Modeling

Geophysical Modeling and Inversion Software:
AMT and CSAMT

EM2D

EM2D is a finite element program which computes apparent resistivity and impedance phase responses of two-dimensional models illuminated by a plane-wave source field. Models may include both subsurface structure and surface topography on cross sections up to one hundred dipole lengths long and twenty five dipole lengths deep. Model sections may have one to nine distinct geologic units, with each unit assigned a resistivity and an optional Cole-Cole IP dispersion.

Modeling results may be calculated for TM mode, TE mode or both source polarizations. Calculated model-response data are stored in a tabular ASCII file format, which facilitates plotting by generic contouring programs.

EM2D is particularly convenient for models which include topography. EM2D can read topographic profile information from a tabular file format and automatically adjusts its finite element mesh to follow the profile. Topographic-correction factors can be calculated using models with a uniform subsurface resistivity or topography and geology can be modeled together to capture the interaction between terrain and structure.

CSINV

CSINV inverts CSAMT or AMT frequency-sounding data into a layered-earth model. For controlled-source soundings, its calculations include the effects of finite transmitter-receiver separation and three-dimensional source fields. CSINV computes accurate impedances for near-field, transition, and far-field controlled-source data. Apparent resistivity and impedance phase data from natural-source or controlled-source scalar, vector or tensor survey configurations may be inverted.

CSINV has two inversion methods, an iterative inversion algorithm that minimizes the root-mean-square difference between observed and calculated data and a controlled random search inversion that minimizes the average absolute deviation between observed and calculated data. Both the iterative inversion and the controlled random search test random variations to the starting model and end up with a group of inverted models. Model parameter errors are estimated from variation between inverted models that fit the data nearly as well as the best result. Neither algorithm changes the number of layers during inversion. Starting models can be automatically generated by CSINV or can be entered or edited manually. If geologic control limits the thickness or resistivity of a particular unit, it is possible to freeze selected model parameters so that they do not change during inversion.

SCSINV

Smooth-model inversion is a robust method for converting CSAMT measurements to models of resistivity versus depth. Observed apparent resistivity and impedance phase data for each station are used to determine the parameters of a layered-earth model. Layer thicknesses are fixed before the inversion starts by calculating source-field penetration depths for each frequency. Layer resistivities are given a uniform starting value based on controlled-source apparent resistivities. During the inversion, layer resistivities are adjusted iteratively until the calculated CSAMT response is as close as possible to observed data, consistent with smoothness constraints. Including smoothness constraints in the inversion limits resistivity variation from layer to layer and produces a model with smoothly varying resistivities.

SCSINV's forward-modeling algorithm can include the effects of finite transmitter-receiver separation and a three-dimensional source field. Source types include a vertically incident plane wave for natural source modeling and grounded electric bipole or horizontal loop sources for CSAMT modeling. Accurate impedance magnitude and phase values are calculated for all frequencies and transmitter-receiver separations. Impedances can be calculated for scalar, vector or tensor survey configurations.

Lateral variation is determined by inverting successive stations along a survey line to produce a grid of smooth-model resistivity. Resistivity values are placed at the midpoint of each layer, forming a column below every station. The columns form an array representing a cross-section of model resistivity. Results for a complete line can be presented in pseudosection form by contouring model resistivities.

Inverting apparent resistivity and impedance phase to smoothly varying model resistivities is an effective way to display the information inherent in CSAMT measurements. Smooth-model inversion does not require any a priori estimates of model parameters. The data are automatically transformed to resistivity as a function of depth. Models with smoothness constraints are complementary to more detailed models incorporating specific geologic information.

SCS2D

Smooth-model inversion is a robust method for converting far-field CSAMT or natural-source AMT data to resistivity model cross-sections. SCS2D inverts observed apparent resistivity and impedance phase data from a line of soundings to determine resistivities in a model cross-section. Either TMmode data, TE-mode or both may be inverted. To start the inversion, the model cross-section is usually given a background resistivity generated from a moving average of apparent resistivity data or a 1d smooth-model inversion section. Specific geologic structure may be added to the background model if there is drill-log or geologic-mapping information available. During the inversion, model-section-pixel resistivities are adjusted iteratively until calculated apparent resistivity and impedance phases are as close as possible to observed data, consistent with model constraints. Model constraints include background-model constraints, which restrict the difference between the inversion model and a background model section, which represents known geology, and smoothness constraints, which limit resistivity variation from pixel to pixel.

To calculate apparent resistivity and impedance phase for a given model section, SCS2D uses a two-dimensional, finite-element algorithm to calculate far-field CSAMT or natural-source MT data. To model areas with rough terrain, the finite-element mesh is draped over an along-line topographic profile. Either TM or TE-mode data can be calculated for scalar, vector or tensor survey configurations for frequencies ranging from less than 0.01 Hz to 10 kHz.

Inverting apparent resistivity and impedance phase to smoothly varying model sections is an effective way to display the information inherent in CSAMT and AMT measurements. As smooth-model inversion does not require any preliminary information about geologic structure, observed data are automatically transformed to a resistivity model cross-section providing an image of the subsurface. Model sections generated with smoothness constraints are complementary to more specialized inversions incorporating specific geologic models.

Utilities

SCSPLOT

SCSPLOT reads Zonge SCS2D inversion-program files (*.mtm and *.mtd) and creates multi-panel plots of inversion-model sections and data pseudosections. Screen plots may be exported to the Windows Print Manager, to Windows metafiles (wmf), to portable network graphics (png) raster image files, to Surfer script and data files or to Oasis montaj control and data files. Output files are given the same filename stem as the source inversion model file, plus a suffix characterizing the plot number, 1 or 2. By default resistivity inversion results are shown on plot 1 and IP results on plot 2, but resistivity and IP plot panels can be combined within a single plot.

MODSECT

MODSECT reads Zonge inversion-program model files and creates color-filled contour plots of inversion-model-section resistivity or IP (one panel per plot). Modsect can read scsinv m1d (CSAMT), steminv m1d (TEM), ts2dip IPM (resistivity/IP) or scs2d .mtm and .mtd (far-field CSAMT/NSAMT) files. Plots may be viewed on screen or exported for hardcopy. Modsect can generate script and data files for use with Surfer v6 or v7. It can also export GeoSoft Oasis montaj control and data files which MODSECTGX.GX will turn into finished plots. Modsect also exports plots directly to the Windows Printer Manger, windows metafiles (wmf) or portable network graphics (png) raster image files. Output files are given the same filename stem as the source inversion model file, plus a one-letter suffix. Resistivity section plot-file names end with a "r" while IP model-section plot-file names end with a "p".

MAPDAT

MAPDAT reads SCSINV and STEMINV *.M1D files, TS2DIP *.IPM or SCS2D *.MTM files, interpolates to a constant depth or elevation and then writes interpolated values to a tabular-format *.MAP file. *.MAP files have a simple spreadsheet format which can be used by Geosoft or Surfer.

Making plan maps requires starting with a consistent grid coordinate system in *.STN files for each line, so that inversion results from multiple lines can be combined. Concatenate *.M1D, *.IPM or *.MTM files for a project area into a single large file. MAPDAT v3.01 can handle up to 16384 stations, ie 128 stations on 128 lines. Type "MAPDAT AUBELL.M1D" to extract an elevation or depth slice from AUBELL.M1D. MAPDAT will place the interpolated values into AUBELL.MAP. When you are generating multiple depth slices, you can rename the *.MAP file to something like ABZ500.DAT to avoid overwriting.

Note that S2DIP and SCS2D model sections extend past data coverage at each end of the survey line, but the model-section extensions are poorly resolved and may include spurious features. MAPDAT does not include automatic clipping on data coverage, so it is worthwhile to trim *.IPM and *.MTM model-sections back to the extent of data coverage before concatenating multiple lines into one large file.

You may select either constant elevation or constant depth slices in MAPDAT. You may also control the number of decimal places used in stations numbers. MAPDAT will read keywords from *.MDE files, parse keywords from the command line or will prompt you to enter values.

Reference Papers

The Inversion of Magnetotelluric Data and the Elimination of Topographic Effects Through Modeling

Rough topography may cause severe distortion in imaging based on AMT data acquired with the electric field oriented perpendicular to geologic strike, ie. TM-mode data. Topographic peaks create high-angle conductive distortion while topographic valleys create high-angle resistive features in TM-mode AMT data. Topographic distortions will be carried through to produce high-angle conductive or resistive artifacts in inversion models unless the imaging procedure accounts for the distortion. SCSINV 1-D resistivity-depth images are affected by 2-D topography, while SCS2D 2-D inversions with models including a topographic profile do not.

TE-mode magnetotelluric data, with the electric field oriented parallel to geologic strike, is distorted less by 2-D topography than is TM-mode data. However collecting TE-mode is generally impractical when collecting closely spaced data along survey lines oriented perpendicular to geologic strike. Continuous AMT production is optimized when electric-field dipoles are positioned along survey lines to collect TM-mode data. In contrast, aligning electric field dipoles perpendicular to survey lines to collect TE-mode data is time consuming (and expensive). As a result, most closely sampled AMT data are collected in the scalar TM-mode. Consequently, this paper focuses mostly on the effects of topography upon the interpretation of TM-mode AMT data, although some TE-mode results are included.

Using Analytic Signal Analysis On Aeromagnetic Data To Constrain AMT Inversions, Sonora San Pedro Basin, Mexico

Airborne geophysical studies on the American side of the San Pedro Valley of Arizona and Mexico have allowed us to map depth to crystalline basement in this area where groundwater is critically important. This basin, whose head lies in northern Mexico, hosts a major US-Mexico migratory bird fly-way. A desire to preserve the surface water in the San Pedro River led to the creation of the San Pedro National Riparian Conservation Area in 1988. To preserve the surface water, one must know something about the aquifer underlying it. On the American side of the basin, time-domain airborne geophysical methods were used to map the relatively conductive groundwater typical of an arid region to depths of 150 - 400 meters in the absence of human cultural interference. In order to better understand the hydrology of the basin as a whole, geophysical surveying has been extended southward into the Sonoran San Pedro Valley of northern Mexico. An airborne magnetic survey in northern Mexico has been processed to depth-to-magnetic-source, and concatenated to a magnetic data set from southern Arizona to show depth to basement for the San Pedro Valley drainage. We then conducted a scalar Audio- MagnetoTelluric (AMT) survey over four different lines in the Sonoran San Pedro basin, and processed these data using a smooth-model inversion to conductivity-vs-depth profiles. As we view the conductivity inversion results, we are in fact visualizing the highly conductive water typical of an arid climate - in effect, we broadly image the saturated sediments. We then used an analytic signal depth-to-source algorithm on magnetic data along the same profiles to constrain the AMT inversion. The result is a unique set of geophysical profiles that clearly show basement structure beneath the Sonoran San Pedro basin to depths of up to 800 meters. These constrained profiles help resolve basement controls on groundwater flow in northern Mexico leading to the US frontier. It is impossible to understand the groundwater regime except in the context of the volcanic and sedimentary history of the region, and neither the geology nor the geophysics can be carried out independently of the other, but the whole together contribute substantially more than the parts.