NS31A-01
A General Approach for Exchanging Structural Information - From A-Priori Data to a new Joint Inversion Algorithm
The inversion of geophysical data is usually non-unique, i.e., a variety of models is able to explain the data.
We restrict the ambiguity assumptions such as smoothness constraints or by additional information.
Often we have only structural information as from boreholes or other geophysical data.
We present a generalized minimization approach, which can be solved by any minimizer such as the Gauss-
Newton method or conjugate gradients.
In addition to the error-weighted residuals between data and model response, a roughness term is included.
The derivative matrix, which can be formulated for any parameterization such as unstructured meshes, is
weighted on model and constraint side by a model matrix and a constraint matrix.
On one hand the model control weights or combines individual model cells and can be used for
reparameterization or active constraint balancing.
On the other hand, a boundary control defines the weight of the individual model boundaries.
For example it can be set to zero for a known or assumed structural change.
The first example is from a ground-water based study where fine-grained and coarse-grained sediments are to
be separated by dc resistivity measurements.
Four boreholes show this materials and a conductive basement at depth.
At the layer positions allowed boundaries are introduced that help to improve the result significantly.
Structural information may also come from other techniques such as seismics.
We present an example where a refraction study has been carried out over a resistive lime stone to estimate the
overlying volume.
The refractor is introduced as allowed change in the resistivity survey and we see a good accordance in most
regions, whereas in other the resistivity shows deviances.
This proves structural information allows for but does not enforce abrupt changes.
The latter is a basis for a new joint inversion algorithm where two tomographic properties are connected
structurally.
We use the techniques of robust modelling, i.e., the iteratively reweighted least squares.
The roughness vector of one parameter is used to determine the weights for the other such that model gradients
in both models are preferred.
An example using dc resistivity and seismic refraction data shows a significant improvement of the images
compared to independent inversion.
Both images are finally combined by means of cluster analysis.
http:www.resistivity.net/
NS31A-02
Fast Numerically Based Modeling for Ground Penetrating Radar
There is a need for computationally fast GPR numerical modeling. This includes circumstances where real time performance is needed, for example discrimination of landmines or UXO's, and in circumstances that require a high number of successive forward problems, for example inversion or imaging. Traditional numerical techniques such as finite difference or finite element are too slow for these applications, but they provide results from general scenarios such as scattering from very complicated shapes with high contrast. Neural networks may fit in the niche between analytical techniques and traditional numerical techniques. Our concept is training a neural network to associate the model inputs of electromagnetic properties of the background and targets, and the size and shape of the targets, with the output generated by a 3-D finite difference model. Successive examples from various electromagnetic properties and targets are displayed to the neural network, until the neural network has adapted itself though optimization. The trained neural network is now used as the forward model by displaying new input parameters and the neural network then generates the appropriate output. The results from the neural network are then compared to results from finite difference models to see how well the neural networks is performing and at what point it breaks down. Areas of poor fit can be addressed through further training. The neural network GPR model can be adapted by displaying additional finite difference results to the neural network, and can also be adapted to a specific field area by actual field data examples. Because of this adaptation ability the neural network GPR model can be optimized for specific environments and applications.
NS31A-03
High resolution seismic imaging at the Odessa (TX) meteorite impact site: the ground-impact geohazard and integration with magnetic gradiometry and time-domain electromagnetics
Comet and asteroid bombardment along with impact crater formation has occurred continuously throughout Earth's history. The greatest impact-related geohazard is due to moderate sized impactors which represent the worst-case trade-off between frequency of occurrence, difficulty of mitigation, and severity of environmental destruction. Impacts in this size range pose a significant regional threat to densely populated areas. The primary objective of this research is to evaluate the ground-impact geohazard via high resolution seismic imaging of the remnants of a moderate-sized impact event, namely that associated with the 50 ka Odessa (TX) Group IAB iron meteorite. We provide high-resolution subsurface images of the 150-m diameter crater, the outcropping crater rim, the ejecta blanket, and the surrounding plain. The subsurface images are analyzed in terms of the deformation and thrusting of the underlying Cretaceous limestone and shale strata and the likely environmental effects caused by the impact. The analysis will build upon and extend our previous geophysical investigations based on magnetic gradiometry and time-domain electromagnetic data.
NS31A-04
Comparison of 1D, 2D and 2.5D Constrained Inversion of Electrical Resistivity Data
Clay-rich till plains cover much of the UK. Such sites are attractive locations for landfills, since the till cover lowers the risk of landfill leachate entering groundwater. However, such tills often contain discrete sand and gravel bodies that can act as leachate flow routes. Such bodies may not be detected by conventional site investigation techniques such as drilling boreholes and trial pitting. A combined geoelectrical survey was carried out at a study site typical of such till plains and close to cliff exposures, which allowed direct mapping of sand bodies. Electrical resistivity tomography (ERT), resistivity cone penetrometry (RCPT) and frequency-domain electromagnetic data were collected. In a previous study, the electromagnetic and RCPT data were used to construct reference models for 2D inversion of the ERT data. The use of these reference models improved the solution models produced by inversion. We showed that the best solution model produced by inversion with a range of reference models could be determined without a priori knowledge of the true geoelectrical structure. This was done by using the area-weighted L2 norm between the solution models and associated reference models as a proxy for the misfit between the solution models and the true geoelectrical structure of the ground. In order to assess the most suitable method for combining invasive and non-invasive measurements, we compare both constrained and unconstrained 1D, 2D and 2.5D inversions of resistivity data collected at the study site. Preliminary results suggest that for 2.5D inversion the true 3D geoelectrical structure of the ground at the field study site is not sufficiently well known for comparison between the solution models and the true geoelectrical structure of the ground to be made. The results of work in progress evaluating layer-depth- constrained 1D inversion will be presented at the meeting.
NS31A-05
Seismic Images of the Complex Fault Systems in the Longitudinal Valley of Eastern Taiwan
The longitudinal valley of eastern Taiwan is the most active fault zone situated in the Island of Taiwan at the collision boundary of two plates. The most significant fault system in the longitudinal valley is the east dipping Coastal Range fault at the eastern side of the valley. However, the other west dipping Central Range fault at the western side the valley is believed existed as well. There are outcrops of the Coastal Range fault exposed at a few locations, but none for the Central Range fault. Although there are other evidences suggest the existence of the Central Range fault, but it is more difficult to locate and trace the Central Range fault than the Coastal Range fault. The slates of the Central Range are overlaid by fluvial deposits in the longitudinal valley; several geophysical methods have been applied to delineate the Central Range fault at the shallow subsurface, but none of them shown direct evidence of the fault. Seismic reflection method was thought inappropriate for applying over the slates of the Central Range; however, the contact of slates and fluvial deposits actually becomes a remarkable target for mapping the fault. In this paper, we will show the first direct evidence of the west dipping Central Range fault by using shallow seismic reflection images. During the survey, several long shallow seismic lines were deployed as wide as possible to cross the longitudinal valley, and seismic energy was stacked around 60 times for penetrating the loose fluvial deposits. After careful data processing, the 24-fold CMP stacked seismic images show clear layering structures of the fluvial deposits in the longitudinal valley for more than 700 meters thick, the remarkable reflection from the top of slates, and the west dipping Central Range fault. In addition, the fronts of the east dipping Coastal Range fault in the valley are clearly imaged as well.
NS31A-06
Analysis of overburden effects in seismic amplitudes in a strcuturally complex scenario of the Gulf of Mexico by 3D ray-tracing
We perform illumination with 3D ray tracing for understanding possible effects of strong heterogeneity in seismic amplitude anomalies observed in the Gulf of Mexico. Credible velocity models in depth are build from integration of velocities obrtained from seismic data and lithological and well information. Real acquisition layouts are used as input in order to simulate illumination maps that can be compared to amplitude maps from actual data at selected target horizons. Geometric and reflection attributres are computyed using the wavefront reconstruction method, a shooting method that advances wavefronts in a smooth background velocity field within boundaries defined by target horizons. The observed lateral variability of ray density for some target horizons of 3D model suggests that morphology of the main geological structures corresponding to the overburden, might have a significant effect on amplitude attributes derived from processed seismic data. Typically, overburden effects are partially addressed in standard seismic processing flows. As a result, it is expected that amplitude anomalies observed in seismic sections that might lead to detection of fluids are hampered by geometric effects predicted by illumination analysis. We discuss situations in which performing this type of studies may lead to a more accurate discrimination of the reflectivity produced by lithological contrasts and/or presence of fluids from other wave propagation effects. An important requirement of illumination studies is the need of a depth-velocity model that is accurate for predicting zones with poor illumination. So we discuss the cost/benefit ratio for carrying out illumination analysis in some specific scenarios.
NS31A-07
Airborne Gravity and Geomagnetism Survey
The State Development and Reform Commission is setting a national project to push the development, the project of airborne remote sensing has been started since last winter, leading by the Chinese Academy of Sciences. To complete a geo-scientific observation system, our dynamic gravimeter and tree-axis magnetometer are included. Our team will run the gravimeter and magnetometer, to obtain the gravity and 3-components geomagnetic data. As a pre-study of this project, we are conducting some land observation. Meanwhile, Tang is taking efforts on the study of paleogravity. Actually igneous rock and sedimentary rock are keeping solid evidence for paleogravity. During the consolidation of the igneous rock, rock¡¯s structures were formed under the original pressure and temperature. The physical parameters, such as density, elastic modulus and porosity were determined. After that the structure were disturbed by later stresses. But if the later stresses were eliminated, rock¡¯s structure as well as the physical parameter will almost return to the original statues. Base on this physical consideration, we can drill a vertical well from a large rock body with old age and take the rock core out, then the additional stress will be eliminated, the rock¡¯s structure will almost reflect the original environment when the igneous rock were formed. From the core cylinder, we cut three pieces of unbroken sample sections 1,2 and 3 from higher position to lower position, and measure their density d1, d2 and d3, elastic modulus E1, E2, E3 and porosity K1, K2, K3, we have the relation connecting the pressure difference and section deformation between sections, i.e., following two equations: dj1hj1g = EjBj - E1B1, j=2,3, where dj1, j=2,3 are the average densities from section j to section 1, hj1, j=2,3 are the heights from section j to section 1 respectively, Bi=(K0 - Ki)/(1- Ki), i=1,2,3 are the relative deformations along cylinder axis for each sections, K0 is the ideal porosity with zero pressure while igneous magma were consolidated. We can find there are only two variables g and K0 unknown in the two equations, so the paleogravity g can be solved out. Preliminary results of paleogravity from Beijing Area¡¯s igneous rocks could be presented. This approach can also be applied to sedimentary rock.
NS31A-08
Dielectric and GPR Characterization of Shallow Carbonate Reservoir Analogs in Central Texas
Albian rudist communities of the Edwards Formation, Fredericksburg Group, occur widely in Central and South- Central Texas. Capped by younger dolostones of the same group, they form important analogs for highly productive hydrated and hydrocarbon-rich reservoirs in arid environments. This study is a part of a larger project covering exposures of Albian rudists and associated depositional facies around Georgetown (Williamson County) in Central Texas. Dielectric measurements of field-collected rock samples of varying clay content were carried out initially under vacuum dried conditions and then under controlled amounts of moisture contents from 1 to 100% of the rock porosity (assuming even and complete saturation of porosity). Plots of the real parts of dielectric constants indicate that the more clay-rich samples exhibit a much greater variation with changing moisture content. Plots of the imaginary parts of the dielectric constants of the samples show a similar trend with a more significant dependency at low moisture content (< 20%), i.e., changes in dielectric losses are more conspicuous with an increase in clay content. These plots indicate that different carbonate units can be differentiated in the field by a Ground Penetrating Radar (GPR) as a result of their different dielectric contrasts arising from a variation of moisture and clay contents. The penetration depths are also calculated for each sample at varying antenna frequencies assuming GPR signal penetration in a homogeneous layer. As expected, this reveals an inverse relation between the clay content and the depth of penetration. Our analyses involve the use of a 400 MHz antenna with the Subsurface Interface Radar (SIR-3000) System by GSSI and GPS data to obtain better knowledge of the architecture and spatial distribution of the rudist mounds in the subsurface. This effort will eventually provide a 3D characterization of the shallow reservoir geometry and its dielectric heterogeneity, useful not only in hydrocarbon exploration but also in hydrological applications in carbonate terrains. Acknowledgement: This research was supported by a GCAGS Student Research Grant to D. Banerjee.
NS31A-09
Shallow Exploration Geophysics Study in Tocuila, Texcoco, Basin of Mexico
Tocuila town is a paleontological and archeological area located in Texcoco Lake at southeastern of Mexico City.
The recent findings of Late Pleistocene megafauna and prehistoric human remains buried nearby Tocuila town
invite us to apply shallow geophysics methodologies to identify and define shallow geological structures
controlling the volcano sedimentary deposits. In this work we present the preliminary results of the seismic and
magnetic exploration campaigns in an area characterized by predominant soft soil deposits. The seismic
refraction exploration arrays were delimited by urban conditions. Profiles consist on 12-24 geophones with 1 m
or 2 m spacing interval along 50 m to 200 m transect in open fields and unpaved roads. We used a Geometrics
StrataVisor NZ seismograph. The magnetic surveying was developed in two phases. The first one was developed
in a 100 X 100 m grid approximately. Several profiles were surveyed and we could recognize high frequency
surface magnetic anomalies coming from quite well identified sources such as isolates metallic structures, or
urban constructions, likewise low-powered lines in the southern edge of this area. In the first phase we used two
Geometrics magnetometers (model G-856). For the second one an overhouser magnetometer with GPS system
integrated was used (GEM-Systems). The combination of the two methodologies allowed us to identify and
delimit pre-hispanic structures with geometries like Tlatels and paleo-channels associated with megafauna
remains. We present preliminary proposed models of shallow structures at range depth between 2-10 m.
http:www.geofisica.unam.mx
NS31A-10
Geoelectrical Characterization of the Punta Banda System: A Possible Structural Control for the Geothermal Anomalies
The lack of sufficient drinking water in México has become a very serious problem, especially in the northern desert regions of the country. In order to give a real solution to this phenomenon the IMPULSA research program has been created to develope novel technologies based on desalination of sea and brackish water using renewable sources of energy to face the problem. The Punta Banda geothermal anomaly is located towards the northern part of Baja California Peninsula (Mexico). High water temperatures in some wells along the coast depicted a geothermal anomaly. An audiomagnetotelluric survey was carried out in the area as a preliminary study, both to understand the process generating these anomalous temperatures and to assess its potential exploitation to supply hot water to desalination plants. Among the electromagnetic methods, the audiomagnetotellurics (AMT) method is appropriated for deep groundwater and geothermal studies. The survey consisted of 27 AMT stations covering a 5 km profile along the Agua Blanca Fault. The employed array allowed us to characterize the geoelectrical properties of the main structures up to 500 m depth. Two main geoelectrical zones were identified: 1) a shallow low resistivity media located at the central portion of the profile, coinciding with the Maneadero valley and 2) two high resitivity structures bordering the conductive zone possibly related to NS faulting, already identified by previous geophysical studies. These results suggest that the main geothermal anomalies are controlled by the dominant structural regime in the zone.