Though some results can be obtained in searching the crevice water in bedrock by using an ordinary radio-active mine locator to make a survey of natural γ ray

yet the feeble γ abnormality

because of the low sensitivity of a mine locator

is very difficult to be discovered

The present apparatus "DTF-1" has a relatively high accuracy and sensitive reaction. Its principle and characteristics are described briefly and the applied effects in fields are also presented.