Louise Pellerin

Correction for the static shift in magnetotellurics using transient electromagnetic soundings

Shallow inhomogeneities can lead to severe problems in the interpretation of magnetotelluric (MT) data by shifting the MT apparent resistivity sounding curve by a scale factor, which is independent of frequency on the standard log-apparent-resistivity versus log-frequency display. The amount of parallel shift, commonly referred to as the MT static shift, can not be determined directly from conventionally recorded MT data at a single site. One method for measuring the static shift is a controlled-source measurement of the magnetic field. Unlike the electric field, the magnetic field is relatively unaffected by surface inhomogeneities. The controlled-source sounding (which may be a relatively shallow sounding made with lightweight equipment) can be combined with a deep MT sounding to obtain a complete, undistorted model of the earth. Inversions of the static shift-corrected MT data provide a much closer match to well-log resistivities than do inversions of the uncorrected data.The particular controlled-source magnetic-field sounding which we used was a central-induction Transient ElectroMagnetic (TEM) sounding. Correction for the static shift in the MT data was made by jointly inverting the MT data and the TEM data. A parameter which allowed vertical shifts in the MT apparent resistivity curves was included in the computer inversion to account for static shifts. A simple graphical comparison between the MT apparent resistivities and the TEM apparent resistivities produced essentially the same estimate of the static shift (within 0.1 decade) as the joint computer inversion.Central-induction TEM measurements were made adjacent to over 100 MT sites in central Oregon. The complete data base of over 100 sites showed an average static shift between 0 and 0.2 decade. However, in the rougher topography and more complex structure of the Cascade Mountain Range, the majority of the sites had static shifts of the order of 0.3 to 0.4 decade. The static shifts in this area are probably due to a combination of topography and surficial inhomogeneities. The TEM apparent resistivity (which is used to estimate the unshifted MT apparent resistivity) does not necessarily agree with either the transverse electric (TE) or the transverse magnetic (TM) MT polarization. TEM apparent resistivity may occur between the two, or may agree with one of the two polarizations, or may lie outside the MT polarizations.

A survey of current trends in near-surface electrical and electromagnetic methods

ElectricalandelectromagneticE&EMmethodsfornearsurface investigations have undergone rapid improvements over the past few decades. Besides the traditional applications in groundwater investigations, natural-resource exploration, and geological mapping, a number of new applications have appeared. These include hazardous-waste characterizationstudies,precision-agricultureapplications,archeological surveys, and geotechnical investigations. The inclusion of microprocessors in survey instruments, development ofnewinterpretationalgorithms,andeasyaccesstopowerful computers have supported innovation throughout the geophysical community and the E&EM community is no exception. Most notable are development of continuous-measurement systems that generate large, dense data sets efficiently. These have contributed significantly to the usefulness of E&EM methods by allowing measurements over wide areas without sacrificing lateral resolution. The availability of theseluxuriantdatasetsinturnspurreddevelopmentofinterpretation algorithms, including: Laterally constrained 1D inversionaswellasinnovative2D-and3D-inversionmethods. Taken together, these developments can be expected to improve the resolution and usefulness of E&EM methods and permit them to be applied economically. The trend is clearly towarddensesurveyingoverlargerareas,followedbyhighly automated, post-acquisition processing and interpretation to provide improved resolution of the shallow subsurface in a cost-effectivemanner.

EMMA - a geophysical training and education tool for electromagnetic modeling and analysis

An interactive modeling and analysis program with a user-friendly graphical interface, for students and professionals in the field of exploration geophysics, has been developed. The ElectroMagnetic Modeling and Analysis program—EMMA—is capable of modeling one-dimensional responses for most electrical and electromagnetic methods and array configurations, including the time-domain, frequency-domain, resistivity, magnetotelluric and borehole methods. EMMA is available on-line, at no charge, from http:∕∕www.hgg.au.dk. An innovation of EMMA is the calculation of the model sensitivity analysis, a feature that is usually only present in inversion codes. The variance of the model parameters depends on the variance of the measured data and the way in which an error is mapped from the data to the model parameters. The measurement situation is realized by ascribing data noise to the model response. This facilitates the calculation of a realistic model parameter analysis. For time-domain data, piece-wise linear wavef...

Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 Mw 7.9 Denali fault earthquake. These data and aftershock seis- micity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25� north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seis- micity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonre- flective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (� 10 Xm), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maxi- mum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after- shock zone.

Mutually Constrained Inversion (MCI) of Electrical and Electromagnetic Data

Summary Mutually constrained inversion (MCI) is a process in which two distinct data sets are inverted to produce two closely related models. Time domain electromagnetic (TEM) and electrical resistivity are two methods, that measure the same fundamental property, resistivity, but have different sensitivity and will not necessarily respond to the earth in the same manner. MCI has many of the properties of joint inversion, the process where two datasets are inverted to produce one model. Poorly resolved parameters are enhanced and invisible layers can be seen. However MCI is more robust, the two resulting models can be independently evaluated, and the best resolved parameters used in the interpretation. The approach is illustrated first on synthetic data. The MCI is used to resolve incompatibilities produced when the resistivity sounding is distorted with near surface inhomogeneities, a static shift, without the need of a special parameter that cannot be measured. A field study demonstrates how a resistive layer, which is important in aquifer characterization, that is either inconsistently detected or unresolved in the separate time domain and resistivity datasets, is well delineated with the MCI.

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October-November 2002, M = 7.9 Denali fault earthquake

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Three‐dimensional inversion of electromagnetic data

The interpretation of electromagnetic (EM) data using one-dimensional (1-D) inversion algorithms can be expected to provide poor resolution of two-dimensional (2-D) and threedimensional (3-D) structures. Unfortunately 2or 3-D inversion of large datasets is computationally costly both in processing time and computer memory. Therefore we approach multidimensional interpretation with an inversion algorithm, based upon a Born approximation to the 3-D integral equation, that iteratively refines a piece-wise 1-D interpretation at a receiver using the data from neighboring receivers. We test the algorithm on the central-loop response of two 3-D models: a simple prism representing the problem of low-contrast resistivity mapping, and a conductive out-flow plume to simulate mapping of a conductive contaminant plume. Discretization of the first test model exactly matches that of the inversion and illustrates the strengths and weakness of the scheme. The second model is discretized such that its response simulates field data. The test results indicate our method does indeed improve a 1-D interpretation and has merit as a tool in multi-dimensional EM interpretation.

A method to estimate hydraulic conductivity while drilling

Abstract A field test and analysis method has been developed to estimate the vertical distribution of hydraulic conductivity in shallow unconsolidated aquifers. The field method uses fluid injection ports and pressure transducers in a hollow auger that measure the hydraulic head outside the auger at several distances from the injection point. A constant injection rate is maintained for a duration time sufficient for the system to become steady state. Exploiting the analogy between electrical resistivity in geophysics and hydraulic flow two methods are used to estimate conductivity with depth: a half-space model based on spherical flow from a point injection at each measurement site, and a one-dimensional inversion of an entire dataset. The injection methodology, conducted in three separate drilling operations, was investigated for repeatability, reproducibility, linearity, and for different injection sources. Repeatability tests, conducted at 10 levels, demonstrated standard deviations of generally less than 10%. Reproducibility tests conducted in three, closely spaced drilling operations generally showed a standard deviation of less than 20%, which is probably due to lateral variations in hydraulic conductivity. Linearity tests, made to determine dependency on flow rates, showed no indication of a flow rate bias. In order to obtain estimates of the hydraulic conductivity by an independent means, a series of measurements were made by injecting water through screens installed at two separate depths in a monitoring pipe near the measurement site. These estimates differed from the corresponding estimates obtained by injection in the hollow auger by a factor of less than 3.5, which can be attributed to variations in geology and the inaccurate estimates of the distance between the measurement and the injection sites at depth.

Hydrogeophysics and the settlement of San Marcos Pueblo, NM: Investigations by the SAGE geophysical field course

San Marcos Pueblo, archaeological site LA 98, was occupied in the 13th century and abandoned after 1680. This was a large community that conducted extensive trade based on local mineral resources. The Summer of Applied Geophysical Experience (SAGE) Field Course has conducted investigations pointing to unique groundwater access as a motivating factor in the pueblo settlement. The Quaternary stratigraphy of the site has been reveled by near-surface seismic refraction profiling. The specific hydrogeologic conditions leading to the reliability of San Marcos Spring are the presence of the Ancha formation, its basal juxtaposition on the Galisteo formation and the erosion of overlying Holocence terrace deposits.

An empirical approach to inversion of an unconventional helicopter electromagnetic dataset

Abstract A helicopter electromagnetic (HEM) survey acquired at the U.S. Idaho National Engineering and Environmental Laboratory (INEEL) used a modification of a traditional mining airborne method flown at low levels for detailed characterization of shallow waste sites. The low sensor height, used to increase resolution, invalidates standard assumptions used in processing HEM data. Although the survey design strategy was sound, traditional interpretation techniques, routinely used in industry, proved ineffective. Processed data and apparent resistivity maps were severely distorted, and hence unusable, due to low flight height effects, high magnetic permeability of the basalt host, and the conductive, three-dimensional nature of the waste site targets. To accommodate these interpretation challenges, we modified a one-dimensional inversion routine to include a linear term in the objective function that allows for the magnetic and three-dimensional electromagnetic responses in the in-phase data. Although somewhat ad hoc, the use of this term in the inverse routine, referred to as the shift factor, was successful in defining the waste sites and reducing noise due to the low flight height and magnetic characteristics of the host rock. Many inversion scenarios were applied to the data and careful analysis was necessary to determine the parameters appropriate for interpretation, hence the approach was empirical. Data from three areas were processed with this scheme to highlight different interpretational aspects of the method. Wastes sites were delineated with the shift terms in two of the areas, allowing for separation of the anthropomorphic targets from the natural one-dimensional host. In the third area, the estimated resistivity and the shift factor were used for geological mapping. The high magnetic content of the native soil enabled the mapping of disturbed soil with the shift term.