Gamma-ray (GR) log values for clean sandstone and shale are 15 API and 115 API, respectively. If GR records 35 API for shaly-sandstone, then the GR index is _____________ (rounded off to one decimal place).
Show Hint
Correct Answer: 0.2
Solution and Explanation
The Gamma Ray Index (also called the shale index, \(I_{GR}\)) is a normalised measure of how 'shaly' a formation reading is between the two log end-members — the clean (shale-free) sand baseline \(GR_{min}\) and the pure shale baseline \(GR_{max}\):
\[ I_{GR}=\dfrac{GR_{log}-GR_{min}}{GR_{max}-GR_{min}} \]
Given: \(GR_{log}=35\) API (the shaly-sandstone reading), \(GR_{min}=15\) API (clean sand), \(GR_{max}=115\) API (shale).
\[ I_{GR}=\dfrac{35-15}{115-15}=\dfrac{20}{100}=0.2 \]
\[ \boxed{I_{GR}=0.2} \]
An index of 0.2 means the interval is only lightly shaly — about 20% of the way from clean sand toward pure shale on the linear GR scale — consistent with the log describing it as 'shaly sandstone' rather than shale.
Top GATE GG Geophysics Questions
In the schematic, line P shows a 1-D geothermal profile. If the heat flow at the base of the mantle increases, which line will reflect the new geothermal profile?
Consider a steady-state heat conduction equation for the Earth’s crust where \( A \) is the heat source and \( k \) is the thermal conductivity. Given the boundary conditions: \( T = 0 \) at the surface and \( Q \) is the heat flux at the surface, Which one of the following options would be the temperature \( T \) at depth \( z \)?
- Assume heat producing elements are uniformly distributed within a 16 km thick layer in the crust in a heat flow province. Given that the surface heat flow and reduced heat flow are 54 mW/m$^2$ and 22 mW/m$^2$, respectively, the radiogenic heat production in the given crustal layer in $\mu$W/m$^3$ is ________ (in integer).
- In a multi-electrode resistivity tomography (ERT) survey, using equally spaced electrodes, which of the given configurations will provide the maximum number of data points?
- In Electromagnetic methods of prospecting, which one of the given options is CORRECT about frequency and type of current source for the Primary field used?
Top GATE GG Geophysical Well Logging Questions
Top GATE GG Questions
Is there any good show __________ television tonight? Select the most appropriate option to complete the above sentence.
As the police officer was found guilty of embezzlement, he was ___________ dismissed from the service in accordance with the Service Rules. Select the most appropriate option to complete the above sentence.
- The sum of the following infinite series is: \[ 1 + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + \frac{1}{5!} + \dots \]
- A thin wire is used to construct all the edges of a cube of 1 m side by bending, cutting, and soldering the wire. If the wire is 12 m long, what is the minimum number of cuts required to construct the wire frame to form the cube?
The figures I, II, and III are parts of a sequence. Which one of the following options comes next in the sequence at IV?
