Geophysics Department TU Clausthal

A Magnetic Profile across a 2D-Model

Students exercise at Geophysics Department TU Clausthal

Applet   ( in separate Window )

User's Manual

DEMO version : No local I/O, Example data or Fieldobservations in HTML-File ( Applet Parameter ) only.

FULL version : Local I/O ( own observed data ) included
( Works with the Appletviewer of a JDK_1.1.x, provided the path to the local files is set with acl.read= and acl.write= in .hotjava/properties. )

The "sand box" restrictions of Applets, especially in browsers as Netscape, Explorer etc., do not apply to Applications running in a Java Runtime Environmet ( JRE ).

The Applet simulates a magnetic survey along an arbitrary orientated profile across a two-dimensional subsurface model of arbitrary strike direction and composed of the surface of a halfspace and up to six 2-d structures of arbitrary shapes in a vertical cross section.

Model calculations are based on Talwani's calculus for two dimensional bodies of homogenious magnetization.

The 2-d simplification approximates numrous practical cases, where the extension of geological structures along their horizontal strike direction is much greater than in a vertical cross section.
The asumption of homogenious magnetization strictly holds for elliptical cross sections only, but implies tolerable errors at least for "weakly" magnetized structures, where the magnitude of a body's internal field is far below the magnitude of the local magnetic field of the earth.

In addition to the 2-d bodies infinite lines of dipoles may be included,
- to represent 2-d stuctures of dimensions in a cross section much less than the shortest distance to the profile,
and
- to facilitate the understanding of the magnetic field surrounding a subsurface structure, the decomposition of the field vector into components of location dependent directions and the superposition of the local field of the earth.

Topographic effects along a profile are simulated by modelling the surface of the halfspace.

Students exercise :
Observation of vertical and horizontal component using a fluxgate magnetometer on a NS-profile, length 38 [m], across the center of a subsurface structure, srike EW and length ca. 10 [m] ( a row of radiators, cast iron, upper edge ca. 0.7 [m] below surface ).

Table of Content

HOWTO :

Graphic display:

Model calculation :

The magnetic field surrounding a subsurface structure is caused by the body's magnetization, composed of

a remanent part ("pemanent magnet") of arbitrary magnitude and orintation,
depending on the mineral content, the thermal and/or tectonic history of the geologic structure and the magnetic field it was exposed to during different states of its history,
and
an induced part parallel to the actual magnetic field of the earth and described by the materials volume susceptibility,
a scaling factor to be applied to the magnitude of the actual magnetic field it is exposed to and again not a "material constant", itself depending on the magnitude of the inducing magnetic field and the history of the geological structure.

For an obsevation point the contributions of all infinitesimal volume elements of a body have to be summed up ( integrated ).
For a homogenious magnetization ( constant in magnitude and orientaion all over the body ) and for a point outside the body this procedure is greatly simplified by approximating the body's surface by plane triangles, and by transforming the volume integral over the structure into an integal over its surface, and further into line integrals along the sides of the triangles.
This, for three dimensional structures, still very time consuming calculation is further accelerated for 2-d bodies, reducing the integration path to the border lines in a vertical cross section ( a set of staight lines forming a closed polygon ).

As a result of the implicit integration along the ( infinite ) strike extension of a model, contibutions of magnetization components parallel to this axis cancel out and the surrounding field parallel to this axis vanishes: the field vector lies in the plane of the vertical cross section.
Consequently only the projection of the magnetization of a body / magnetic moment of a line of dipoles into this plane has to be taken into account.

For the simplified model of a homogenious line of dipoles the term "remanent" ( REM ) is used to indicate a magnetic moment of arbirary magnitude and direction, the term "induced" ( IND ) to indicate a magnetic moment parallel to the local magnetic field of the earth and proportional to its magnitude.

Within the dimensions of a survey area the magnetic field of the earth is assumed to be constant, neglecting changes of magnitude with height, ranging from -15 [nT/km] at the magnetic equator to -30&nbs;[nT/km] at the magnetic poles, and with latitude of up to about +4.5 [nT/km] at about 45 [deg] of magnetic latitude.

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Coordinates :

The location of the survey area on the globe is specified in geographic coordinates
longitudeLON [deg], reference meridian = Greenwich ) and
latitudeLAT [deg], positive on the northern hemisphere ).

Distances are measured in meters[m] )
in a horizontal plane :
S = parallel, and
Q = perpendicular to the profile,
and in a vertical plane across the subsurface model :
X = perpendicular to the strike direction of the model,
Z = pos. downward.

Angles are measured in degrees[deg] )
horizontally from geographic north ( = local meridian ) positive to east ( declination, DEC and azimuth, AZIM ),
vertically from a horizontal plane positive downward ( inclination, INC ).

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Components of the magnetic field :

The magnetic field is described by its
magnitudeT, TOT, units : nano Tesla, [nT] ),
inclination angleINC ) from the local horizontal plane, and
declination angleDEC ) from geographic north.

This corresponds to a vertical component

Z = T * sin(INC), positive downward,

and a horizontal component

H = T * cos(INC), pointing to magnetic north.

The horizontal component may be split up into ( geographic ) north and a east components

N = H * cos(DEC) and E = H * sin(DEC),

for practical use affording a fairly precize absolute orientation of the magnetic sensors ( H = 20000 [nT] affords a precission of about 10 seconds of arc to keep errors in E component below 1 [nT] ).

Within the dimensions of a survey area the magnetic field of the earth is assumed to be constant, neglecting changes of magnitude with height, ranging from -15 [nT/km] at the magnetic equator to -30&nbs;[nT/km] at the magnetic poles, and with latitude of up to about +4.5 [nT/km] at about 45 [deg] of magnetic latitude.

In most field surveys the magnitude T is observed ( ! without any information about the orintation of the field ! ) and / or the change in vertical component DEL_Z ( affording a relatively simple and fast sensor orientation via bubble levels ).

To observe changes DEL_H of the horizontal component of the magnetic field, the sensors mostly are orientated towards magnetic north, thus neglecting small local variations of this direction due to the superimposed field of magnetized subsurface structures.

For surveys focused on shallow subsurface structures the effect of regional field changes and of "slow" field variations with time ( diurnal variations ) may be suppressed by observing an approximation to the gradient of the magetic field, i.e. by observing the magnitude T simultaniously or within a few seconds ( typically 3 - 4 [sec] )

at two different heights above the surface
or
at two points with a fixed spacing in profile direction

The gradients of vertical component Z or horizontal component H, selectable from the menue panel, are for information only without great importance for practical cases because of the difficulties in sensor orientaton.

Evtl. from horizontally narrow spaced field observations, corrected for diurnal variations, the horizontal gradient can be calculated from the processed observations.

To Table of Content

HOWTO :

Applet / Dialogue window ( screenshot ) :

The button DATA I/O opens / closes a window to load and to list geometry information of a profile and a set of data observed along the profile,

NO INP / CMP 1 ... selects one of up to three components of the data set for display and comparison to values calculated for a subsurface model.

The text fields to the right describe the data source ( file name / example xx / Applet param. ) and the component selected ( name, sensor height etc. ).

The button MODEL I/O opens / closes a window to load and to list geometry information and magnetic properties of a model of the earth's surface and of up to six subsurface structures ( 2-d bodies / dipole lines ).

The buttons SAVE ACT / REST SVD MOD allow to save the actual state of a model and to restore a previously saved state.

From two button arrays ( SENSOR / GRADIENT COMPONENT ) up to three components of the calculated magnetic field may be selected for display in the window PRO ( see below ).

The row SCALING provides three independent scaling factors for the coordinates of the profile, the dimensions of the model and for the height of the magnetic sensors above surface.

The row SHOW ( bottom, yellow ) shows / hides four seperate windows to display and adjust various survey, profile and model parameters :

SRV = geographic ccordinates of the survey area and resulting local magnetic field of the earth,
MAP = orientation and extend of the 2-d structures and of the profile in a horizontal plane,
MOD = position, shape and magnetic properties of the model structures in a vertical cross section,

and to display observed and calculated field values :

PRO = magnetic observations along the profile, and field values, calculated from the actual survey, profile and model parameters adjustable in the above windows.

HELP and INFO refer to the windows MAP, MOD and PRO :

HELP displays informations to the actually possible mouse interactions,
and
INFO displays the coordinates of the actual cursor position.

STOP ( Applet ) closes all windows,

EXIT ( Application ) exits the program.

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Model calculations are performed for 5 different sensor positions :

one central sensor ( MIDDLE ),
2 sensors equally spaced aboveUPPER ) and belowLOWER ),
and
2 sensors ( height = MIDDLE ) at the same distance in positive"foreward" ) and negative profile direction"backward" ).

Height and spacing of the sensors are displayed and adjusted in the H_SENSOR panel :

the height is adjusted preserving the vertical ( and horizontal ) spacing by dragging the M-position
and
the vertical and horizontal spacing is varied preserving the M-position by dragging either U / L-position of the sensors,
using the left mouse button,

the L-position is dragged preserving the U-position and vice versa
using the the right button

! Positions L, M and U always are above the point of observation, positions L / U and "forward" / "backward" are symmetrical to M !

The vertical gradientD/DZ ) is approximated
by the difference LOWER - UPPER ( ! z = pos. downward ! ),
by the difference "foreward" - "backward",
both devided by the sensor spacing and "repesentative" for the central sensor position ( height = MIDDLE ).

To avoid scaling problems in the graphic display ( window PRO ), only differences to the undisturbed local magnetic field ( DEL_T, DEL_Z and DEL_H ) are displayed and the component selection is restricted to either one row or one column of the two button arrays SENSOR / GRADIENT COMPONENT.

The horizontal component DEL_H is calculated for different "orientations of the sensors" :
H = MAG N parallel to the local horizontal field,
H = GEO N pointing to geogr. north,
H = S_PRO along the profile and
H = Q_PRO across the profile.

The components DEL_Z and DEL_H directly represent the respective components of the additional field of the subsurface structure, whereas DEL_T is the difference in magnitude of the "disturbed" and the "undisturbed" magnetic field ( total intensity, T ).
The total values for all three components may be estimated by adding the corresponding values for the undisturbed local field, listed in the window SRV.

! Loading observed data and selecting one of the observed components ( see DATA I/O and CMP 1 ... ) disables the panel H_SENSOR, the button arrays SENSOR / GRADIENT COMPONENT and the scaling buttons PROFILE and H_SENS, since the respective parameters are given in the dataset loaded !

To Table of Content

Window DATA I/O :

to erase, load and dump field observations,
containing survey and geometry information of a profile and observations of up to three components taken along this profile :
CLEAR DATA
erases all field observations,
EXAMPLE OBS
loads example data ( coded in program ),
INP APPL PARAM ( Applet only )
loads field observations from calling HTML-file,
INP FROM FILE ( FULL version only )
loads field obsevation from a local ASCII-file,
OUTPUT OBS / REG / O - R
dumps observations, a calculated regional field and the difference OBS - REG.

! If write-accsess to the local disc is denied, OUTPUT ... lists the respective data to the screen / the JAVA console !

and
to list characteristic parameters of the profile set and the observations in a text area
( CLR TEXT / STATUS / SURVEY / HEADER / DATA, if the number of observations exceeds 500, DSP NXT / SKP FWD / SKP BWD is enabled ).

COMP 1 / 2 / 3
selects the components for OUTPUT ... ,

TRC I/O
allows to "trace" all I/O actions in the text area,

and

HIDE
closes the window DATA I/O.

To Table of Content

Window MODEL I/O :

to load and dump model parameters
( coordinates and orientation of model, number, type, shape and mag. properties of structures ) :
DEFAULT MODEL
EXAMPLE MOD
loads example models ( coded in program ),
INP APPL PARAM ( Applet only )
loads model data from calling HTML-file,
INP FROM FILE ( FULL version only )
loads model data from a local ASCII-file,
OUT ACT / SVD MODEL
dumps the actual / previously saved model parameters.

! If write-accsess to the local disc is denied, OUT ... lists the respective data to the screen / the JAVA console !

and
to list characteristic parameters of the actual or a previously saved model in a text area ( CLR TEXT / ACT MOD / SVD MOD ).

TRC I/O
allows to "trace" all I/O actions in the text area,

and

HIDE
closes the window DATA I/O.

To Table of Content

Window SRV :

The local magnetic field, based on the geographic coordinates of the survey area and on the date of the field observations, is approximated by a series of spherical harmonics, trucated at 10th degree / order.
The coefficients used here are distributed by the National Geophysical Data Center, USA, for the International Geomagnetic Reference Field ( IGRF ) model of the International Association of Geomagnetism and Aeronomy ( IAGA ).

The postion of the survey area is adjusted with the mouse by dragging the end point of the corresponding radius ( blue ) :
the geogr. longitude on an equatorial section of the globe ( upper left ),
the geogr. latitude on a section containing the local geographic meridian ( upper right ).

A menue panel below the graphic display allows to select
the date of the field survey ( 01-jan-1900 ... 31-dec-2009 )
and
the altitude of the survey area ( 200 [m] below NN ... 2000 [m] above NN ),

and below MODE :

DIA allows to adjust all parameters ( i.e. coordinates, date and altitude ) needed to calculate the local field,

CLZ selects coordinates and altitude of Clausthal and observed local field values coded in the programm ( idependent of date ),

OBS applies parameter and / or local field values suplied by the field data input and disables their modification / calculation.

In the lower part of the window magnitude and direction of the local field are listed and displayed in a local horizontal plane ( lower left ) and a local vertical plane ( lower right ).

To Table of Content

Window MAP :

A map of the survey area in cartesian coordinatesgeogr. E [m], geogr. N [m] ) shows the projection to a local horizontal plane
of the profile ( blue )
and
of the subsurface model2-d bodies = yellow areas + green reference lines, dipole lines = green lines only ),
structures not included in field calculations are represented by gray areas / short reference lines.
( reference line = projection of the "center of gravity" in a vertical cross section of a structure, see Window MOD. )

! The map is intended to show the horizontal extend and orientation of the profile and the subsuface model, i.e. the depth of structures overlapping in the projection is not taken into account, and the fact of hiding one structure by another only reflects their position in a structure table processed in ascendig order !

The reference line of the model ( magenta ) represents the projection of the "center of gravity" of the model and an arrow ( +X, magenta ) points to the pos. x-direction of the model cross section ( Window MOD ).

The origin of the coordinate system ( = 0 [m] E, 0 [m] N ) corresponds to geographic longitude ( LON ) and latitude ( LAT ) of the survey, that are listed in a headline, together with the declination angle of the local field.

Positioning the pointer over the letters DEC, the left button shows and the right button hides a red arrow pointing to magnetic north ( mag. N = direction of the horizontal component of the "undisturbed" local field ).

Positioning the mouse pointer over the intersction point of profile and model reference line, displays the coordinates of the intersection point ( E / N_REF [m] and S_MOD [m] ).

Menue field ( above the graphic display ) :

MODEL, PROFILE and ROT BOTH
enable various mouse interactions :

model and/or profile can be rotated,
the length of the profile can be adjusted at both end points ( left mouse button ),
and
the profile can be moved as a whole ( right button ).

UNDO
toggle switch to undo / reapply last change.

! If observed data are loaded and one of the observed components is selected ( see DATA I/O and CMP 1 ... ), the profile geomerty is taken from the dataset loaded, and modification of the profile is disabled !

Zoom is enabled permanently :
the left mouse button sets a display window ( rectangle, gray ), and
the right button resets to autom. scaling.

To Table of Content

Window MOD :

A vertical cross section along the x-axis of the model ( magenta, +X, in MAP window ) shows
the subsurface structures ( in general z >= 0 [m] ),
the earth's surface ( in general z = 0 [m] or relief ),
the vertical sensor positions ( blue lines, in general z <= 0 [m] or above relief ),
and
a projection of the profile into the cross section plane.

The shape of a 2-d body is displayed as a filled polygon :
yellow with magenta border lines if included in field calculation,
gray and black if excluded,
( see below : INCLUDE / EXCLUDE STRUCT ).

The "center of gravity" of a 2-d body / the postion of a dipole line is marked green
by a crosshair if "selected" and included in calculations,
and
by a small cross if not ( see below : STRUCT 1 ... and INCLUDE / EXCLUDE STRUCT ).

The "center of gravity" of the model is marked magenta
by a crosshair if none of the 2-d bodies / dipole lines is "selected",
and
by a small cross if one of the 2-d bodies / dipole lines is "selected" ( see below : STRUCT 1 ... ).

The horizontal positions of "centers of gravity" are marked on the x-axis of the cross section and on the projection of the profile, they correspond to the reference lines in the MAP window.

Induced magnetization and "induced" dipole moment
are marked green ( origin = "center of gravity" ),
remanent magnetization and "remanent" dipole moment
are marked red ( origin = end point of induced mag. / mom. ).
! Scaling of magnetization / dipole moment is arbitrary but uniform for all magnetizations / moments !

A dialog panel ( yellow ) above the graphic display enables various mouse interactions, applied to the model as a whole ( M O D E L ) or to selected 2-d bodies / dipole lines ( STRUCT 1 ... ) :

ALLOW ZOOM ONLY
any changes of model parameters are inhibited.
MOVE MOD / STR
move the model or a selected structure by dragging the respective crosshair line.
ROTATE / SHEAR
rotateleft mouse button ),
shear horizontally or vertically ( right button )
the model or a selected 2-d body by dragging the appropriate crosshair line
( Fixpoint = resp. "center of gravity" ).
Horizontal and vertical shear is included to meet geological problems like dipping structures with a relatively well known horizontal top border or a vertical contact to some neighbouring sructure but unknown dip angle, that may be varied preserving the volume of a body ( here : area of a shape ) and its horizontal / vertical boundaries.
MODIFY POINT
move a vertice of a selected polygon ( 2-d body ),
delete it by positioning it over one of its neighbours ( min = 3 vertices/shape ),
and
insert a new vertice on a border line of the polygon ( max = 32 vertices/shape ).
MAGNETIZATION
adjust the magnitude of induced or remanent magnetization / moment
or
the inclination of remanent magnetization / moment.
The result is applied to the structure selected or to all structures of identical type ( 2-d body / dipol line ).
EXCLUDE / INCLUDE STRUCT
all structures or a selected 2-d body / dipole line are excluded from / included in field calculation,
to facilitate the understanding of the contribution of individual bodies / dipole lines to the resulting superposition of the fields of all structures.
( A structure selected via STRUCTURE 1 ... is automatically included in the field calculation, SURFACE has to be included explictly by INCLUDE. )
COPY STRUCT
copies a selected 2-d body / dipole line right to the existing model.
adds a 2-d body / dipole line with default parameter values right to the existing model.
REMOVE STRUCTURE
removes a selected 2-d body / dipole line from the model's structure table.

UNDO
toggle switch to undo / reapply last change.

FIELD
shows the surrounding field ( light gray lines ) and the effective magnetic moment ( magenta ) of a selected dipole line.

Topography ( selected by SURFACE ) :

MOVE MOD / STR ( horizontally only ) and MODIFY POINT allow to model the 2-d topography of the earth's surface, along which ( z = z(surf) - sensor height ) the model calculation can be performed ( PRO=SRF ).

MAGNETIZATION and INCLUDE / EXCLUDE STRUCT allow to include in / exclude from the calculation the contribution of the topography.
( The in general very weak magnetization of the halfspace is scaled independently for graphic display. )

In addition to all modes of opreation of the mouse pointer listed above, the ZOOM mode is enabled :

a zooming rectangle, preserving the aspect of the graphic display area, may be dragged ( left mouse button ),
and
the scaling may be reset to a value derived from the maximum extend of the model and the sensor height ( right button ).

Positioning the mouse pointer over the "center of gravity" of the model or of one of the structures, displays coordinates ( Z / X_REF in cross section plane, S_PRO = intersection of reference line and profile ) and magnetic properties ( MAG / MOM and INC = magnitude and inclination of the projection onto the crossection plane of total magnetization / magnetic moment ).

To Table of Content

Window PRO :

The window displays observed data and / or data calculated for the actual survey, profile and model parameters.

If no observed component is selected for display, ( see DATA I/O ), up to three field components selected from the SENSOR / GRADIENT COMPONENT menue are displayed.

Menue panel above graphic display :

enables the adjustment of the model position with respect to the profile,
and
enables the menue items AUTO and APPLY, if only one structure contributes to the field calculation ( see STRUCT 1 ... / SURFACE and EXCLUDE / INCLUDE STRUCT in the MOD window ) :

AUTO fits the calculated component to the field obsevations of the profile range displayed ( see Zoom ) by adjusting the magnitude of magnetization / moment,
APPLY applies the result to the structure
( scaling ind. and rem. magnetization / moment ).

UNDO
toggle switch to undo / reapply last change.

If observed data are loaded and one of an observed components is selected, a second menue panel is shown below the graphic display :

Selectable by OBS / CALC DATA / OBS + CALC
blue squares represent the field observations,
and
a red curve shows the corresponding calculated component.

REG FIELD POLYNOME NONE / DEG 0 ...
selects the dgree ( max 6 ) of a polynome approximating a regional field resulting from deeper structures and superimposing the observed data.

Observations estimated to represent the background field are selected as input to a least square fit by dragging a rectangle using the left mouse button, and marked by filled squares,
observations to be excluded from calculations are selected using the right button.

SHOW FCT / SHOW REG / FCT - REG selects the display mode for the background polynome :
SHOW FCT = hide polynome, data unchanged as read in / calculated,
SHOW REG = show polynome, used as bias for the calculated data,
and
FCT - REG = subtract polynome from observed data.

Zoom ( allways enabled ) :
The left mouse button selects a profile range ( below the display area ), an amplitude range ( left to the dispay area ), the right button resets to auto scaling.

To Table of Content

Class files are available

for the DEMO version ( Applet )
as
zip file and as .tar.gz file,

and

for the FULL version ( Applet + Application )
incl. example files for model parameters and field observations
as zip file and as .tar.gz file.

The Java Runtime Environment ( JRE ) needed to run the Application is available for several platforms at Sun Microsystems, Inc..

Both versions include a HTML-file containing

<PARAM name="MES" value="...">
and
<PARAM name="MOD" value="...">

to demonstrate the input of field observations and model parameters via INP APPL PARAM.
The FULL version includes two example files for input of model parameters and field observations via INP FROM FILE.

To Table of Content

Input formats for field obsevations and model parameters
ASCII-files and Applet Parameter

Originally the formats were developed for disc files. They are organized in lines of header and description data,
and for field observations a record of coordinates and observed mag. components
1 line/station, max. 9 columns/line,
where max. 3 values can be input as mag. components.

Reading data via Applet Parameter, two conventions concerning the display of HTML texts have to be taken into account :

strings of type "< ... >" are interpreted to format the HTML text, therefore the "tokens" "<XXX>", used in the data files, have to be replaced by "#XXX>".
( In data files both forms are allowed&nbp;)

The characters "CR" and "LF" ( carridge return, line feed ) are ignored :
in data lines containing field observations the constant number of columns/line has to be coded in a header line
( see below CLP=, allowed but ignored in data files ),
and
for all other lines ( typically of variing length ) beginning with "#XXX>" or "!" the end of line has to be marked by "|" ( pipe symbol, vertival bar ).

Example : Profile survey as Applet Parameter

<PARAM name="MES" value="
#TIT> Gel. Prakt 03 FLUXGATE Z + H  10 [m] E|
#DAT> 27/JUN/2003|
#QPR> 10.0 [m] E|
#FMT>   S_PRO  DEL_Z  DEL_H|
#HMS>          +1.20  +1.20|
#SOD> S=1=0. C=2-3=DEL_Z=1.2=DEL_H=1.2 L=10.3=51.8=600. CPL=3|
38   -167    249
36   -550     42
34   -500      3
33   -459    -15
*      *      *
*      *      *
10    -50     -8
8    -45     16
4     -7    -18
0      5      2
#EOD>|">

Most of the Header lines starting with "#XXX> contain informations for the user and are ignored by the program,

except :
The "#DAT>" line contains the date of observation, tranferred to SRV for local field calculation,

and
the "#SOD>" line describes the format of the field observations starting immediatly below :

"S=1=0."
=> coordinate S_Pro ( along the profile ) in column 1, azimuth A_S2n = 0 [deg] ( S pos. -> N ),
"C=2-3=DEL_Z=1.20=DEL_H=1.20"
=> observed components in columns 2 to 3,
=> col. 2 = Delta_Z, sensor height H_Mes = 1.20 [m],
=> col. 3 = Delta_H direction mag. N, sensor height H_Mes = 1.20 [m],
( alternative "C=2,3" ... , and for Delta_H :
"DEL_H" = "DEL_M" => magn. N,
"DEL_G" => geogr. N,
"DEL_Q" => across the profile ),
"L=10.3=51.8=600."
=> coordinates of the survey area ( longitude, latitude and altitude above NN, here Clausthal, see window SRV ),
( alternative :
"L=CLZ" => coordinates and lokal field for Clausthal,
or
"N=48700.=0.=67.0" => total intensity, declination and inclination in the survey area, here Clausthal ),
"CPL=3"
=> 3 culumns/line in data record.
A "#SHF>" line within the data record
marks the start of a profile section overlapping the previous section by at least 3 stations and containing observations offset by a constant amount to avoid a range overflow.
The offset value is calculated from a least square fit of a polynomial in the region of overlap and is applied to the observations of all subsequent profile sections.

"FLD_T", "FLD_Z" and "FLD_H" ( incl. _M, _G, _S u. _Q )
=> total, vertical and horizontal intensity,
"DT/DZ=sensor_height=sensor_spacing
=> vertical gradient of total intensity, approximated by 2 observations in different heights,
"DEL_T"
=> Delta_T ( changes in total intensity rel. to base station ),
+ evtl. "O=off1=off2..."
=> reference values for all components listed in "C=..." ( evtl. 0. )

"Q=n=ddd.d"
=> coordinate Q_Pro in col. n, azimuth A_Q2n = ddd.d
( A_Q2n = A_S2n +/- 90 [deg],
in general + 90 => S, Q, Z pos. downward = "right hand system" ),
"T=n=DDD"
=> time of observation T_Pro in col. n, dimension DDD :
HOR or none => hours decimal,
MIN => minutes,
SEC => seconds,
HMS => "hhmmss"

Example : Model as Applet Parameter

<PARAM name="MOD" value="
!     Title / Name|
#TIT> START MODEL|
!|
!          N_Ref   E_Ref   Z_Ref   A_X2n   F_Scl   N_B/D|
#MOD>      +0.00    0.00   +0.00   +0.00    1.00       1|
!|
!B/D-Num   X_Off   Z_Off    Kapa   X_Rem   Z_Rem   N_Pnt|
#SRF>       3.50   +0.00   2.e-4      0.      0.       2|
!|
!PNT_Num   X_Pnt   Z_Pnt|
#PNT>  1   +0.80    0.20|
#PNT>  2   -0.80    0.00|
!|
!B/D_Num   X_Off   Z_Off    Kapa   X_Rem   Z_Rem   N_Pnt|
#BDY>  1    5.00   +1.50      0.  -4500.  +7200.       4|
!|
!PNT_Num   X_Pnt   Z_Pnt|
#PNT>  1   +0.22   -0.96|
#PNT>  2   -0.27   -0.47|
#PNT>  3   -0.14   -0.34|
#PNT>  4   +0.35   -0.83|">

Only lines starting with "#XXX>" are interpreted,
the character "!" marks comment lines, containing user informations to facilitate generating / editing of a model.

"#TIT>"
=> title / name of a model ( arbitrary text for output / listing ),
"#MOD>"
=> parameters concerning the entire model,
N / E / Z_Ref [m] = origin, relative to geogr. coordinates and local earthsurface of survey,
A_X2n [deg] = azimuth of model x-direction ( prpendicular to strike direction ),
F_Scl = scaling factor ( see SCALING, Applet dialogue ),
N_B/D = number of structures ( max. 6 2D-bodies / dipole lines, except max. 1 suface relief SRF ),
"#SRF>", "#BDY>" and "#DIP>"
=> surface relief ( optional, no number ), 2D-body and dipole line ( + order number ),
X / Z_Off [m] = origin, relative to N/E/Z_Ref ( MOD ),
Kapa = susceptibility,
X / Z_Rem = X- / Z-component of the remanent magnetization,
N_Pnt = number of vertices ( max. 32, min. 3 / min. 2 for SRF ),
"#PNT>"
=> vertice, ( SRF and BDY only )
PNT_Num = order number of vertice
X / Z_Pnt = coordinates, relative to X/Z_Off ( SRF / BDY )
! Vertices are counted ccw., for SRV : right to left !

Model calculation :
D_X = X(N_Ref,E_Ref,A_X2n) + X_Off(SRF/BDY) + X_Pnt - X(S_Pro,Q_Pro,A_S2n)
D_Z = Z_Ref + Z_Off(SRF/BDY) + Z_Pnt + H_Mes