Resistance measurement

Hello,
Could you help me with the homework, please...

For the conventional resistance measurement used four electrodes: two current (A, B) into the country "deserts" and the current I between two measuring (M, N) is then measured voltage AU, resistivity, which is the goal of our measurement is determined according to Ohm's law; in its formula is needed to add a so-called. arrangements constant k, which depends on the distance of the individual electrodes. The calculated value is then referred to as "apparent resistivity" ρz, because it describes the whole "block" hemisphere, where penetrated current:

ρz = k * AU / I

Depth of this measurement is approximately 1/3 of the distance between the electrodes A and B. How far AB hypothetically you chose if you think that the resistance measurements would allow to determine the presence of a ditch.

Stephen Tashi
The general subject of this post is "electrical ground probing" ( Described in http://www.reeve.com/Documents/Ground Testing R1-1.pdf)
However, the statement of the problem is unclear
Can you show a diagram?
What type of book poses this problem? - a geology book? a general physics book? an archaeology book?

Stephen Tashi
The depth of the ditch given? Is the width of the ditch given?

He's going to archaeological research the castle, which
will be preceded by geophysical measurements. The challenge is also localization
said ditch. According to available information,
it is almost certain that trench was carved into the rock, its depth could be around 6 meters and width of 6-8 meters. We can assume that filling the trench extinct forms loamy - stony material with a share flushes cultural layers.

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Stephen Tashi
If this is elementary algebra you would say 2 = (1/3) X and solve for X.

If this is an engineering course, you might have to use more equations.

No, that is homework from high school

Please, its depth is 6 meters and width 6-8 meters. How far AB hypothetically you chose if you think that the resistance measurements would allow to determine the presence of a ditch. Formula is ρz = k * AU / I. What am I substitute. Please, help me...

Stephen Tashi
You must reveal all the known information and perhaps other formulas.
Which values are known?
Is the voltage of the battery known? AU = voltage? = ?
Do you know a formula that tells the value of k as a function of the distance between the electrodes? k = (constant)(distance between electrodes) ?

mfb
Mentor
Where in the sketch is the ditch?

He's going to archaeological research the castle, which
will be preceded by geophysical measurements. The challenge is also localization
said ditch. According to available information,
it is almost certain that trench was carved into the rock, its depth could be around 6 meters and width of 6-8 meters. We can assume that filling the trench extinct forms loamy - stony material with a share flushes cultural layers.

For the conventional resistance measurement used four electrodes: two current (A, B) into the country "deserts" and the current I between two measuring (M, N) is then measured voltage AU, resistivity, which is the goal of our measurement is determined according to Ohm's law; in its formula is needed to add a so-called. arrangements constant k, which depends on the distance of the individual electrodes. The calculated value is then referred to as "apparent resistivity" ρz, because it describes the whole "block" hemisphere, where penetrated current:

ρz = k * AU / I

Depth of this measurement is approximately 1/3 of the distance between the electrodes A and B. How far AB hypothetically you chose if you think that the resistance measurements would allow to determine the presence of a ditch.

On the picture http://nicco35.rajce.idnes.cz/nastenka/#9.jpg I find the distance between points A and B. Please, help me