Electromagnetic
Methods
ELECTROMAGNETIC CONDUCTIVITY
EMIX 34 Plus is
designed for the interpretation of EM conductivity data acquired with the
Geonics Ltd. EM31, EM34 or EM38 instruments. Forward and Inverse modeling is
done on individual sounding stations and the results are displayed in profiles
form. The results are in terms of apparent conductivity response.
EMIX 34 Plus computes the forward
model using the full solution described by H.P. Patra and K. Mallick in
Geosounding Principles, 2 (1980) published by Elsevier. The inversion
calculation uses the Inmanstyle ridge regression approach of nonlinear least
squares curve fitting. Geometric soundings using the vertical magnetic dipole (VMD)
or horizontal magnetic dipole (HMD) or both modes are supported either in terms
of change in coil separationfrequency or instrument height.
EMIX 34 Plus interprets and
presents data acquired as a profile. Profiles can differ with respect to
frequency, coil height and spacing, in the horizontal coplanar or vertical
coplanar coil configurations. You may use different instruments, or the same
instrument in different configurations, or both methods to collect data. The use
of multiple combinations produces data sets interpretable as soundings.
EMIX 34 Plus lets you divide the
profile data into five segments along the profile and then fix the individual
model parameters for each segment. This allows flexibility in integrating known
geologic or geophysical information into the interpretation.
Layered Interpretation allows the user to
interpret sounding data at each station in terms of a model with up to five
plane layers. Results are in terms of apparent conductivity response for each
configuration of frequency, spacing and coil height. This data is presented as
apparent conductivity at low induction number, although the forward calculations
account for the deviation from the low induction number assumption. The
quadrature response of the earth is converted to apparent conductivity using eq.
(2) from McNeill (Geonics Limited Technical Note TN6. 1980) The in phase
component of the field is ignored.
EMIX 34 Plus features the
interactive graphics interpretation mode; you move from station to station
along the profile establishing the starting forward model. You can invert the
sounding at this time if you choose.
Inverse modeling allows the user to obtain a
model which best fits the data in a least squares sense. This is done using
ridge regression to adjust the parameters of a starting model in an iterative
manner.
You may constrain some of the parameters of the
layered starting model so that they will not be adjusted by the inversion.
Layers are fixed across sections of the model, not on a point by point basis.
Starting models can have up to 5 layers; the maximum number of fixed segments is
5.
Inverse modeling of layered models can be
carried out in batch mode. Accurate results are achieved even if the low
induction number assumption is not satisfied.
EMIX 34 Plus provides a data
logger interface to data files created by the Geonics Ltd. data logger and an
ASCII interface using the Geosoft XYZ format.
Interactive Graphics Interpretation screen
showing full data set and individual sounding with calculated response
EMIX VLF is a presentation an
processing package for VLF data. VLF field data can be presented as profiles and
contour maps. Data can be filtered using either the FRASER or HJELT filters.
Results are displayed as either contour maps, profiles or contoured profile
sections. The main focus of the programs is to provide an interactive graphics
analysis tool for users with VLF data.
MATHEMATICAL BASIS
Apart from displaying VLF data EMIX VLF provides
the user with two data filters. The FRASER filter converts crossovers of the
correct polarity into peak responses by differencing successive values of the
tiltangle along the survey profile.
The HJELT filter profiles are obtained by using
the linear fit theory to solve the integral equation for the current
distribution, assumed to be located in a thin horizontal sheet of varying
current density, situated everywhere at a depth equal to the distance between
measurement stations.
EMIX VLF was designed to read inphase and
quadrature VLF data from instruments and files into a grid based data set. Easy
to use editors allows for keyboard entry of data collected on a non digital
instrument or for the editing of recorded data. Data sets are then saved in a
binary file for quick and easy retrieval.
The program allows for the export of VLF data
into XYZ ASCII files and printing of complete datasets as paginated text reports
to any of the printers supported.
The main feature of EMIX VLF is the interactive
graphics viewing screens. The first option is the single map view that allows
the user to view a contour map of any of the following datasets:
Inphase data
Quadrature data
Amplitude
Phase
Fraser inphase
Fraser quadrature
Fraser amplitude
Hjelt inphase
buttons Interactive
Graphics Screen showing contour map of data
The second interactive graphics screen allows
users to display two maps side by side with interactive profile selection from
the map displays. Any of the datasets available for viewing in the single map
mode is available for either of the maps in the dual maps display mode.
Interactive
Graphics Screen showing contour map of data with selected profiles in top
windows
Features available in both the interactive
graphics screen include:
Full Zoom and Replot capabilities
Direct hardcopy output of displayed screen
Contour line on/off toggle
Data posting on/off toggle
Contour color and value editing
Interactive
Graphics Screen showing contour map of data wit Hjelt Depth Sections of selected
profiles
EMIX VLF also allows for the export of plot and
grid files to IXCAD. IXCAD enables users to edit and annotate the plot files,
shade the grid files as color or grayshade images.
EMIX MM Plus allows
for forward and inverse modeling of EM data acquired with instruments such as
the SCINTREX EM4 GENIE/HLEM or APEX Parametrics MAXMIN systems in terms of
layered earth models and thin plate conductors. EMIX MM PLUS can interpret data
from both the MAXMIN and GENIE systems.
The total number of layers for layered earth
models are 7. The program calculations allow for VARIABLE coil spacings and
also takes the instrument height into consideration. A floating constant
compensate for the spacing errors in TX and RX separation.
EMIX MM Plus interprets and
presents data collected in profile form in terms of both layered earth models
and thin plate conductors. Support for the SCINTREX EM4 GENIE/HLEM is now
standard with this package. The program uses data acquired in the HLEM mode.
Interactive Graphics Screen showing profile
data and layered model results
EMIX MM Plus lets you divide the
data into five segments along the profiles and then fix individual model
parameters for each of the four segments.
The Genie option was developed to accommodate
the measurements of the Scintrex Genie HLEM Instrument. This instrument is
useful in rugged terrain, since it measures total field at two frequencies and
combines them to form the "Genie Anomaly". In addition to allowing EMIX MM Plus
to model this Genie Anomaly, it is also better for MaxMin and other instruments
which measure inphase and quadrature components of the secondary field. The
program now has forward and inverse modeling for conductors, instead of a direct
inversion.
Interactive Graphics Screen showing profile
data with interpreted conductor models
The dip, depth and conductance of the thin sheet
are specified, and the argand diagrams are interpolated to find the inphase and
quadrature values. The Wesley algorithm is used to find the curve shape, using
the depth and the angle. The curve is then scaled to yield the inphase and
quadrature responses.
In case of the Genie, this is done for the low
and the high frequencies and combined to form the Genie Anomaly. This means a
starting model can be specified, or manually interpret without using inversion
is possble. It also means that there is a single interpretation for dip angle
and width. There is no longer a need to edit the argand interpretation, since
there is now a forward modeling mode.
The response for conductor models can be
calculated for all the frequencies without copying the conductor to all the
frequencies, effectively "filling" in the synthetic data with the response from
the selected conductors. The program does a forward calculation of the
conductors to adjust for the frequency and then an inversion to adjust for the
base values. Conductors can also be copied specified frequencies.
Plot of profile data and synthetic calculated
responses for both layered earth and conductor models. The bottom window shows
the appropriate models with labels that was added using the interactive label
option.
Calculated layered earth responses or "filled"
conductor responses from the field data can be "removed" from the data by
subtracting the synthetic responses from the field data.
