Groundwater exploration using combined controlled-source and radiomagnetotelluric techniques

Geophysics ◽  
2005 ◽  
Vol 70 (1) ◽  
pp. G8-G15 ◽  
Author(s):  
Laust B. Pedersen ◽  
M. Bastani ◽  
L. Dynesius
Keyword(s):  
Transfer Functions ◽  
Three Dimensional ◽  
Groundwater Exploration ◽  
Impedance Tensor ◽  
Two Dimensional ◽  
Controlled Source ◽  
Reflection Seismic ◽  
Seismic Sections ◽  
Clay Layers ◽  
Clay Lenses

Radiomagnetotelluric (RMT) (14–250 kHz) combined with controlled-source magnetotelluric (CSMT) (1–12 kHz) measurements were applied to the exploration of groundwater located in sandy formations at depths as great as 20 m below thick clay lenses. A combination of approximately 30 radio frequencies and controlled-source frequencies is essential for penetrating the thick clay layers. The electromagnetic transfer functions of impedance tensor and tipper vectors point toward a structure that is largely two-dimensional, although clear three-dimensional effects can be observed where the sandy formation is close to the surface. The determinant of the impedance tensor was chosen for inversion using two-dimensional models. The final two-dimensional model fits the data to within twice the estimated standard errors, which is considered quite satisfactory, given that typical errors are on the level of 1% on the impedance elements. Comparison with bore-hole results and shallow-reflection seismic sections show that the information delivered by the electromagnetic data largely agrees with the former and provides useful information for interpreting the latter by identifying lithological boundaries between the clay and sand and between the sand and crystalline basement.

scholarly journals Magnetotelluric regional strike

Geophysics ◽  
1982 ◽  
Vol 47 (6) ◽  
pp. 932-937 ◽  
Author(s):  
T. D. Gamble ◽  
W. M. Goubau ◽  
R. Miracky ◽  
J. Clarke
Keyword(s):  
Baja California ◽  
Weighting Function ◽  
Three Dimensional ◽  
Geothermal Field ◽  
New Method ◽  
Weighted Sums ◽  
Impedance Tensor ◽  
Two Dimensional ◽  
Cerro Prieto

A new method for determining regional strikes from the magnetotelluric impedance tensor Z and tipper T is presented. It involves the minimization of weighted sums of the squared magnitudes of elements of Z or T over all frequencies and all stations of interest. When applied to data from the Mexicali Valley, Baja California around the Cerro Prieto geothermal field for a particular weighting function, the method yielded orientations that agree to within 2.9 degrees for three lines with a total of 16 stations. The consistency of orientations can be attributed in part to the pronounced two‐dimensional (2-D) geologic characteristics of the area, but the techniques also proved stable at stations near three‐dimensional (3-D) inhomogeneities.

scholarly journals Magnetotelluric analysis using Mohr circles

Geophysics ◽  
1993 ◽  
Vol 58 (10) ◽  
pp. 1498-1506 ◽  
Author(s):  
F. E. M. Lilley
Keyword(s):  
Mechanical Stress ◽  
Three Dimensional ◽  
Dimensional Structure ◽  
Impedance Tensor ◽  
Two Dimensional ◽  
Skew Angle ◽  
The Real ◽  
Mohr Circle ◽  
Quantitative Expression

The Mohr circle, most commonly met in the analysis of mechanical stress, is used to depict magnetotelluric impedance information, taking the real and quadrature parts of magnetotelluric tensors separately. The magnetotelluric concepts of two‐dimensionality, three‐dimensionality, skew and anisotropy are then all given quantitative expression on a diagram, as are various magnetotelluric invariants. In particular, a new invariant, the “central impedance,” becomes evident in a discussion of effective impedances. Some insight is gained into impedance rotations, and an anisotropy angle is defined, analogous to skew angle. Mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two‐dimensional structure. Generally, the application of shear or twist results in an impedance tensor with a Mohr circle of typical three‐dimensional form.

scholarly journals Integral equation modeling of three‐dimensional magnetotelluric response

Geophysics ◽  
1981 ◽  
Vol 46 (2) ◽  
pp. 182-197 ◽  
Author(s):  
Sam C. Ting ◽  
Gerald W. Hohmann
Keyword(s):  
Integral Equation ◽  
Cross Section ◽  
Apparent Resistivity ◽  
Transfer Functions ◽  
Three Dimensional ◽  
Equation Modeling ◽  
Impedance Tensor ◽  
Surface Charges ◽  
Text Mode ◽  
Magnetic Transfer

We have adapted a three‐dimensional (3-D) volume integral equation algorithm to magnetotelluric (MT) modeling. Incorporating an integro‐difference scheme increases accuracy somewhat. Utilizing the two symmetry planes of a buried prismatic body and a normally incident plane wave source greatly reduces required computation time and storage. Convergence checks and comparisons with one‐dimensional (1-D) and two‐dimensional (2-D) models indicate that our results are valid. We show theoretical surface anomalies due to a 3-D prismatic conductive body buried in a half‐space earth. Instead of studying the electric and magnetic fields, we have obtained impedance tensor and magnetic transfer functions by imposing two different source polarizations. Manipulation of the impedance tensor and magnetic transfer functions yields the following MT quantities: apparent resistivity and phase, impedance polar diagrams, tipper direction and magnitude, principal directions, skew, and ellipticity. With our preliminary analyses of these MT quantities, we have found that three‐dimensionality is usually revealed by all of them. Furthermore, we have recognized two pairs of complementary parameters: apparent resistivity and phase, and skew and ellipticity. Because of surface charges at conductivity boundaries, low‐frequency 3-D responses are much different from 1-D and 2-D responses. Thus, in many cases 3-D models are required for interpreting MT data. Although an overall 3-D MT interpretation is still not practical due to high computer costs and the complicated structure of the true earth, combined 2-D and 3-D modeling can be applied to yield a gross 3-D structure, which is composed of a cross‐section and its strike extent. In doing so, we suggest that the cross‐section be obtained from higher frequency 2-D E‐perpendicular [Formula: see text] mode modeling, and that the strike extent be derived by matching with lower frequency E‐parallel [Formula: see text] mode results due to corresponding 3-D models. In addition, we have indicated that some simple 3-D features, e.g., location above conductive zone, corners, and symmetry lines, can easily be recognized from the surface MT response.

Magnetotelluric soundings across the Pemberton Volcanic Belt, British Columbia

1980 ◽  
Vol 17 (2) ◽  
pp. 161-167 ◽  
Author(s):  
H. Dragert ◽  
L. K. Law ◽  
P. O. Sule
Keyword(s):  
British Columbia ◽  
Transfer Functions ◽  
Three Dimensional ◽  
Volcanic Belt ◽  
Two Dimensional ◽  
Magnetotelluric Soundings ◽  
Western Cordillera ◽  
Profile Line ◽  
Conductivity Structure ◽  
Short Period

Magnetotelluric soundings were carried out at three sites about 30 km apart in a line crossing the Lillooet Valley near Pemberton, British Columbia, to determine the regional conductivity structure and its relationship to the Meager Creek geothermal area. Telluric data were limited to a period band of 10–500 s, while geomagnetic data were recorded in this band as well as a longer period band of 5 min–3 h.Single-station vertical transfer functions indicate that no simple, two-dimensional conductivity discontinuity crosses the profile line or its immediate extension. Thus, the Pemberton volcanic line of plutons does not have an associated two-dimensional continuous conductivity anomaly. Impedance tensors calculated from the short period data are dominated by three-dimensional effects and surface structure effects at the western and eastern sites respectively. The results from the central site indicate a macro-anisotropy in horizontal conductivities, possibly produced by local fracture zones. One-dimensional modelling suggests a vertical conductivity structure consisting of a highly resistive upper crust 15–25 km thick, underlain by a conductive region. Though this conductive layer beneath Pemberton is more resistive, a similar layer has been noted in previous studies in southern British Columbia and appears to be a feature of the western Cordillera.

Controlled source tensor magnetotellurics

Geophysics ◽  
1991 ◽  
Vol 56 (9) ◽  
pp. 1456-1461 ◽  
Author(s):  
Xiaobo Li ◽  
Laust B. Pedersen
Keyword(s):  
Half Space ◽  
Transfer Functions ◽  
Unit Vector ◽  
Impedance Tensor ◽  
Wave Excitation ◽  
Controlled Source ◽  
Practice Analysis ◽  
Homogeneous Half Space ◽  
Dipole Sources ◽  
Layered Half Space

Impedance tensor and tipper vectors, known to connect the electromagnetic surface components for plane‐wave excitation, are shown to be uniquely defined for horizontal electric or horizontal magnetic dipole sources. Two independent source polarizations are needed for their estimation in practice. Analysis of impedance tensors and tipper vectors for a layered half‐space shows that the impedance tensor can be antidiagonalized by rotating the measurement system so that one of the measurement directions coincides with the direction to the transmitter dipole. The tipper vector points towards the transmitter dipole. In the static limit, all transfer functions are real, and simple analytic results for a homogeneous half‐space show that impedance elements are proportional to the inverse of the product of conductivity and distance between source and receiver, while the tipper vector is a unit vector pointing towards the transmitter dipole.

High Resolution Microscopy of Multi-Layered Biological Crystals

1982 ◽  
Vol 40 ◽  
pp. 80-83
Author(s):  
H.A. Cohen ◽  
T.W. Jeng ◽  
W. Chiu
Keyword(s):  
High Resolution ◽  
Low Dose ◽  
Three Dimensional ◽  
Data Interpretation ◽  
Two Dimensional ◽  
Biological Objects ◽  
Density Maps ◽  
Density Map ◽  
Complex Crystal ◽  
High Resolution Microscopy

This tutorial will discuss the methodology of low dose electron diffraction and imaging of crystalline biological objects, the problems of data interpretation for two-dimensional projected density maps of glucose embedded protein crystals, the factors to be considered in combining tilt data from three-dimensional crystals, and finally, the prospects of achieving a high resolution three-dimensional density map of a biological crystal. This methodology will be illustrated using two proteins under investigation in our laboratory, the T4 DNA helix destabilizing protein gp32*I and the crotoxin complex crystal.

Instrumentation For Quantitative Microscopy

1977 ◽  
Vol 35 ◽  
pp. 206-209
Author(s):  
B. Ralph ◽  
A.R. Jones
Keyword(s):  
Volume Fraction ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Automatic Data ◽  
Phase Volume Fraction ◽  
Statistical Confidence ◽  
Resultant Data ◽  
Automatic Data Collection ◽  
Third Dimension ◽  
Evolution Of Microstructure

In all fields of microscopy there is an increasing interest in the quantification of microstructure. This interest may stem from a desire to establish quality control parameters or may have a more fundamental requirement involving the derivation of parameters which partially or completely define the three dimensional nature of the microstructure. This latter categorey of study may arise from an interest in the evolution of microstructure or from a desire to generate detailed property/microstructure relationships. In the more fundamental studies some convolution of two-dimensional data into the third dimension (stereological analysis) will be necessary.In some cases the two-dimensional data may be acquired relatively easily without recourse to automatic data collection and further, it may prove possible to perform the data reduction and analysis relatively easily. In such cases the only recourse to machines may well be in establishing the statistical confidence of the resultant data. Such relatively straightforward studies tend to result from acquiring data on the whole assemblage of features making up the microstructure. In this field data mode, when parameters such as phase volume fraction, mean size etc. are sought, the main case for resorting to automation is in order to perform repetitive analyses since each analysis is relatively easily performed.

A linearity test for thick specimens imaged by HVEM over a 180° angular range

1991 ◽  
Vol 49 ◽  
pp. 552-553
Author(s):  
Yu Liu
Keyword(s):  
Phase Contrast ◽  
Three Dimensional ◽  
Angular Range ◽  
Two Dimensional ◽  
Back Projection ◽  
Line Integral ◽  
Exponential Law ◽  
Transmission Electron ◽  
Absorption Law ◽  
Linearity Test

The image obtained in a transmission electron microscope is the two-dimensional projection of a three-dimensional (3D) object. The 3D reconstruction of the object can be calculated from a series of projections by back-projection, but this algorithm assumes that the image is linearly related to a line integral of the object function. However, there are two kinds of contrast in electron microscopy, scattering and phase contrast, of which only the latter is linear with the optical density (OD) in the micrograph. Therefore the OD can be used as a measure of the projection only for thin specimens where phase contrast dominates the image. For thick specimens, where scattering contrast predominates, an exponential absorption law holds, and a logarithm of OD must be used. However, for large thicknesses, the simple exponential law might break down due to multiple and inelastic scattering.

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