Resistivity/depth mapping with airborne electromagnetic survey data

Geophysics ◽  
1983 ◽  
Vol 48 (2) ◽  
pp. 181-196 ◽  
Author(s):  
Klaus‐Peter Sengpiel
Keyword(s):  
Salt Water ◽  
Mapping Method ◽  
Shielding Effect ◽  
Single Frequency ◽  
Salt Water Intrusion ◽  
Horizontal Gradient ◽  
Model Parameters ◽  
Apparent Depth ◽  
Contour Maps ◽  
Airborne Electromagnetic

Using the homogeneous half‐space as a universal interpretation model, all of the secondary field data obtained with a single‐frequency airborne electromagnetic (EM) system that satisfies the superposed dipole condition can be converted to the model parameters [Formula: see text] (apparent resistivity) and [Formula: see text] (apparent depth). These parameters have been investigated for their behavior above various conductivity models and at various flight altitudes, first for theoretical examples and then for several applications in the field. The values of [Formula: see text] and [Formula: see text] are good approximations of the true resistivity and true depth of an extended, buried conductor only where the shielding effect of the cover is small. Moreover, a depth value has a meaning only within the lateral limits of a target conductor. A method is described to locate these lateral limits and to select acceptable depth and resistivity values by means of the “area of [Formula: see text],” which is derived from the horizontal gradient of log [Formula: see text] and the maxima of [Formula: see text]. The results of the resistivity/depth mapping method are presented in the form of two contour maps. Examples of the practical application of the method, over known sulfide ore bodies and over a salt water intrusion, show that reliable data can be obtained on the depth, dip, and extent of these kinds of conductors, as well as on the approximate resistivity of the conductors and the host rock.

Crosswell electromagnetic inversion using integral and differential equations

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 899-911 ◽  
Author(s):  
Gregory Newman
Keyword(s):  
Inverse Problem ◽  
Field Data ◽  
Green's Functions ◽  
Green’S Functions ◽  
Salt Water ◽  
Single Frequency ◽  
Model Parameters ◽  
Inversion Algorithm ◽  
Before And After ◽  
Using Data

The crosswell electromagnetic (EM) inverse problem is solved with an integral‐equation (IE) formulation using successive Born approximations in the frequency domain. Because the inverse problem is nonlinear, the predicted fields and Green’s functions are continually updated. Updating the fields and Green’s functions relates small changes in the predicted data to small changes in the model parameters through Fréchet kernels. These fields and Green functions are calculated with an efficient 3-D finite‐difference solver. Since the resistivity is invariant along strike, the 3-D fields are integrated along strike so the 2-D kernels can be assembled. At the early stages of the inversion, smoothing of the electrical conductivity stabilizes the inverse solution when it is far from convergence. As the solution converges, this smoothing is relaxed and more effort is made to reduce the data misfit. Bounds on the conductivity are included in the solution to eliminate unrealistic estimates. The robustness of the inversion scheme has been demonstrated with synthetic and field data that are underdetermined from the standpoint of the smooth models being sought. Two synthetic examples with added Gaussian noise were considered, including data arising from an IE solver. This IE solver is different from the one embedded in the inversion algorithm and has provided a stronger check on the scheme. The synthetic examples show it is more difficult to reconstruct a target’s conductivity than its geometry at a single frequency. The inversion scheme has been successfully tested using data collected at the Richmond‐field site near Berkeley, California, where it has imaged a salt water plume injected into the interwell region. The data in this experiment consisted of two sets of measurements, taken before and after the injection of 50 000 gallons of 1 Ωm salt water. Findings show that underdetermined inversion using small amounts of field data can be sufficient to produce useful, but smoothed, maps of the conductivity. The data in this instance need be only single frequency and single component.

Approximation for Salt-Water Intrusion in Unconfined Coastal Aquifer

Ground Water ◽  
1980 ◽  
Vol 18 (2) ◽  
pp. 147-151 ◽  
Author(s):  
B. K. Panigrahi ◽  
A. Das Gupta ◽  
A. Arbhabhirama
Keyword(s):  
Coastal Aquifer ◽  
Salt Water ◽  
Salt Water Intrusion ◽  
Water Intrusion

Simulated annealing for airborne EM inversion

Geophysics ◽  
2007 ◽  
Vol 72 (4) ◽  
pp. F189-F195 ◽  
Author(s):  
Changchun Yin ◽  
Greg Hodges
Keyword(s):  
Simulated Annealing ◽  
Random Walk ◽  
Boltzmann Distribution ◽  
Model Parameters ◽  
Local Minima ◽  
Model Configuration ◽  
Airborne Electromagnetic ◽  
Starting Model ◽  
Airborne Em ◽  
Cooling Schedule

The traditional algorithms for airborne electromagnetic (EM) inversion, e.g., the Marquardt-Levenberg method, generally run only a downhill search. Consequently, the model solutions are strongly dependent on the starting model and are easily trapped in local minima. Simulated annealing (SA) starts from the Boltzmann distribution and runs both downhill and uphill searches, rendering the searching process to easily jump out of local minima and converge to a global minimum. In the SA process, the calculation of Jacobian derivatives can be avoided because no preferred searching direction is required as in the case of the traditional algorithms. We apply SA technology for airborne EM inversion by comparing the inversion with a thermodynamic process, and we discuss specifically the SA procedure with respect to model configuration, random walk for model updates, objective function, and annealing schedule. We demonstrate the SA flexibility for starting models by allowing the model parameters to vary in a large range (far away from the true model). Further, we choose a temperature-dependent random walk for model updates and an exponential cooling schedule for the SA searching process. The initial temperature for the SA cooling scheme is chosen differently for different model parameters according to their resolvabilities. We examine the effectiveness of the algorithm for airborne EM by inverting both theoretical and survey data and by comparing the results with those from the traditional algorithms.

scholarly journals GIS analysis of effects of future Baltic Sea level rise on the island of Gotland, Sweden

2016 ◽  
Author(s):  
Karin Ebert ◽  
Karin Ekstedt ◽  
Jerker Jarsjö
Keyword(s):  
Baltic Sea ◽  
Sea Level Rise ◽  
Sea Level ◽  
Coastal Aquifers ◽  
Salt Water ◽  
Global Perspective ◽  
Salt Water Intrusion ◽  
The Baltic Sea ◽  
Water Intrusion ◽  
The Baltic

Abstract. Future sea level rise as a consequence of global warming will affect the world's coastal regions. Even though the pace of sea level rise is not clear, the consequences will be severe and global. Commonly the effects of future sea level rise are investigated for relatively vulnerable development countries; however, a whole range of varying regions need to be considered in order to improve the understanding of global consequences. In this paper we investigate consequences of future sea level rise along the coast of the Baltic Sea island of Gotland, Sweden, with the aim to fill knowledge gaps regarding comparatively well-suited areas in non-development countries. We study both the quantity of loss of infrastructure, cultural and natural values for the case of a two metre sea level rise of the Baltic Sea, and the effects of climate change on seawater intrusion in coastal aquifers, causing the indirect effect of salt water intrusion in wells. We conduct a multi-criteria risk analysis by using Lidar data on land elevation and GIS-vulnerability mapping, which gives formerly unimaginable precision in the application of distance and elevation parameters. We find that in case of a 2 m sea level rise, 3 % of the land area of Gotland, corresponding to 99 km2, will be inundated. The features most strongly affected are items of touristic or nature values, including camping places, shore meadows, sea stack areas, and endangered plants and species habitats. In total, 231 out of 7354 wells will be directly inundated, and the number of wells in the high-risk zone for saltwater intrusion in wells will increase considerably. Some values will be irreversibly lost due to e.g. inundation of sea stacks and the passing of tipping points for sea water intrusion into coastal aquifers; others might simply be moved further inland, but this requires considerable economic means and prioritization. With nature tourism being one of the main income sources of Gotland, monitoring and planning is required to meet the changes. Seeing Gotland in a global perspective, this island shows that holistic multi-feature studies of future consequences of sea level rise are required, to identify overall consequences for individual regions.

Inversion of helicopter electromagnetic data to a magnetic conductive layered earth

Geophysics ◽  
2003 ◽  
Vol 68 (4) ◽  
pp. 1211-1223 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Magnetic Permeability ◽  
Field Data ◽  
Model Parameters ◽  
Relative Importance ◽  
Magnetic Polarization ◽  
Inversion Algorithm ◽  
Layered Earth ◽  
Electromagnetic Data ◽  
Airborne Electromagnetic ◽  
True Values

Inversion of airborne electromagnetic (EM) data for a layered earth has been commonly performed under the assumption that the magnetic permeability of the layers is the same as that of free space. The resistivity inverted from helicopter EM data in this way is not reliable in highly magnetic areas because magnetic polarization currents occur in addition to conduction currents, causing the inverted resistivity to be erroneously high. A new algorithm for inverting for the resistivity, magnetic permeability, and thickness of a layered model has been developed for a magnetic conductive layered earth. It is based on traditional inversion methodologies for solving nonlinear inverse problems and minimizes an objective function subject to fitting the data in a least‐squares sense. Studies using synthetic helicopter EM data indicate that the inversion technique is reasonably dependable and provides fast convergence. When six synthetic in‐phase and quadrature data from three frequencies are used, the model parameters for two‐ and three‐layer models are estimated to within a few percent of their true values after several iterations. The analysis of partial derivatives with respect to the model parameters contributes to a better understanding of the relative importance of the model parameters and the reliability of their determination. The inversion algorithm is tested on field data obtained with a Dighem helicopter EM system at Mt. Milligan, British Columbia, Canada. The output magnetic susceptibility‐depth section compares favorably with that of Zhang and Oldenburg who inverted for the susceptibility on the assumption that the resistivity distribution was known.

Sign in / Sign up

Export Citation Format

Share Document