Gravimetry ========== -------------- Theory ------ Gravity surveys aim to derive a sub-surface model of density differences, based on the gravitational attraction of the underlying rock masses. From the inferred density differences, an interpretation in terms of lithology can be made. Gravity measures the gravitational acceleration :math:`\vec{g}` [m/s\ :math:`^2`] of the Earth, in most cases the vertical component of the gravity vector, :math:`g_z`, with the help of a gravimeter. Gravimetry is a potential field method. For further details, see script of lecture *Angewandte Geophysik II* from Georg Kaufmann. **Material properties** - density :math:`\rho` [kg/m\ :math:`^3`], defined as the ratio of mass :math:`m` [kg] and volume :math:`V` [m\ :math:`^3`]: :math:`\rho = \frac{\displaystyle m}{\displaystyle V}` Density as a macroscopic property depends on the density of the rock type, :math:`\rho_{rock}` [kg/m\ :math:`^3`], the density of the air-filled pore space, :math:`\rho_{air}` [kg/m\ :math:`^3`], and the density of the water-filled pore space, :math:`\rho_{infill}` [kg/m\ :math:`^3`]. Defining porosity :math:`\Phi` [-] (0-no pores, 1-only pores) and water content :math:`\Theta` [-] (0-no water, 1- only water), we arrive at: :math:`\rho = \left[ \rho_{rock} (1-\Phi) + \rho_{air} \Phi (1-\Theta) + \rho_{infill} \Phi \Theta \right] \left[ 1 - \alpha T + \frac{\displaystyle P}{\displaystyle K_T} \right]` with :math:`T` [C] temperature, :math:`P` [Pa] pressure, :math:`\alpha` [1/K] thermal expansion, and :math:`K_T` [1/Pa] isothermal compression. Instrument(s) ------------- 1. **Scintrex CG6:** The Scintrex CG-6 relative gravimeter uses a quartz sensor to measure the vertical component of the gravity field. .. figure:: images/Scintrex_CG6.jpg :alt: *CG6*, photo Scintrex *CG6*, photo Scintrex `homepage of manufacturer `__ Assembly -------- **Important:** Handle with extreme care! - The CG-6 is ready to use. Note that the gravimeter is always on! While storage (in a save space), the gravimeter needs to be connected to the power supply (15V DC). This charges the two Lithium batteries, which are located in the two front slots of the unit. - The gravimeter should be stored on the tripod, and leveled to keep the sensor in a horizontal position. - For transport, use the soft case and transport the gravimeter with great care. Configuration ------------- - **Switching on:** To check the status of the gravimeter, simply press the *central* key on the key pad, the screen should turn on. The status of the two lithium batteries can be seen on the top (left and right corners). - On the top line, **date** and **time** is also shown. - The second line shows a **timer**, the **status** of the gravimeter (should be *idle*), and the **number of cycles** for measuring. - The second block displays saved reading and below the horizontal line the *current reading* in mGal, togehter with standard error and standard deviation. - In the third block, the **orientation** of the gravimeter is indicated, both *numerically* (in arcsec), and *graphically*. The arrows indiate, in which direction the tripod screws need to be turned. The arrows become green, when the tilt threshold (currently set to 20 arcset) is reached. - The bottom block shows the **menu**. - **LIST:** Contains a pre-defined set of station names, which are stored in the file ``stations.txt`` in the root-directory of the CG-6. - **RECORD:** Starts the recording sequence for a station. Text changes from **idle** to **recording**. Do not touch instrument any more and wait for completion, shown again as **idle**. - **RECALL:** Recall the recorded stations. - **USB:** If CG-6 is connected to computer, activate to list the gravimeter hard disc as storage device on computer. - **SETTINGS:** - **SYSTEM:** System settings, leave untouched. - **SURVEY:** Enter survey name and operators before starting a new project. Choose *station cycle* as numeric and *increment* as 1. The points *Cycles*, *Measure length*, *Record delay* define, how often a measurement is repeated, how long a single measurement takes (in s), and after which time the measurement starts (in s). We usually choose **3** *Cycles*, a *Measure length*\ of **30s**, and a *Record delay* of **5s**. - **CORRECT:** Defines which corrections will be applied. The choice is *Temperature*, *Drift*, *Tide*, *Level*. We use *Temperature* and *Level* **ON** and *Drift* and *Tide* **Off**. - **STATION:** Manually define a station name. - **GPS:** Check status of GPS unit (*searching*, *locked*). Once the satellite lock is established, press **synctime**, then the correct date and time will be used from the GPS signal. - **SLEEP:** sleep mode, turn screen off. -------------- Field measurement ----------------- During a gravity survey, survey points are measured either along pre-defined profiles or as irregular points in the field. At each point :math:`i`, several measurements need to be taken: 1. Geographical coordinates :math:`(\phi,\theta)` (as GPS point, e.g. with a hand-held GPS unit) as longitude :math:`\phi` and latitude :math:`\theta`, 2. Elevation :math:`h_i` [m] (from leveling) as height above datum, 3. Date and time :math:`t_i` [yyyy.mm.dd/hh:mm:ss] (clock), 4. Gravity reading :math:`g_{obs}` [mGal] (e.g. scale units in case of the Lacoste-Romberg gravimeter). Note that the CG-6 records all these data by itself (coordinates, elevation and time from the GPS signal), but often both coordinates and especially elevation are not accurate enough for gravity processing. Thus, these values need to be obtained from leveling. **Important:** The initial setting has been done as described above, especially the date and time taken from a valid GPS signal, and setting the *station cycle* to numeric. - First check, if GPS is running (menu **GPS**). If here is an error, reboot the CG-6 by pulling both batteries out. - Next, define a *new* survey (menu **SETTING->SURVEY**). Otherwise, the last used survey is used (unfortunately, the survey name is not indicated on the screen). - When we start mapping, we define our **base station**. We need to memorise, which station number we use here. Go to the **GPS** menu, and use **GET_POS** to assign the current position to the station. Then start the measurement by pressing **RECORD**. The gravimeter starts a recording sequence based on the pre-defined parameter settings. - Move to the next station. Always assign the new GPS coordinates first by going to the **GPS** menu and pressing **GET_POS**. Data download ------------- Data can be downloaded from the CG-6 directly by connecting it with an USB cable to a computer. The internal disk should be visible, once you choose the **USB** menu. **Important!** Always unmount the external device on your computer, before you pull out the USB cable. Post processing --------------- **Important:** Use ``GRAViMAG`` web interface for processing data! - The project file downloaded from the CG-6 unit can be directly imported in ``GRAViMAG``. See **GRAViMAG**\ :math:`^2` section… - You need to replace coordinates and elevation with your leveling data! Georeferencing -------------- The CG-6 gravimeter automatically records GPS positions with its in-build GPS. However, the **elevation** from the GPS unit is rather poorly determined. We therefore need a **properly levelled** station position list. We record coordinates (e.g. hand-held GPS) and level the elevation differences between stations. This information is then stored and pre-processed. Create a file with your GPS coordinates and the corresponding backward and forward views from leveling. The file should be in this format: .. code:: quote INFO from dummy lat lon elevation BASE 421 none 51.638526 10.406761 383.66 INFO from to lat lon backward forward DATA 421 422 51.638186266645789 10.406670151278377 7.32 13.28 DATA 421 423 51.638171933591366 10.406761597841978 7.32 23.28 DATA 421 424 51.638181824237108 10.406874166801572 7.32 34.79 - **INFO** lines: Not needed, ignored while reading, but useful for information - **BASE** line: Base point, with name, and **known** coordinates and elevation - **DATA** lines: Points levelled, **from** and **to** mark the two adjacent points, **lat** and **lon** the coordinates of the **to** point, and **backward** and **forward** the leveling readings (in dm!). This file can be processed with the ``python`` function ``createGravityCoordElevation.py``.