# IsoBal Tutorial

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AN ISOBAL TUTORIAL This handout, first prepared by Katherine Giles of New Mexico State University, will familiarize you with the program ISOBAL which calculates Airy isostasy for two columns. To run ISOBAL, follow the directions at the top of the first problem set. What you type is shown in bold italic type. Once you are in ISOBAL, you will see this screen: At the top of the screen are four commands: D_epth, L_eft, R_ight, and Q_uit. They can be implemented by typing the first letter of the command. Watch this line – the available commands change depending on where you are in the process. Type: d (not case sensitive) This command allows you to change the compensation depth, in other words, the depth (in km) to which calculations will be made. The default value is 100 km. Type: 100 Enter Keep a compensation depth of 100 km for now. Type: L L_eft refers to the reference column, which will be displayed on the left of the screen. This column defines your concept of “ordinary” lithosphere. The column of interest (the active column which is displayed on the right) will be balanced against this standard column. Isobal Tutorial Geos 440/540 Page 2 of 5 Type: S By choosing S_tandard, you have chosen a standard column with the following structure: 0-10 km rho=2670 kg*m-3 10-20 2870 20-30 2950 3300 On -the screen, z = depth of the layer, t = thickness of the layer and rho = density of the. layer. Notice that depths ...

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AN ISOBAL TUTORIAL This handout, first prepared by Katherine Giles of New Mexico State University, will familiarize you with the program ISOBAL which calculates Airy isostasy for two columns. To run ISOBAL, follow the directions at the top of the first problem set. What you type is shown inbold italictype. Once you are in ISOBAL, you will see this screen:
At the top of the screen are four commands: D_epth, L_eft, R_ight, and Q_uit. They can be implemented by typing the first letter of the command. Watch this line – the available commands change depending on where you are in the process. Type:d(not case sensitive) This command allows you to change the compensation depth, in other words, the depth (in km) to which calculations will be made. The default value is 100 km. Type:100  EnterKeep a compensation depth of 100 km for now. Type:LL_eft refers to the reference column, which will be displayed on theleft of the screen. This column defines your concept of “ordinary” lithosphere. The column of interest (the active column which is displayed on the right) will be balanced against this standard column.
Isobal TutorialGeos 440/540Page 2 of 5 Type:SBy choosing S_tandard, you have chosen a standard column with the following structure: 0-10 kmrho=2670 kg*m-3 10-20 2870 20-30 2950  3300 On -the screen, z = depth of the layer, t = thickness of the layer and rho = density of the. layer. Notice that depths below sea level have positive values. Elevations above sea level have negative values'. For example, a 2 km high mountain range would have a z value of -2.
The total mass (m) of this column is displayed at the bottom of the screen and is equal to 315900 kg*m-3. Type:RUsing the R_ight command allows you to construct a column to be balanced with the standard column. A. Inputting a Column Say you want to see whether the following column for a 2 km high mountain range is in isostatic balance., t = 1 kmrho- 2400kg*m-3  202670  .152950
Isobal TutorialGeos 440/540Page 3 of 5 The first z value is the elevation of the column. Since the mountain range is 2 km high, the z value is -2. Type:-2 EnterThis enters the value and moves you to the rho (density)column. Type:2400 EnterYou now have a choice whether you want to enter values in terms of depth (z) or layer thickness (t). Since the data table gives values in terms of layer thickness,choose t (entering depths is the default). Type:tEnter Notice that the program automatically calculates the depth of the layer. Now type in the rest of the data. Type:2670  Enter  20  Enter  2950  Enter 3300 Enter (The last density you enter should be the mantle density)Now you have entered all the data and need to exit the column. Type:eThe E_nd command allows you to exit from the column. Notice: because the compensation depth has already been specifiedthe programautomatically calculates the mantle thickness. Also notice m= 317850. A note on correcting errors.: The backspace key will erase everything but the first digit or letter of an entry. If the first digit is incorrect, you must reenter the value. In order to do this choose E_nd, then R_ight. If an entry is correct, typeEnter. If an entry is incorrect, type in the correct value. Type:cNow the program calculates delta m (the mass difference between the active column and the reference column), delta g(the Bouguer gravity anomaly of the active column if the reference column is zero), and delta sigma (the isostatic stress of the active column). Since delta m is not equal to 0 in this case, the active column is not in balance.
Isobal TutorialGeos 440/540Page 4 of 5 Note that a reference column (on the left) can be constructed by the same kinds of commands.
B. Creating a Column in Isostatic Balance In many situations, we can assume the active column is in isostatic balance, or at least ask what would it take for this to happen. If the active column isnt already in isostatic balance, you must change one of the data values within it to achieve balance. You can enter ? instead of a number for one of the values, and then the program will calculate the value that will make the column balance. For example, assume the density for the third layer doesnt allow balance. Type:rEnter(enough times to reach rho for the third layer.Notice this time, you are checking depths).Enter?for the density, theneto fill out the column down to compensation depth. Then C_alculate and the program will calculate rho3. Type:cTo balance in this situation, the third layer must have a density of 2820 kg/m3.
Isobal Tutorial
Geos 440/540
Page 5 of 5
The ? can be used for any entry, but can only be used ONCE in the right-hand column. (One equation for isostatic balance, can only solve for one variable.) It wont work in the left hand reference column, which you have to specify completely. Other possibilities include: variable to be calculated:Placement of ?: elevation z thickness of layert density rho density of mantlelast rho To save a screen of results, use the W_rite command (this only can be used after the C_alculate command) . See the problem instructions for advice on just where the file containing your results might go. To quit the program, use the Q_uit command.