Extracting Residual Anomaly Profile from Bouguer Anomaly Map

How to extract Residual Anomaly profile from Bouguer Anomaly Map?

It has been known that Potential fields do not occur in isolation as most of these geophysical dataset are interacted with other sources. Then it is the key task for you as a Geophysicist or interpreter to isolate these datasets so that to make interpretation clear and reasonable. This rule is the same as in gravity geophysics. However it may be difficult or challenging to isolate the unwanted sources so that we can have our target sources. There are many advanced techniques you can apply to do this, however this post is intended to give you the basic insight on how you can produce your residual profile from Bouguer Anomaly Map, just through the Excel spreadsheets to assist the graphical Method.

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The following are the five (5) steps that you can utilize to clear this task.

1. Acquire your Gravity Map

This is the first step, because it defines what kind of map you want to work with. You should make sure that your Bouguer anomaly map is corrected for various gravity Data reduction and plotted on a contour map. For our case we have the figure below, its simple contoured bouguer anomaly map (contour interval, 50 g.u) so that you can grasp the basic idea easily.

2. Identify direction of your Profile

After having your Bouguer anomaly map it is the time now you have to identify which direction in your map your profile will be running. Remember to follow the Golden Rule of defining your profile orientation.

If you have the hard paper map you have to define two points then trace the straight line joining those points such A to A' in this case see figure above. Otherwise you have to scan it, so that you can digitize it either in GIS software ,surfer or any useful software.

3. Create your Bouguer Anomaly Profile.

>> Extract the Bouguer anomaly values, either manually or digital. This depends on your version of your Bouguer anomaly map whether it is hard paper or digitized. Then we will have the x and y values as shown in the table 1 below with Contour interval 50 g.u

Table 1: Bouguer Anomaly values (y) in g.u and contour line positions (x)

>> Plot the profile of the values in the table 1 above by scatter plots, to have the profile as shown in the figure 1 below.

Figure 1: A simple Bouguer Anomaly Profile

4. Extract the Residual Values

We have to extract the residual values, these values are the difference between observed bouguer anomaly and the estimated regional gradient ( redline on the figure below).

>> Perform trend analysis by simply inserting the trendline (redline) such as Linear trending by least square method. This is shown in the figure 2 below.

Figure 2: Linear trend Analyzed Bouguer Anomaly Profile.

>> Obtain the corresponding values of y by inserting the trendline equation in the function bar as shown in the screenshot below.

>> Drag down the column to have all values of y and corresponding x values.

>> Obtain the residual values by set up the formula in function bar as shown in the screenshot below

>> Drag down throughout the column, to have all residual values as shown in table 2 below

Table 2: Extracted Residual values (y) and corresponding contour line positions (x)

5. Plot the Residual Profile.

To finish this task we have to plot residual values against the contour line position as shown in the table above, by scatter plotting and our end resulted profile is shown in the figure 3 below