a) In order to set up a pre-unconformity geology, select Read History from the File menu, and select the file q2.his (click on Overwrite when the time comes) which creates a default stratigraphy with a default Tilt event . This produces a block diagram which looks something like this:

b) Now we need to import an unconformity, so add an unconformity event to the history window. The unconformity event definition window contains many types of parameters, first of all concentrate on the Surface Shape group box:

•In order to change from a flat unconformity surface to one defined by triangulation, select DXF Surface (instead of Flat Plane) from the Menu list button. Now select the file uncon2.dxf . At this point the actual X, Y & Z dimensions of the bounding box (see below) for the surface will be automatically filled into the X,Y and Z dimensions area.

A view of the topography of the surface projected onto the best fit plane can be obtained by clicking on the View button. High points are bright, and low points are dark. Note that the topography of this surface has been defined to be a minimum around the edge of the defined area, because the triangulated surface has to merge with a flat plane outside the defined area.

•On the assumption that we actually want a generally horizontal unconformity surface, switch to the Orientation group box, and set the Dip to 0° and Dip Direction to 0°.

•The origin of DXF surfaces in Noddy is defined to be the lower North-East corner of the bounding box, so in order for the unconformity to occur within the default block volume, set the X Position parameter in the Position group box to 9000, the Y Position parameter at 9000 and the Z Position to 2500.

•Click on the Stratigraphy Group Box Icon , delete all the layers but one, and edit the Density and Susceptibility of this layer to 2.7 and 1.35e-03 respectively. Finally exit from the Unconformity definition window.

•This block diagram is rotatable by dragging the cursor in the window, and scalable by dragging the cursor with the shift key held down.

•To produce a block diagram that shows the geology immediately below the unconformity surface, click in the diagram with the right mouse button, which brings up a window that allows you to make any of the rock types transparent:

d) You are now ready to calculate the resulting gravity and magnetics images by selecting Calculate Anomalies->Anomalies from the Geophysics menu, this will ask you for a file name for the results of this calculation. Once the calculation is complete both the gravity and magnetics images will be displayed.

e) The outcrop pattern of the unconformity is clear in parts of the magnetics image, but is unclear in other areas, and is very hard to pick at all in the gravity image. If you are NOT running Windows 3.11 (which doesn’t allow enough memory to be allocated), you can enhance the unconformity contact by performing a 1st vertical derivative calculation on the magnetics image. To do this select Vertical Derivatives->First->Magnetics from the Geophysics menu and selecting the file name you have just saved, and then typing in a new file name for the 1VD image.