Seismic Tomography Experiment Research Articles

Seismic refraction tomography in San Luis, headward Coca River erosion zone

In the context of the headward erosion of the Coca River, we have carried out a seismic tomography experiment in the San Luis town at the river bank. San Luis is located directly over the eroded debris avalanche, but this resists mass movement, probably due to the presence of volcanic breccia deposits.We have developed a procedure to attack seismic tomography’s regularization problem. This procedure consists in using regularization by smoothing. The smoothing parameter is chosen whit the L-curve criterion and gives the optimal solution. To draw the L-curve, we have varied the vertical and horizontal smoothing parameters and plotted the root mean square error of the model versus the three norms between the a priori and the resulting models. We have tested the Euclidean, taxicab, and infinity norms in our procedure to better detail the optimal solution. We have tested the regularization strategy in a synthetic model with a similar dimension to the problem in our survey.The results of tomography allow describing the geology strata structure below the village. Two stratigraphic units resulting from debris avalanche deposits are overlayed in a bedrock unit that seems to give structural strength to prevent river erosion in San Luis. We have identified this bedrock with the volcanic breccia at 20 m depth. The breccia P wave seismic velocity obtained with the tomography is 1.2 km/s.The parameters of velocity and structure resulting from our seismic survey allow a hazard diagnostic and future solutions for the San Luis settlement.

Joint tomography of multi-cross-hole and borehole-to-surface seismic data for karst detection

Two-dimensional (2D) cross-hole seismic tomography is widely used in karst exploration. However, this method only obtains subsurface geological structures from the profiles of two adjacent wells and cannot acquire geological information from the lateral profile direction. The tomographic imaging area and accuracy are also limited owing to sparse ray density in the near-surface area. This approach is therefore not conducive to the spatial identification and evaluation of subsurface karst structures. To address this problem, we conducted multi-hole combined seismic tomography experiments and obtained high-density and high-quality seismic spatial data using an optimized observation system and acquisition parameters. We propose a new method of joint tomography of multi-cross-hole and borehole-to-surface seismic data. We performed 2D seismic tomography inversion by combining cross-hole data with borehole and surface data based on the theory of seismic tomography, and then used the data fusing from multiple wells based on the theory of kriging method. A pseudo-three-dimensional (3D) spatial velocity field of the experimental site is established. The results show that (1) the combined borehole and surface data provide richer ray-tracing information than the cross-hole dataset alone, (2) the pseudo-3D spatial velocity field and spatial distribution of seismic anomalies (karst) can be obtained using the proposed method, and (3) a comparison with actual borehole logging data verifies the effectiveness and accuracy of the proposed method in obtaining subsurface geological structures and detecting the spatial distribution of karst, which has significant practical and application potential.

Modal analysis of Rayleigh waves using classical MASW-MAM approach: Site investigation in a reclaimed land

Surface wave tests such as the multi-channel analysis of surface waves (MASW) and microtremor array measurements (MAM) offer a fast and convenient way of measuring the shear wave velocity (Vs) of the ground for geotechnical site investigation due to their non-intrusive nature. However, it has been widely reported that the accuracy and resolution of the Vs profile depend on the construction of the dispersion image and its interpretation. While many advanced computational techniques have been proposed to process surface waves into a final Vs profile, we opted for the classical ‘picking and inverting’ approach in the current study. This is in view of the lower required computation power as well as the relatively uniform geology of the site. Surface wave tests (MASW, MAM and horizontal-to-vertical spectral ratio) were carried out at a remote reclaimed land in Singapore. A systematic methodology for the modal analysis of Rayleigh waves based on combined MASW and MAM tests is presented. The MAM dispersion image was constructed using the phase shift method, the extended spatial autocorrelation (ESAC), and the refraction microtremor (ReMi) method but a meaningful dispersion curve could only be obtained using the phase shift. Due to the modes of energy in the low frequency regime being unclear in the dispersion image, an idealized synthetic model of the site was set up to guide the mode identification process. The combined dispersion curve and HVSR profile were jointly inverted to construct the final Vs profile, which agreed quite well with the seismic cross-hole tomography experiments and bore-log information. It is also demonstrated that the inversion of the fundamental mode alone led to slight errors and low resolution in the final velocity profile, even for such a simple soil stratigraphy.

About this title ‐ The Eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010

The 1995 to present eruption of Soufrière Hills Volcano on Montserrat is one of the most important and best-studied eruptions of an explosive andesitic volcano. This volume presents scientific findings from the period between 2000 and 2010; it follows on from Memoir 21, which focused on the early years of activity between 1995 and 1999. In addition to descriptions and analysis of the growth, collapse and explosions associated with lava domes, there are papers on the deformation of the volcano caused by the deep magma, the petrology and geochemistry of the lavas and associated gases. Of particular note are: an overview of the insights into the deep structure of the volcano that resulted from a major international seismic tomography experiment; and an analysis of the quantitative risk assessment process that has run now for most of the eruption, the longest such continuous assessment in the world.

Crustal construction and magma chamber properties along the Eastern Lau Spreading Center

The L-SCAN active-source seismic tomography experiment maps the crustal structure and magmatic system along a 120-km-long section of the back-arc Eastern Lau Spreading Center (ELSC), where the ridge undergoes abrupt changes in morphology and composition associated with increasing proximity to the Tofua volcanic arc. Using this dataset, we picked ~197,000 P-wave travel times from 57 seismic airgun lines recorded at 83 ocean bottom seismograph stations, and inverted for a 3-D P-wave velocity image. The seismic images reveal a prominent, but narrow, seismic low velocity volume (LVV) located beneath all surveyed ridge segments, consistent with the high temperatures and melt of a crustal magmatic system. Crustal magmatic systems thus underlie spreading axes even where previous seismic reflection surveys did not detect magma lens reflectors, accounting for the heat source of high-temperature hydrothermal vents in these areas. The top of the LVV closely follows the ridge axis and steps across three overlapping spreading centers. As the offset of the overlap increases, the LVV becomes increasingly discontinuous across the ridge limbs. Surprisingly, the LVV is as much as twice as wide, but deeper, in the northern part of the ridge system where the crust is thinner, as compared to the LVV beneath the southern segments, where the crust is relatively thicker. The width of the LVV may be modulated by the degree of deep hydrothermal activity or temporal variations in melt supply, and thus may not correlate directly with the average melt flux as indicated by crustal thickness. Over the past 4Myr, the location of the ridge has swept across different mantle compositional domains, and the crust produced at the ridge formed a zoned pattern. The interpretation is that thick, highly porous, volcanic layers with felsic compositions cap regions of thick crust; thinner volcanic layers of basaltic composition cap regions of thinner crust. The zonal pattern indicates that the influence of slab-derived water on crustal construction has substantially decreased over time.

Mafic intrusions east of Svalbard imaged by active-source seismic tomography

A seismic refraction and reflection tomography experiment was performed across the igneous province east of Svalbard which is a part of the Cretaceous High Arctic Large Igneous Province. Seismic travel times from 12 ocean bottom seismometers/hydrophones deployed along a 170km line are inverted to produce smooth 2D images of the crustal P-wave velocity and geometry of the acoustic basement and Moho. The inversion of travel times was complemented by forward elastic wave propagation modeling. Integration with onshore geology as well as multichannel seismic, magnetic and gravity data have provide additional constraints used in the geological interpretation. The seismic P-wave velocity increases rapidly with depth, starting with 3km/s at the sea floor and reaching 5.5km/s at the bottom of the upper sedimentary layer. The thickness of this layer increases eastward from 2km to 3.5km. On average the P-wave velocity in the crystalline crust increases with depth from 5.5km/s to 6.8km/s. The crustal thickness is typical for continental shelf regions (30–34km). Finger-shaped high-velocity anomalies, one reaching 12% and two of 4–6% velocity perturbation, are obtained. These velocity anomalies are concomitant with Lower Cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs, traced towards the northern Barents Sea passive continental margin. We interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the Arctic Amerasia Basin.

Active Source Seismic Experiment Peers Under Soufrière Hills Volcano

Characterizing internal structures of active volcanoes remains an enigmatic issue in geosciences. Yet studies of such structures can greatly improve hazard assessments, helping scientists to better monitor seismic signatures, geodetic deformation, and gas emissions, data that can be used to improve models and forecasts of future eruptions.Several passive seismic tomography experiments—which use travel times of seismic waves from natural earthquakes to image underground structures—have been conducted at active volcanoes (Hawaii's Kilauea, Washington's Mount St. Helens, Italy's Etna, and Japan's Unzen), but an inhomogeneous distribution of earthquakes compromises resolution. Further, if volcanic earthquakes are dominantly shallow at a given location, passive methods are limited to studying only shallow features. Thus, active source experiments—where seismic waves from the explosion of deliberately set charges are used to image below the surface—hold great potential to illuminate structures not readily seen through passive measures.

Velocity structure and gas hydrate saturation estimation on active margin off SW Taiwan inferred from seismic tomography

This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.

Characteristics of the seismicity of Vesuvius and Campi Flegrei during the year 2000

This paper describes the characteristics of the seismicity in the volcanic Neapolitan area during the year 2000 recorded by the monitoring seismic network of the Osservatorio Vesuviano. In particular, a detailed analysis of the seismicity of Vesuvius is presented. We compared the seismic velocity models available for the Vesuvius area locating the earthquakes recorded in the year 2000 and on the basis of the results, we introduce for routine earthquake location the new velocity model obtained by the seismic tomography experiments (TomoVes) performed in the area. We also determined the focal mechanisms and analysed the seismicity rate, comparing the results with those obtained for the past years. After the introduction of the new acquisition system at the Osservatorio Vesuviano, a re-calibration of the duration magnitude scale was necessary to avoid biases related to the different instrumental response. Consequently, we re-calibrated the magnitude relation used for the Vesuvius earthquakes, obtaining a new formula to be used for the earthquakes recorded by the new acquisition system. Finally, we give a description of the seismic activity in the Campi Flegrei area during the summer of 2000.

Finite-frequency tomography using adjoint methods-Methodology and examples using membrane surface waves

SUMMARY We employ adjoint methods in a series of synthetic seismic tomography experiments to recover surface wave phase-speed models of southern California. Our approach involves computing the Frechet derivative for tomographic inversions via the interaction between a forward wavefield, propagating from the source to the receivers, and an 'adjoint' wavefield, propagating from the receivers back to the source. The forward wavefield is computed using a 2-D spectral- element method (SEM) and a phase-speed model for southern California. A 'target' phase- speed model is used to generate the 'data' at the receivers. We specify an objective or misfit function that defines a measure of misfit between data and synthetics. For a given receiver, the remaining differences between data and synthetics are time-reversed and used as the source of the adjoint wavefield. For each earthquake, the interaction between the regular and adjoint wavefields is used to construct finite-frequency sensitivity kernels, which we call event kernels. An event kernel may be thought of as a weighted sum of phase-specific (e.g. P) banana- doughnut kernels, with weights determined by the measurements. The overall sensitivity is simply the sum of event kernels, which defines the misfit kernel. The misfit kernel is multiplied by convenient orthonormal basis functions that are embedded in the SEM code, resulting in the gradient of the misfit function, that is, the Frechet derivative. A non-linear conjugate gradient algorithm is used to iteratively improve the model while reducing the misfit function. We illustrate the construction of the gradient and the minimization algorithm, and consider various tomographic experiments, including source inversions, structural inversions and joint source-structure inversions. Finally, we draw connections between classical Hessian-based tomography and gradient-based adjoint tomography.

Seismic Amplification within the Seattle Basin, Washington State: Insights from SHIPS Seismic Tomography Experiments

Recent observations indicate that the Seattle sedimentary basin, under- lying Seattle and other urban centers in the Puget Lowland, Washington, amplifies long-period (1-5 sec) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound (SHIPS) experiments to better characterize the seismic hazard the basin poses. The 3D tomographic models, which resolve features to a depth of 10 km, for the first time define the P- and S-wave velocity structure of the eastern end of the basin. The basin, which contains sedimentary rocks of Eocene to Holocene, is broadly symmetric in east-west section and reaches a maximum thickness of 6 km along our profile beneath north Seattle. A comparison of our velocity model with coincident amplification curves for weak ground motions pro- duced by the 1999 Chi-Chi earthquake suggests that the distribution of Quaternary deposits and reduced velocity gradients in the upper part of the basement east of Seattle have significance in forecasting variations in seismic-wave amplification across the basin. Specifically, eastward increases in the amplification of 0.2- to 5-Hz energy correlate with locally thicker unconsolidated deposits and a change from Crescent Formation basement to pre-Tertiary Cascadia basement. These models de- fine the extent of the Seattle basin, the Seattle fault, and the geometry of the basement contact, giving insight into the tectonic evolution of the Seattle basin and its influence on ground shaking.

Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics

Recent results of high-resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of Western Pacific and East Asia. The most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation (corner flow) processes in the mantle wedge. The combination of the two processes may have caused the back-arc spreading in the Lau basin, affected the morphology of the subducting Philippine Sea slab and its seismicity under southwest Japan, and contributed to the formation of the continental rift system and intraplate volcanism in Northeast Asia, which are clearly visible in our tomographic images. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent (a) small mantle plumes, (b) upwellings associated with the slab collapsing down to the lower mantle, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab. Combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite + water, we proposed a petrologic model for arc volcanism. Arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water-rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge. A large amount of fluids can be released from hydrated MORB at depths shallower than 55 km, which move upwards to hydrate the wedge corner under the fore-arc, and never drag down to the deeper mantle along the slab surface. Slab dehydration reactions at 120 km depth are the antigorite-related 5 reactions which supply water-rich fluids for forming the volcanic front. Phase A and Mg-surssasite breakdown reactions at 200 and 300 km depths below 700 °C cause the second and third arcs, respectively. Moreover, the dehydration reactions of super-hydrous phase B, phases D and E at 500–660 km depths cause the fluid transportation to the mantle boundary layer (MBL) (410–660 km depth). The stagnant slabs extend from Japan to Beijing, China for over 1000 km long, indicating that the arc–trench system covers the entire region from the Japan trench to East Asia. We propose a big mantle wedge (BMW) model herein, where hydrous plumes originating from 410 km depth cause a series of intra-continental hot regions. Fluids derived from MBL accumulated by the double-sided subduction zones, rather than the India–Asia collision and the subsequent indentation into Asia, are the major cause for the active tectonics and mantle dynamics in this broad region.

Electrical conductivity and seismic velocity structures of the lithosphere beneath the Fennoscandian Shield

Abstract Key results from the BEAR (Baltic Electromagnetic Array Research) soundings and the SVEKALAPKO Seismic Tomography Experiment (SSTE), which explored geophysical properties of the upper mantle beneath the Fennoscandian Shield, are summarized here. These two projects formed the two key geophysical experiments in EUROPROBE's SVEKALAPKO project. The major result obtained from the teleseismic tomography indicates P- and S-wave velocity variations of up to 4%, as compared with the IASP'91 global model. The positive P-wave velocity anomaly seems to extend down to 300 km, without any indication of an asthenospheric low-velocity layer. This is corroborated by surface-wave analysis. The Archaean-Proterozoic suture zone has no continuation at upper mantle depths. Instead, a laterally and vertically heterogeneous structure of the subcontinental lithospheric mantle in the contact zone of Archaean and Proterozoic domains beneath the SVEKALAPKO area has been revealed by both teleseismic and local event studies. Comparison between stacked receiver functions from the TOR (Teleseismic Tomography of TORnquist Zone) and SSTE arrays indicates that the difference between arrival times of the converted P410 and P660 phases increases below the Shield as a result of a cooler upper mantle. The asthenosphere beneath Central Europe terminates at the southern edge of the shield and cannot be identified in the SVEKALAPKO data. The variations in Moho depths from 30 km to 50 km (locally, more), known from earlier deep seismic sounding (DSS) work, were corroborated by the receiver function analysis. The BEAR (Baltic Electromagnetic Array Research) experiment data indicate that an upper mantle conducting layer is required in some places beneath Fennoscandia. In the northern part of the Shield the upper mantle conductor is located at a depth of c. 170 km. Magnetotelluric data exhibit strong anisotropic behaviour, in particular in the central parts of the Fennoscandian Shield. The present state of modelling implies, however, that isotropic 3D crust and upper mantle explain nearly all observed anisotropic features and that very minor anisotropy of the upper mantle is required to explain the data. The crust is strongly variable electrically, ranging from resistive areas to highly conducting elongated structures. The conductances (= conductivity x layer thickness) obtained range from a few Siemens to several tens of thousands of Siemens. The conductive structures delineate boundaries between Archaean and Proterozoic crustal units and are inferred to image relics of subduction processes. In the central parts of the Shield, the lower crust is conducting, with dipping structures extending far to the NE below the Archaean-Proterozoic boundary. Thus, the present interpretation of electromagnetic BEAR and tomographic data from the Fennoscandian Shield demonstrates that the structure of the upper mantle beneath the Shield is much more heterogeneous than was supposed when the project started. Models of the evolution of lithosphere must be revised to accommodate lateral and vertical heterogeneity in the upper mantle.

Rock microstructure in the deep extension of the Nagamachi-Rifu fault revealed by analysis of collocated seismic and magnetotelluric data: Implication of strong deformation process

Quantitative analysis of collocated seismic velocity tomography and electromagnetic experiments is developed to elucidate the structure of the deep extension of the Nagamachi-Rifu fault, northeastern Japan. P and S wave seismic velocities obtained from a dense seismic network are examined and a ratio of spatial variation in P and S wave velocities d ln V s /d ln V p is chosen as a proxy for the influence of pore geometry. The analysis shows that the deep extension of the Nagamachi-Rifu fault reveals the d ln V s /d ln V p values exceeded 1.1. Such large values of d ln V s /d ln V p cannot correspond to equilibrium pore geometry, at which the interfacial energy is at a minimum, and indicate regions with non-equilibrium state where non-isotropic stress prevents the equilibrium pore geometry to be achieved. To specify a fine distribution of porosity and connectivity of micropore in the region, we carry out the joint analysis of the seismic velocities with the electrical resistivity data obtained by the magnetotelluric survey crossing the Nagamachi-Rifu fault. It is shown that the region at 10–17 km depths at about 20–40 km to the northwest from the hypocenter of the M5.0 earthquake occurred in 1998 exposes the highest connectivity among the adjacent areas, suggesting a strong deformation process.

A crosswell seismic experiment for nickel sulphide exploration

A crosshole seismic tomography experiment was conducted in the Kambalda nickel district to image the space between two dipping exploration boreholes separated by 80 m. A downhole electromagnetic (EM) survey had disclosed a conductor (possible mineralisation) between the holes. Excellent data quality was obtained from explosive sources at 2-m increments and recorded on a hydrophone array at 2-m spacing. Over 3200 travel times were converted into a velocity tomogram. A velocity low was found to coincide with the EM anomaly. Laboratory measurements on rock and core from Kambalda suggested that the velocity low was massive nickel sulphide. A subsequent drillhole was targeted on the seismic and EM anomaly. Unfortunately, no mineralisation was found. However, the sonic log of the hole gave an almost identical velocity profile to that extracted from the tomogram, and an inductive log run in the hole revealed a high conductivity. The cause of the velocity and conductivity anomaly was found to be due to chemical alteration within the host ultramafics.

A three‐dimensional study of a crustal low velocity region beneath the 9°03′N overlapping spreading center

Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge‐axis discontinuities and magma supply, we conducted a three‐dimensional seismic reflection and tomography experiment at the 9°03′N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid‐crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

Space and Time Behavior of Seismic Activity at Mt. Vesuvius Volcano, Southern Italy

We analyze the space and time behavior of seismicity at Mt. Vesuvius during the last 20 yr to characterize the seismic regime of the volcano during the present quiescent period. The new results on the volcano structure inferred from active seismic tomography experiments, the newly implemented 10-yr arrival time catalog, and a high-quality digital waveform data set have been analyzed. The background seismicity is concentrated near and beneath the Mt. Vesuvius crater, with depths lying above and below the discontinuity, which marks the transition from the shallow alluvium/volcanic sediments and the Mesozoic carbonate basement. The focal mechanisms of microearthquakes show variable stress-axis orientations as a function of depth, although there is evidence for a clustering around roughly the north–south to vertical directions for the tension axes and east-southeast–west-northwest to vertical directions for the pressure axes. The statistical analysis of the seismic catalog confirmed the tendency of background seismicity to cluster in time, according to a trigger model (as denoted by Vere-Jones and Davies, 1966), that is, the generalized Poisson process. A significant increase of the average seismic energy release with time is observed, which is related to the occurrence of several M D > 3 events in the past 10 yr, accompanied by intense swarm activity. This is consistent with the decrease of the b -value from about 2 to 1 during the same period. The ( M D > 3) events are located in the same area and depth range of the whole seismicity, and their fault-plane solutions also show variable stress-axis and nodal-plane orientations. In particular, the moment tensor inversion of P and S waveforms from the largest earthquake in the catalog ( M D 3.6, on 9 October 1999) shows no significant departure from a pure shear, double-couple mechanism, thus suggesting a dominant tectonic-like fracture mechanism. The decrease of parameter b with time is interpreted as a dominant effect of fluid pressure variations on the present seismic regime at Mt. Vesuvius, which could be driven by the progressive cooling of the volcanic system.

Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge.

Subsurface fault imaging using crosshole tomographic methods

Crosshole seismic tomography experiments were performed to image the subsurface structure between pairs of wells in three locations at Ano Liosia Municipality. Seismic caps were used as downhole sources and a 24-channel geophone cable provided the downhole receivers. The crosshole data were processed with a SIRT traveltime transmission tomography package to produce a velocity model. The crosshole velocity section exhibits rapid lateral and vertical velocity changes that are not resolved by surface methods. The results indicate that crosshole tomography can potentially be used to image steeply dipping beds or complex structures, such as small faults or cavities.

Crustal magma plumbing within a segment of the Mid-Atlantic Ridge, 35°N

We report the results of a seismic tomography experiment which images the three-dimensional nature of the crustal melt delivery system beneath a segment of the slow-spreading Mid-Atlantic Ridge. In the lower crust (>3.5 km depth) near the segment center, inversion of first-arriving crustal P-waves reveals a pair of vertical pipe-like (<10-km-diameter) low-velocity anomalies (−0.4 km/s). In the upper crust, these two features, which are physically isolated from each other below 3 km, both connect to a 10-km-wide, 45-km-long, axis-parallel, low-velocity zone (−0.2 km/s). Three higher-amplitude low-velocity anomalies (−0.6 km/s) are observed in the upper crust (<2 km depth), and are located directly beneath seafloor volcanic features. We interpret the overall image to represent the thermal/melt signature of a magma feeding system in which focused injections of magma from the mantle travel upward until they intersect the brittle-ductile transition, where they are then diverted along-axis to supply shallow intrusive bodies and seafloor eruptions along much of the ridge segment.