenging wide-angle land data set from a geologically complex region in Japan. The survey line is crooked (up to 500 m deviation from the 2D line over the 16 km line length), and the area contains significant topographic relief (up to 400 m) and near-surface weathering layers. The lack of reflections previously prevented conventional reflection processing. We demonstrate that 2D acoustic waveform tomography is capable of extracting a reliable velocity model from refractions and wide-angle reflections with carefully designed data-preconditioning and inversion strategies. Sources and receivers are projected onto a 2D plane without preserving offsets: we eliminate traces where errors in offsets are large. We restrict the inversion to phase information at lowest frequencies, and discretize the model with a fine grid, in order to minimize the effects of the crooked line and the topography. We use balanced data amplitudes (using surface-consistent static deconvolution) to conduct the source estimation. We extensively validate the obtained velocity model by comparisons with a well log and observed waveforms, through scrutiny of the resulting reverse-time-migration image. An additional test with 2.5D waveform tomography demonstrates that 2D waveform inversion is adequate for these data.

SUMMARY Modern wide-angle surveys are often multi-fold and multi-channel, with densely sampled source and receiver spacings. Such closely spaced data are potentially amenable to multi-channel techniques involving wavefield propagation methods, such as those commonly used in reflection data processing. However, the wide-angle configuration requires techniques capable of handling very general wave types, including those not commonly used in reflection seismology. This is a situation analogous to that faced in cross-borehole seismics, where similar wave types are also recorded. In a real cross-borehole example, we compare pre-stack migration, traveltime tomography and wavefield inversion. We find that wavefield inversion produces images that are quantitative in velocity (as are the tomograms) but are of significantly higher resolution; the wavefield inversion results have a resolution comparable to that of the (qualitative) pre-stack migration images. We seek to extend this novel development to the larger-scale problem of crustal imaging. An essential element of the approach we adopt is its formulation entirely within the temporal frequency domain. This has three principal advantages: (1) we can choose to ‘decimate’ the data, by selecting only a limited number of frequency components to invert, thus making inversion of data from large numbers of source positions feasible; (2) we can mitigate the notorious non-linearity of the seismic inverse problem by progressing from low-frequency components in the data to high-frequency components; and (3) we can include in the model any arbitrary frequency dependence of inelastic attenuation factors, Q(ω), and indeed solve for the spatial distribution of Q. An initial synthetic test with an anomaly located within the middle crust yields a velocity image with the correct structural features of the anomaly and the correct magnitude of velocity anomaly. This is related to the fact that the reconstruction is obtained from forward-scattered waves. Under these conditions, the method thus behaves much like tomography. A second test with a deeper, more extensive anomaly yields an image with the correct velocity polarity and the correct location, but with a deficiency in low and high wavenumbers. In this case, this is because the reconstruction is obtained from backscattered waves; under these conditions the method behaves not like tomography, but like migration. A more extensive test, based on a large wide-angle survey in south-eastern California and western Arizona, demonstrates a real potential for high-resolution imaging of crustal structures. Although our results are limited by the acoustic approximation and by the relatively low frequencies that we can model today, the images are sufficiently encouraging to warrant future research. The problem of local minima in the objective function is the most significant practical problem with our method, but we propose that appropriate ‘layer’ stripping methods can handle this problem.

White clover (Trifolium repens L.) stands in the southeastern USA survive during summer drought predominantly through persistence of stolons. The decay and death of stolons during the hot, humid summer may be partially due to fungal diseases. This study determined the effect of summer fungicide applications on stolon survival of 'Regal' and 'Louisiana S-1' white clover compared with drought-tolerant Brown Loam Syn. No. 2 germplasm (BLSyn). The three white clover entries were grown in the field on a Catalpa silty clay (fine, montmorillonitic, thermic Fluvaquentic Hapludoll) at Mississippi State, MS, in three separate 1-yr studies. During summer stolon dormancy, plots of each entry were either untreated or sprayed biweekly with benomyl [methyl 1-(butylcarbamoyl)-2-benzimidazole-carbamate]. In 2 of 3 yr, BLSyn had 23 to 63% greater ground cover, 33 to 55% greater stolon density, and 30 to 47% greater relative live stolon length following the summer drought than Louisiana S-1. In all 3 yr, BLSyn had greater relative live stolon length (7-18%) following the summer drought than Regal. Plots treated with benomyl had 14 to 52% greater stolon density (in 2 of 3 yr), 14 to 40% greater stolon growing point density, and 9 to 27% greater relative live stolon length than untreated plots. Benomyl treatment gave less of an increase in relative live stolon length of BLSyn than in the two cultivars. These results suggest that fungal pathogenesis may reduce white clover stolon survival during summer dormancy, and that greater fungal disease resistance could be part of the mechanism of improved summer survival in BLSyn white clover.

The resolution of a particular solution to a given inverse problem may be quantified by resorting to a posteriori resolution indicators. In constrained least-squares inversions, for weakly nonlinear problems, the authors compute resolution kernels that relate the true model parameters to the estimated ones. These kernels reflect the significant effect on resolution that regularizing operators have. Since these operators are normally chosen to control the effect of noise, the kernels predict the degradation of the solutions in the presence of noise. For real problems with data errors, the knowledge of the resolving kernels enables us to quantify confidence in any parameter estimate, and gives a clear physical picture of the resolution; the resolution (indirectly) depends on the noise level in the data. The resolving kernels are an indication of the averaging of the true model parameters. The question arises as to whether these averages are biased or not. Popular methods, such as SVD truncation, or the use of a priori covariance operators, produce biased averages. In contrast, finite-difference regularizing operators systematically produce unbiased averages and prove to be a superior regularization technique in this respect. The authors tested the use of finite-difference regularizing operators using a simulation case study, in which they synthesized full waveforms for a simulated multi-offset 'VSP' survey, and inverted the picked traveltimes for 1D anisotropic parameters. The model resolution matrix for this case study was shown to be a reliable resolution indicator and it gave invaluable information on the essence of this inverse problem.

In October and November 1999, Vale carried out a series of crosshole seismic investigations in the Eastern Deeps deposit at Voisey’s Bay, Labrador, Canada, with the goal of mapping the continuity of a mineralized massive sulfide zone. Traveltime Tomography revealed a significant velocity contrast between the target zone and surrounding country rock. To add detail to the traveltime images Waveform Tomography was applied. Significant velocity contrasts led to difficulties in constructing an anisotropy model through Traveltime Tomography, and thus our starting model for Waveform Tomography did not satisfy the half-cycle first break criterion. A new approach for determining the best-fit anisotropy model was developed based on waveform fit. We apply a Monte Carlo type method for finding the best-fit 1-D anisotropy model for use in Waveform Tomography. Using simulated annealing, we minimize the norm of the phase residuals in the frequency domain through random perturbations to the anisotropy model.

A 3D ground penetrating radar (GPR) survey was performed at the Haughton impact structure, Devon Island, Canada over heterogeneous carbonate-dominated impact melt breccias. The survey location was chosen for the purpose of qualifying the effect of a scattering medium on the depth of useful imaging in the context of planetary exploration. This survey, and associated 2D, common mid-point (CMP) and error control surveys, show that the depth of penetration is less on the brecciated unit than desired despite the electrical suitability of the material to GPR.

In many areas the geological interpretation of seismic images can benefit from quantitative, high resolution velocity images. Waveform Tomography can yield the appropriate images provided the input data contain large offsets and low frequencies, such as those available from a suite of Ocean Bottom Seismographs (OBSs). We provide an illustration in the Nankai subduction zone, where a full interpretation requires both the delineation of detailed crustal fault structures and a quantitative velocity map. Previous prestack migrations have revealed key structures, including a megasplay fault, and two distinctive low velocity zones. However the migration sections are negatively affected by the influence of the sea bottom topographic relief. In this study we applied acoustic Waveform Tomography to the OBS data using the traveltime tomography to provide a suitable starting model; synthetic wavefields generated from the final model reproduced wide-angle reflections from the megasplay fault. Our image clearly delineated major geological features including the megasplay fault, thrusts within the accretionary prisms, and an apparent connectivity of two low velocity zones. By comparing the image with a slice from the 3D migration volume, we confirmed the reliability of our image, and the improvement enabled by the use of wide-angle data with Waveform Tomography.

Cross‐borehole data were acquired in the surface crown pillar of a massive sulfide ore mine. The data consist of five, two‐dimensional (2-D), cross‐borehole panels, each with approximately 900 source‐receiver pairs. The panels were located within the crown pillar at either side of and within a major subvertical fault zone that intersects the orebody. An initial analysis of the data indicates that the bedrock containing the orebody is seismically anisotropic. A rigorous analysis of the traveltimes using anisotropic velocity tomography confirms the initial assessment that anisotropy exists within the crown pillar rock mass. Anisotropic velocity tomography is the generalization of tomographic methods to anisotropic media. As in any geophysical problem, the data are insufficient to completely resolve the distributions of the rock properties at all scale lengths; we use external constraints on the roughness of the final solution to ensure an algebraically well‐posed problem. Plots of the data residuals (the “traveltime surfaces”) are an essential tool in determining an optimal level of constraint. Of equal importance are plots of the relationship between the solution roughness and the rms level of the residuals. The final results of anisotropic velocity tomography are a set of images (tomograms) of the velocity and selected anisotropy parameters for the five panels. Our images do not contain the distortions typically exhibited when using isotropic tomography in anisotropic media. The velocity tomograms clearly show the geometry of the overburden contact at the top of the bedrock. The anisotropy tomograms show a decrease in anisotropy with depth on two of the panels. They also show a decrease in anisotropy with proximity to the fault zone. These features of the seismic velocity anisotropy are consistent with observations of fracture orientation and distribution. The results of the crosshole data interpretation contribute to the overall site investigation and provide a reliable interrogation of the bulk properties of the rock mass.

Waveform tomography yields sub-wavelength scale velocity resolution through formal inversion of the recorded seismic wavefield. Velocity estimation through waveform inversion works best where wide angle (large offset) refractions and reflections are available in the input data, where low frequencies with good signal-to-noise rations are available, and where sources and receivers are adequately coupled and are reliably consistent. Waveform tomography results have been obtained by our group for both Marine and Land data settings. Marine seismic data, in most cases, lend themselves well to waveform tomography. Waveform tomography, especially where OBS recording are available, is ideally suited to provide geologically significant velocity images of deeper structures. Land seismic data, in contrast, pose significant challenges for waveform tomography particularly when topographic relief, weathering, and near-surface conditions are severe. The challenges afforded by land data mean that waveform tomography can require extensive manual intervention and repeated parameter testing, driving the costs up dramatically.

SUMMARY In principle, crosshole, traveltime tomography is ideal for directly detecting and measuring seismic anisotropy. The traveltimes of multiple rays with wide angular coverage will be sensitive both to inhomogeneities and to anisotropy. In practice, the traveltimes will depend only on a limited number of the anisotropic velocity parameters, and the data may not be adequate even to determine these parameters uniquely. In addition, trade-offs may exist between anisotropy and inhomogeneities. In this paper, we use the linear perturbation theory for traveltimes in general, weakly anisotropic media to discuss the dependence of traveltimes in 2-D crosshole tomographic experiments on the anisotropic parameters. In a companion paper, we apply the results to synthetic and real data examples. We show that when measurements are restricted to a 2-D plane, the qP and qS traveltimes depend on subsets of the complete set of 21 anisotropic velocity parameters. Formulae are developed for the differential coefficients of the traveltimes with respect to these parameters in piecewise homogeneous and in linearly interpolated models. It is shown how in a generally oriented model element, the local parameters are related to the same parameters in the global model. The parameters that can be determined from 2-D tomographic data do not in general determine the full nature of the anisotropy. Rather, these parameters serve only to describe the intersection of the slowness sheet with the 2-D plane. Since many models may fit this description, additional information on symmetry properties and orientations is required. For example, if a priori information suggests that the anisotropy is transversely isotropic (TI), then we can determine some of the TI parameters and some information on the orientation of the axis of symmetry. Formulae are given relating the general parameters to those of a TI system with general orientation of the symmetry axis. The general formulae for qS traveltimes are intrinsically more complicated than those for qP. In the qS case, the traveltime perturbation depends on the polarization, which in turn depends on the perturbation. This makes the general problem non-linear even for small perturbations. However, the mean qS traveltime and the traveltime dependence on various subsets of parameters are linear. Although linear perturbation theory is invalid for qS rays, degenerate perturbation theory is valid for the calculation of the traveltimes and could be used in a non-linear inversion scheme.

SUMMARY Cross-borehole seismic data have traditionally been analysed by inverting the arrival times for velocity structure (traveltime tomography). The presence of anisotropy requires that tomographic methods be generalized to account for anisotropy. This generalization allows geological structure to be correctly imaged and allows the anisotropy to be evaluated. In a companion paper we developed linear systems for 2-D traveltime tomography in anisotropic media. In this paper we analyse the properties of the linear system for quasi-compressional waves and invert both synthetic and real data. Solutions to the linear systems consist of estimates of the spatial distributions of five parameters, each corresponding to a linear combination of a small subset of the 21 elastic, anisotropic velocity parameters. The parameters describe the arrival times in the presence of weak anisotropy with arbitrary symmetries. However, these parameters do not, in general, describe the full nature of the anisotropy. The parameters must be further interpreted using additional information on the symmetry system. In the examples in this paper we assume transverse isotropy (TI) in order to interpret our inversions, but it should be noted that this final interpretation could be reformulated in more general terms. The singular value decomposition of the linear system for traveltime tomography in anisotropic media reveals the (expected) ill-conditioning of these systems. As in isotropic tomography, ill-conditioning arises due to the limited directional coverage that can be achieved when sources and receivers are located in vertical boreholes. In contrast to isotropic tomography, the scalelength of the parametrization controls the nature of the parameter space eigenvectors: with a coarse grid all five parameters are required to model the data; with a fine grid some of the parameters appear only in the null space. The linear systems must be regularized using external, a priori information. An important regularization is the expectation that the elastic properties vary smoothly (an ad hoc recognition of the insensitivity of the arrival times to the fine-grained properties of the medium). The expectation of smoothness is incorporated by using a regularization matrix that penalizes rough solutions using finite difference penalty terms. The roughness penalty sufficiently constrains the solutions to allow the smooth eigenvectors in the null space of the unconstrained problem to contribute to the solutions. Hence, the spatial distribution of all five parameters is recovered. The level of regularization required is difficult to estimate; we advocate the analysis of a suite of solutions. Plots of the solution roughness against the data residuals can be used to find ‘knee points’, but for the fine tuning of the regularization one has little recourse but to examine a suite of images and use geological plausibility as an additional criterion. The application of the regularized numerical scheme to the synthetic data reveals that the roughness penalty should include terms that penalize high gradients addition to penalizing high second derivatives. Only when this constraint was included were the features of the original model recovered. The inversions of the field data yield good images of the expected stratigraphy and confirm previous estimates of the magnitude of the anisotropy and the orientation of the symmetry axis. The solutions further indicate an increase in anisotropy from the top to the bottom of the survey region that was not previously detected.

Conventional acquisition and processing of seismic data from thrust-fold belt tectonic settings (e.g., the Canadian Foothills) is often inadequate in producing a reliable velocity model and corresponding sub-surface image. Near-surface geology and data qu

White clover (Trifolium repens L.) stands commonly decline within 2 to 3 yr after establishment, due to insects, fungi, nematodes, viruses, and other factors. This study determined the effect of application of insecticide and fungicide, and infectation of southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, alone and in combinations, on white clover persistence, yield, and crude protein (CP) concentration

During June, 2004, BP carried out a crosswell seismic survey in the Rocky Mountains. The crosswell data were acquired in finely layered clastic sediments in the Mesozoic section. The reservoir sands are known to vary laterally in quality, and the crosswell survey was conducted in order to provide a high resolution, sub-seismic image of the target. High-resolution velocity images were obtained using Waveform Tomography. This was particularly challenging due to low signal-to-noise ratios, high intrinsic attenuation, and significant, variable, an-elliptic anisotropy with a vertical transverse isotropy (TIV) symmetry. The final Waveform Tomography results show we can resolve layers down to the order of 4 m in thickness, and that we can accurately track these layers across the target zone. A subtle change in dip in the geology is revealed below a known unconformity. The variability of the anisotropy played a critical role, and the development of fully anisotropic Waveform Tomography is a logical next step.

Several shallow, engineering-scale seismic surveys were carried out in Seven Sisters, Manitoba, between 1999 and 2001. The surveys were designed to locate three buried riprap targets, located within a constructed clay embankment as a blind test of geophysical imaging methods. While previous studies used near-offset reflection methods to image the targets, in this paper we use Waveform Tomography of the long-offset, refracted arrivals to image the seismic velocity and the seismic attenuation. The refraction surveys were carried out with a 2 m spacing on both source and receiver positions, with 70 source positions and 48 receiver positions respectively. The targets were buried approximately 7 m below the surface of the constructed embankment. Using weight-drop data, with frequencies between 20 and 130 Hz, we were able to resolve the sub-wavelength targets using the seismic-Q images. A very good fit between model and observed data is an indication of the reliability of the results.

In the structurally complex Canadian foothills, conventional seismic data processing is inadequate, given the effects of steep dips, rough topography, and near-surface weathering. Depth imaging is often necessary, and an accurate estimation of the macro-velocity model is essential. Waveform Tomography (i.e., traveltime tomography followed by frequency-domain waveform inversion) of long-offset data provides a possible solution. In order to investigate this, we generated synthetic data in a geologically realistic model exhibiting fold-thrust sheets, steeply dipping structures, and topographically elevated carbonate outcrops. The method shows promise for resolving the near-surface geological structures in this environment. Real data acquired with long-offsets and low-frequency sensors may provide the necessary input for Waveform Tomography to succeed in the Canadian foothills.

Waveform Tomography, when implemented in the frequency-domain, potentially yields images of the instrinsic attenuation from seismic waveform data. The attenuation (or its inverse, the seismic Q value) is strongly related to useful geological variables such as rheology, fluid flow, pore fluid content and fractures. Since phase and amplitude anomalies in recorded data are also caused by velocity structure (due to geometrical and scattering effects), it is critical to assess inversion strategies as to their ability to resolve these effects. In this study, frequency domain Waveform Tomography was implemented with the subspace-search method, and evaluated with synthetic tests. We compared two sets of strategies: first, velocity and attenuation models were updated jointly at each iteration (“simultaneous inversion”). In a second test, (“sequential inversion”), a velocity model alone was first inverted followed by simultaneous inversion. While the predicted waveforms from both strategies agreed with the observed data, only the sequential inversion strategy imaged attenuation structure well in the presence of small-scale velocity heterogeneities. This highlights the strong dependence of attenuation imaging on the quality of the velocity model.

[1] Crosshole seismic experiments were conducted to study the in-situ properties of gas hydrate bearing sediments (GHBS) in the Mackenzie Delta (NW Canada). Seismic tomography provided images of P velocity, anisotropy, and attenuation. Self-organizing maps (SOM) are powerful neural network techniques to classify and interpret multi-attribute data sets. The coincident tomographic images are translated to a set of data vectors in order to train a Kohonen layer. The total gradient of the model vectors is determined for the trained SOM and a watershed segmentation algorithm is used to visualize and map the lithological clusters with well-defined seismic signatures. Application to the Mallik data reveals four major litho-types: (1) GHBS, (2) sands, (3) shale/coal interlayering, and (4) silt. The signature of seismic P wave characteristics distinguished for the GHBS (high velocities, strong anisotropy and attenuation) is new and can be used for new exploration strategies to map and quantify gas hydrates.

We present a study of the ability of efficient waveform tomography to recover 2-D velocity models from a synthetic surface experiment. We show that tomographic, frequency domain waveform inversion using only a subset of the available frequencies can give results equivalent to waveform inversion with many frequencies, for a low starting frequency, using the 2003 CCSS dataset and crustal velocity model. Aliasing effects due to sparse survey geometries begin to affect the result only if sources are spaced with an interval larger than or equal to 3 x Nyquist, provided receiver spacings are unaliased. We investigate this effect further using increased starting frequencies, and find that waveform tomography can produce results consistently superior to traveltime tomography at frequencies up to approximately 3.0 Hz for this dataset.

The JAPEX/JNOC/GSC et al. Mallik 3L-38 and 4L-38 observation wells were part of a crosshole seismic study. Both wells are 1160 m deep and located 42.5 m from the central 1188 m deep production research well (Mallik 5L-38). The crosshole seismic experiments provided images of the gas-hydrate-bearing interval at depths between 900 m and 1100 m. Portions of the surrounding stratigraphic sequences were also imaged. During a production experiment and after the baseline survey, 3 time-lapse monitors surveys were conducted to evaluate the seismic detection of gas hydrate dissociation. Early one-dimensional and two-dimensional studies of the P-wave propagation showed that the notably saturated gas-hydrate-bearing sediments are concordant with high velocities, strong attenuation and some seismic anisotropy. According to modelling studies, the dissociation created by the production test will result in only slight modifications in the seismic data. These effects will only be detected through advanced processing.

The author recently contacted a number of leading research groups, many of whom provided examples of the success of the frequency-domain approach to waveform inversion.

Optimization of intravascular radiation to reduce stenosis following coronary angioplasty requires the ability to predict the patterns of radiation dose distribution. This investigation evaluated the agreement between Monte Carlo simulations and experimental radiation dose measurements for a radioisotope liquid-filled balloon catheter in a tissue equivalent phantom.Direct measurements of the radiation dose from Re-186 liquid-filled balloons were made using thermoluminescent dosimeters (TLDs) and radiochromic film. Monte Carlo simulations were carried out using the Monte Carlo N-Particle code system (MCNP4B).The Monte Carlo generated dose values agreed with the experimentally determined results within the statistical uncertainty. A slightly higher penetration was indicated by regression analysis for the TLD data relative to the MCNP4B prediction that may be due to experimental configuration anomalies. For this balloon catheter, approximately 55 mCi of Re-186 will deliver 15 Gy at a 0.5 mm depth in tissue equivalent material in 5 min.Correlation between experimentally measured dose values and Monte Carlo computation supports the position that MCNP4B simulations constitute a valuable tool for investigating various clinical therapy designs. The agreement between Monte Carlo calculations and experiments provide confidence in applying MCNP4B to more sophisticated geometries of interest, and other methods of intravascular radiation dose delivery.