Magnetics ========= -------------- Theory ------ Magnetic surveys are used to identify sub-surface structures, which have considerable magnetization (induced or, more importantly, remanent magnetisation). Examples of such structures are magmatic or metamorphic rocks containing magnetite (e.g. basalt, granite, gabbro, …) or man-made objects as metal artefacts (farming, ammunition) or burned loam (archaeology). Magnetic surveys measure the strength and the direction of the Earth’s magnetic induction :math:`\vec{B}` [nT], either as magnitude :math:`T` [nT] and inclination :math:`I` [:math:`^{\circ}`] and declination :math:`D` [:math:`^{\circ}`], or as the three components :math:`B_x`, :math:`B_y`, and :math:`B_z` [nT]. There are several types of magnetometers: Fluxgate magnetometers measure one of the components of the magnetic induction, mostly the vertical component Bz. Proton precession magnetometers measure the magnitude of the total magnetic field, :math:`T`. For further details, see script of lecture *Angewandte Geophysik II* from Georg Kaufmann. Material properties ------------------- - Magnetisation :math:`\vec{M}` [A/m], defined as the ratio of magnetic dipole moment :math:`\vec{m}` [Am\ :math:`^2`] and volume :math:`V` [m\ :math:`^3`]: :math:`\vec{M} = \frac{\displaystyle m}{\displaystyle V}`. While the definition looks similar to our definition of density, we note that in the case of magnetization we already use the concept of a model, the dipole moment, representing the sub-surface. The magnetization as material properties can have two causes: (i) an external magnetic field of strength :math:`H` [A/m] induces electric currents in the material, which in turn create a magnetic field. Here, :math:`H` can be regared as the Earth’s magnetic field. This sort of magnetization is only present, when the external field is present, and thus is called induced magnetization :math:`\vec{M}i`. (ii) The second cause is the intrinsic property of materials, with the electron spin creating a magnetic field. This magnetization will also remain without external field, and thus is called remanent magnetization :math:`\vec{M}r`. Both induced and remanent magnetization add up to the total magnetization: :math:`\vec{M} = \vec{M}i + \vec{M}r` -------------- Instrument(s) ------------- 1. **GEM-Systems GSM-19:** The GSM-19TW from GEM Systems is an Overhauser proton precession magnetometer. It is shipped with a console, two sensors, a GPS antenna, several rods for assembly either as measuring pole or on a backpack frame, and a set of connecting cables. .. figure:: images/GEMSystems_GSM19.jpg :alt: *GSM-19*, photo GEM-Systems *GSM-19*, photo GEM-Systems `homepage of manufacturer `__ Assembly -------- **Important:** Handle with extreme care! - The GSM-19 is equipped with two sensors and a GPS antenna. The GPs antenna cable has a small yellow ring. The sensors cables are twin cables each, with a black and a gray cable. - The poles can be used as hand-held pole or mounted to the backpack. - For hand-held poles, use the base pole (rubber end) as start, then attach other poles, together with sensors in desired heights. - For backpack mounting, fix a pole on the rack, then a sensor. - as final assemblage, fix the GPS antenna to the top of the pole construction. - connect the cables to the sensors, to the GPS antenna, and the other side to the GSM-19 logger (plugs are labelled). **Note:** The GSM-19 can be used with *single* sensor or with *two* sensors. If the two-sensor setup is chosen, be careful to connect the lower sensor to the correct plug on the GSM-19 device (lower). Configuration ------------- **Important:** We use the GSM-19 to walk along pathways or profiles. This usage is described here, for other (re.g. raster) surveys consult the user manual. - Swith the unit on by pressing **B**. The main screen is displayed. .. figure:: images/GSM19_screen01.png :alt: Main screen Main screen - Erase all data from the previous survey by pressing **4** and **5** simultaneously. The unit asks for confimation, press **e** and **r** simultaneously. - By pressing **A**, the survey mode is activated. If this option is chosen right after booting the unit, the GPS signal is searched for: .. figure:: images/GSM19_screen02.png :alt: GPS search GPS search - Once enough GPS satellites are found, the unit is ready for survey and displays the following screen: |GPS search|\ Note that the unit displays **mobile** as survey mode. This option is kept from the previous use. To change the operation mode, press **C-change**, then toogle to the correct mode with the **B** or **F**-buttons, then save with **C**. -------------- Field measurement ----------------- - Start the survey by pressing **A**. The unit now shows UTM coordinates in the lower line, and the number of satellites seen. .. figure:: images/GSM19_screen05.png :alt: GPS search GPS search - We now move from point to point and make a measurement by pressing **any** key. After pressing **any** key, the unit takes makes a measurement cycle. After the successful cycle, the measurment is displayed. In the top line, the *total-field value* (in nT) is displayed, the difference of this value to the previous point (in nT), and a *quality factor*. Here, the first digit from 0-9 shows whether the time necessary for a quality reading has been reached. The higher the better. The second digit ranging form 0-9 provides data for the reliability of the measurement – the higher the better. “99” is a HQ measurement. .. figure:: images/GSM19_screen06.png :alt: GPS search GPS search - Once the profile is mapped out, press **1** and **C** simultaneously to end recording. Data download ------------- Download of the data is done with the GEM-Systems software *GEMLink*. - Attach the GSM-19 to the serial port (via a serial-to-usb converter), start the GEMLink software. The port should be identified automatically. - From the GSM-19 unit, press ``1-send``, then choose ``D`` for default format. The data sould now be transferred to the laptop. - Save the transferred data to a file. Post processing --------------- **Important:** Use ``GRAViMAG`` web interface for processing data! The data exported from the GSM-19 unit with ``GEMLink`` can be directly imported into ``GRAViMAG``. **NOTE:** Sometimes you need to check the exported data file, there should be **no** empty line(s) at the start of the file. .. |GPS search| image:: images/GSM19_screen03.png