Geophysical Department TU Clausthal

Students exercise at Geophysics Department TU Clausthal

Applet   ( in separate Window )

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 ).
see Download

The Applet simulates a magnetic survey along an arbitrary orientated profile and in a horizontal plane ( earth's surface ) above a dipole of arbitrary magnetic moment and position.

It demonstrates the anomalies to be observed for different components of the magnetic field and for various configrations and magnetic properties of a dipole,
and allows to adjust the parameters of a dipole to fit a calculated anomaly to field obsevations along a profile.

( Students exercise : A mag. Dipole is moved along a straight line 0 ... 7 [m], obsevation of vertical and horizontal component using a fluxgate-magnetometer in a fixed position at about 3.5 [m], sensor about 1.3 [m] above profile ).

Comments :

Coordinates
Components

HOWTO :

Applet / Dialogue Window
Sensorheight ...
Dialogue DATA I/O
Dialogue MODEL I/O

Graphic display:

Window SRV
Window MAP
Window MOD
Window PRO

Download

Data Format

Coordinates :

The location of the survey area on the globe is specified in geographic coordinates

longitude ( LON [deg], reference meridian = Greenwich ) and

latitude ( LAT [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 containing the dipole :

X = horizontal and

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 ).

Components of the magnetic field :

The magnetic field is described by its

magnitude ( T, TOT, units : nano Tesla, [nT] ),

inclination angle ( INC ) from the local horizontal plane, and

declination angle ( DEC ) from geographic north.

This corresponds to a vertical component

and a horizontal component

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

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

( vertical gradient, DT/DZ )

or

at two points with a fixed spacing in profile direction

(  horizontal gradient, DT/DS ).

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.

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. ).

IND / REM DIPOLE selects a dipole parallel to the local magnetic field or of arbitrary orientation,

The buttons SAVE ACT / REST SVD MOD allow to save the actual parameters of a dipole ( position, magnitude and direction of the magn. moment ) 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, for the depth of the dipolel 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 dipole parameters :

SRV = geographic coordinates of the survey area and resulting local magnetic field of the earth,

MAP = orientation and position of the dipole and of the profile in a horizontal plane, and
calculated anomaly for one component ( marked red in SENSOR / GRADIENT COMPONENT ),

MOD = depth and magnetic properties of the dipole in a vertical 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 dipole 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.

Panel H_SENSOR / Menue SENSOR/GRADIENT COMPONENT :

Model calculations are performed for 5 different sensor positions :

one central sensor ( MIDDLE ),

2 sensors equally spaced above ( UPPER ) and below ( LOWER ),

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 gradient ( D/DZ ) is approximated

by the difference LOWER - UPPER ( ! z = pos. downward ! ),

the horizontal gradient ( D/DS )

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 !

Window DATA I/O :

A menue panel allows

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.

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 ).

Window MAP :

A map of the survey area in cartesian coordinates ( geogr. E [m], geogr. N [m] ) shows the projection to a local horizontal plane

of the profile ( blue )

and

of dipole ( moment = magenta, positon = crosshair, cyan ).

The origin of the coordinate system ( = 0 [m] E, 0 [m] N ) corresponds to geographic longitude ( LON ) and latitude ( LAT ) of the survey, listed in a headline, together with the declination angle ( DEC ) 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 pointer over the reference point of the dipole, its actual parameters ( coordinates and mag. moment ) are listed.

Menue field ( above the graphic display ) :

MAP ...

displays the calculated anomaly for the component marked red in SENSOR / GRADIENT COMPONENT.

the left mouse button selects an amplitude window in the color scale ( right to the map ),
the right button resets to automatic scaling.

DIPOLE and PROFILE

enable various mouse interactions :

the dipole can be moved parallel and / or perpendicular to the profile,

the horizontal projection of the moment can be rotated ( REM DIPOLE ),

and

the length of the profile can be adjusted at both end points.

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.

Window MOD :

A vertical cross section containing the dipole ( +X = DEC DIPOLE, magenta in MAP window ) shows

the depth and inclination of the dipole ( X = 0 [m] = const )

the vertical sensor positions ( blue lines, z < 0 [m] )

and

a projection of the profile into the cross section plane.

Menue panel ( above the graphic display ) :

SHOW FIELD

shows the field ( light gray lines ) surrounding the dipole.

ADJ DIPOLE

enables the mouse activities :

the dipole can be moved vertically,

the magnitude of the moment and

the inclination ( REM DIPOLE ) can be adjusted.

UNDO

toggle switch to undo / reapply last change.

Zoom is enabled permanently :

the left mouse button sets a display window ( rectangle, gray ), and

the right button resets to autom. scaling.

Window PRO :

The window displays observed data and / or data calculated for the actual survey, profile and dipole 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 display :

ADJ MOD

enables the adjustment of the dipole position parallel to the profile ( Q_DIP = const ),

and

enables the menue items AUTO and APPLY, if both calculated and observed values are displayed ( see below OBS + CALC ).

AUTO fits the calculated component to the field obsevations of the profile range displayed ( see Zoom ) by adjusting the magnitude of magnetic moment,

APPLY applies the result to the dipole.

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 / or

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 is enabled permanently :

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

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

To Table of Content

To Table of Content

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

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=15."
=> coordinate S_Pro ( along the profile ) in column 1, azimuth A_S2n = N 15 [deg] E,

"C=2-3=DEL_Z=1.38=DEL_S=1.38"
=> observed components in columns 2 to 3,
=> col. 2 = Delta_Z, sensor height H_Mes = 1.38 [m],
=> col. 3 = Delta_H in profile direction, sensor height H_Mes = 1.38 [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=CLZ"
=> coordinates and local field for Clausthal, see window SRV,

( alternative :
"L=10.3=51.8=600." => coordinates of the survey area, here Clausthal,
and / 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 recoerd

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.

Additional components implemented :

"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. )

Additional coordinates implemented :

"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"

To Table of Content

Additional Applets : Keller-clz.de