GATE-2018 (24) :Wyllie – Time average equation

92. The positions of the maximum anomaly and the minimum anomaly of 183 gamma and -13 gamma respectively are located at the distance coordinates 673m and 1073m respectively along a magnetic profile across a two- dimensional sheet-like body. The anomaly of value 170gamma occurs at the distance coordinate 700m. The depth of the sheet is

1. 400 m

2. 283 m 

3. 141 m 

4. 100 m

Solution:

For a Semi-infinite horizontal thin sheet or fault magnetic anomaly, the half-width is half the distance between the maximum and minimum values of the anomaly.  

Maximum anomaly = 183 gamma at coordinate 673m 

Minimum anomaly = -13 gamma at coordinate 1073 m

Form the above point, the distance between maximum and minimum is =400m

Half width (X_1/2) = the distance between maximum and minimum / 2 

Half width (X_1/2) = 400 /2 
                            = 200 m

The relation between half-width (X_1/2) and depth (Z) is thin sheet

Z = 1.0 X_1/2

Z= 1.0 * 200

Z= 200 m

(Note : Answer is given by 100m )

(Note: I don't know the exact relationship between 2D sheet half width and depth, if any one knows please tell me. That's way i didn't mentioned 2D sheet) 


Reference : Gravity and Magnetic Exploration by William J.Hinze, Ralph R.B.Von Frese  and Afif H.Saad