Development of Forward and Inversion Schemes for Cross-Borehole Ground Penetrating Radar

Tomography is an imaging technique to develop a representation of the internal features of material using a penetrating wave, such as an electromagnetic wave. The calculation method used is an example of an inverse problem, which is a system where the input and the output are known but the internal parameters are not. These parameters can be estimated by understanding the responses of a penetrating wave as it passes through the unknown media. A forward problem is just the opposite; the internal structure and input penetrating wave is known and the output is determined. For both forward and inverse problems, raytracing is needed to define the raypath through the medium and inversion techniques are used to minimize the error for a discretized matrix of material properties. To assess various inversion techniques for use in shallow karst conditions, three synthetic karst geology models, each with increasing complexity, were generated. Each model was analyzed using forward modeling techniques to compare the calculated tomograms from known geometry and material properties. Gaussian Raytracing with LSQR inversion technique performed the best. This technique, Gaussian Raytracing with LSQR, was then applied to an inversion problem; cross-borehole ground penetrating radar data was collected at a karst geology field site and tomograms were produced. The resulting tomography confirmed information detailed in the driller\u27s logs and features between boreholes were identified. This confirmed that cross-borehole ground penetrating radar is an applicable technique for use in geotechnical site characterization activities in karst areas

Strategies for applying active seismic subglacial till characterization methods to valley glaciers

Thesis (M.S.) University of Alaska Fairbanks, 2017Subglacial materials play an important role in glacier dynamics. High pore-pressure, high porosity (dilatant) tills can contribute to high basal motion rates by deforming. Amplitude Variation with Angle (AVA) analysis of seismic reflection data uses the relationship between basal reflectivity and reflection incidence angle to characterize the subglacial material. This technique can distinguish between dilatant tills and less-porous, non-deforming (dewatered) tills due to their distinctive reflectivity curves. However, noise from crevasses and glacier geometry effects can complicate reflectivity calculations, which require a source amplitude derived from the bed reflection multiple. We use a forward model to produce synthetic seismic records, including datasets with and without visible bed reflection multiples. The synthetic data are used to test source amplitude inversion and crossing angle analysis, which are amplitude analysis techniques that do not require absolute reflectivity calculations. We and that these alternative methods can distinguish subglacial till types, as long as reflections from crevasses do not obscure the bed reflection. The forward model can be used as a planning tool for seismic surveys on glaciers, as it can predict AVA success or failure based on crevasse geometries from remote sensing data and glacier bed geometry from radar or from a worst-case-scenario assumption of glacier bed shape. Applying lessons from the forward model, we perform AVA on a seismic dataset collected from Taku Glacier in Southeast Alaska in March 2016. Taku Glacier is a valley glacier thought to overlay thick sediment deposits. It has been the subject of numerous studies focusing on its ice-sediment interactions. Our analysis indicates that Taku Glacier overlies unconsolidated tills with porosity values greater than 33 %, though because of uncertainties due to the lack of a bed reflection multiple, it is possible that the tills are not dilatant

Sub-slab mantle anisotropy beneath south-central Chile

Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3 s and local S delay times are only 0.2 s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2 s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth

Advancements in seismic tomography with application to tunnel detection and volcano imaging

Thesis (Ph.D.) University of Alaska Fairbanks, 1998Practical geotomography is an inverse problem with no unique solution. A priori information must be imposed for a stable solution to exist. Commonly used types of a priori information smooth and attenuate anomalies, resulting in 'blurred' tomographic images. Small or discrete anomalies, such as tunnels, magma conduits, or buried channels are extremely difficult imaging objectives. Composite distribution inversion (CDI) is introduced as a theory seeking physically simple, rather than distributionally simple, solutions of non-unique problems. Parameters are assumed to be members of a composite population, including both well-known and anomalous components. Discrete and large amplitude anomalies are allowed, while a well-conditioned inverse is maintained. Tunnel detection is demonstrated using CDI tomography and data collected near the northern border of South Korea. Accurate source and receiver location information is necessary. Borehole deviation corrections are estimated by minimizing the difference between empirical distributions of apparent parameter values as a function of location correction. Improved images result. Traveltime computation and raytracing are the most computationally intensive components of seismic tomography when imaging structurally complex media. Efficient, accurate, and robust raytracing is possible by first recovering approximate raypaths from traveltime fields, and then refining the raypaths to a desired accuracy level. Dynamically binned queuing is introduced. The approach optimizes graph-theoretic traveltime computation costs. Pseudo-bending is modified to efficiently refine raypaths in general media. Hypocentral location density functions and relative phase arrival population analysis are used to investigate the Spring, 1996, earthquake swarm at Akutan Volcano, Alaska. The main swarm is postulated to have been associated with a 0.2 km\sp3 intrusion at a depth of less than four kilometers. Decay sequence seismicity is postulated to be a passive response to the stress transient caused by the intrusion. Tomograms are computed for Mt. Spurr, Augustine, and Redoubt Volcanoes, Alaska. Relatively large amplitude, shallow anomalies explain most of the traveltime residual. No large amplitude anomalies are found at depth, and no magma storage areas are imaged. A large amplitude low-velocity anomaly is coincident with a previously proposed geothermal region on the southeast flank of Mt. Spurr. Mt. St. Augustine is found to have a high velocity core

Borehole seismic methods for opencast coal exploration

Surface seismic techniques lack the resolution to image the top 100m or so of the earth's surface necessary for opencast coal exploration. The work reported in this thesis is the development of borehole seismic methods making use of the closely spaced boreholes that are routinely drilled by British Coal. The first method investigated was to use a tomographic technique to observe any reduction in seismic velocities above old workings, and hence infer the presence of old workings. In order to obtain clear images of the subsurface, it was necessary to interpret the field data for the presence of head waves, and to pick the later arrival direct waves for the tomographic inversions. However, independent data obtained from uphole surveys showed that there was no measurable reduction in the seismic velocity above old workings for strata below the water table, and the tomographic method was abandoned in favour of borehole seismic reflection methods. Fifteen hole-to-surface seismic reflection surveys were acquired using down- hole explosive charges as sources and a linear spread of surface geophones passing through the borehole position as receivers. A complete package of processing software was developed for processing the data, and eight of the surveys are presented in this thesis. The final migrated and stacked sections delineate a washout and faulting at both large and small scales. The vertical resolution of the data is high due to the wideband temporal frequencies in the data, typically up to 300Hz.The hole-to-surface method is compared to the crosshole seismic reflection method, which was developed in parallel by M. J. Findlay. The relative merits of the two techniques are discussed, and suggestions are made to improve the acquisition of the data to make both methods applicable to a wider variety of problems. Although the vertical resolution of the hole-to-surface method is lower than the crosshole method, this could be more than compensated for by extending the hole- to-surface method to three-dimensions, using areal arrays of surface geophones around the borehole

Continuous reservoir modeling updating by integrating experimental data using an ensemble Kalman Filter

The continuous researvoir model updating is widely used to calibrate reservoir simulation models to production data, but many challenges remain. First, few real field data are available to test the new history matching method, and most of the data sets are synthetic cases. Second, computational cost may be high when using non-Gaussian priors or nonlinear models. Third, with large complex models, the simulation runs and history matching method require huge memory allocations. This dissertation achieves a continuous reservoir model updating workflow with a meter-scale , two-phase flow experiment. Both production and seismic data are collected in the experiment. Because the data are high-frequency sequential data with noise, the EnKF method is used to efficiently integrate them. To better understand the problem, scaling analysis is done on the capillary transition zone. Two new dimensionless numbers are introduced-capillary time and capillary length. We found that for different models, if their capillary time and gravity number are equal, the capillary length would be the same. The scaling analysis results help us find a proper flow rate for the sand tank experiment. Two experiments are conducted to test the workflow and the EnKF method. In the first one, both the production and seismic data are collected and analyzed. The production data have large errors in the flow rate and they are integrated to improve reservoir models using EnKF method. The history matching results are in an acceptable range which demonstrate that even if the observation data has large error, the EnKF method still works. In the second experiment, the errors of flow rate are reduced by measuring manually with a graduated cylinder. Because the data quality are much better in the second experiment, the observations can be matched easily

Real-time In-situ Seismic Tomography in Sensor Network

Seismic tomography is a technique for illuminating the physical dynamics of the Earth by seismic waves generated by earthquakes or explosions. In both industry and academia, the seismic exploration does not yet have the capability of imaging seismic tomography in real-time and with high resolution. There are two reasons. First, at present raw seismic data are typically recorded on sensor nodes locally then are manually collected to central observatories for post processing, and this process may take months to complete. Second, high resolution tomography requires a large and dense sensor network, the real-time data retrieval from a network of large-amount wireless seismic nodes to a central server is virtually impossible due to the sheer data amount and resource limitations. This limits our ability to understand earthquake zone or volcano dynamics. To obtain the seismic tomography in real-time and high resolution, a new design of sensor network system for raw seismic data processing and distributed tomography computation is demanded. Based on these requirements, three research aspects are addressed in this work. First, a distributed multi-resolution evolving tomography computation algorithm is proposed to compute tomography in the network, while avoiding costly data collections and centralized computations. Second, InsightTomo, an end-to-end sensor network emulation platform, is designed to emulate the entire process from data recording to tomography image result delivery. Third, a sensor network testbed is presented to verify the related methods and design in real world. The design of the platform consists of hardware, sensing and data processing components

Crustal structure of the Baltic shield beneath the Sea of Bothnia; BABEL line 6

As part of the 1989 BABEL project, Durham University recorded large quantities of high resolution wide-angle data from six deep seismic lines shot in the Gulf of Bothnia. Fifteen analogue and three digital seismic recording stations were used, located in Sweden around the Sea of Bothnia. The wide-angle data is of very high quality due to the low noise and good transmission of seismic energy through the cratonic crust of the Baltic Shield. BABEL line 6 is a 240 km long, north-south profile in the western half of the Sea of Bothnia that runs almost parallel to several deep seismic refraction profiles previously acquired in Sweden and Finland. A crustal model has been developed for this line from P- and S-wave wide-angle data recorded at five of the online recording stations and from the normal-incidence data. This model has been generated by raytracing methods using a modified version of the BEAMS? gaussian beam package. Wide-angle arrivals are interpreted as diving rays and wide-angle reflections and as 'diffraction-type' arrivals from offsets on reflecting boundaries. The wide-angle reflections appear to correspond to the boundaries between high and low reflectivity zones in the normal-incidence data. The 'diffraction-type' arrivals appear to be related to surface features such as the Ljusnan tectonic zone and the Aranda rift. There is no evidence for a Moho trench similar to those seen on neighbouring profiles. Several high velocity layers are required in the model. These may be the related to the numerous post-orogenic intrusions that have occurred in the Svecofennian region of the Baltic Shield. Traditional approaches to modelling wide-angle data assume that the crust consists of a series of extensive, quasi-horizontal, layers. There are several problems with this approach. In particular it does not explain the patchy reflectivity seen in the normal-incidence data or the rapid variation in amplitude seen along wide-angle arrivals in high resolution data sets. Apparently continuous wide-angle arrivals may be generated from a series of short length reflectors. These show an amplitude variation similar to that seen in the experimental data. This suggests that wide-angle arrivals may arise from the assemblages of reflectors that are often imaged on normal-incidence data