Mr Vikalp,

It is a common practice to install geophones in the three main directions of the Earth: Vertical, North and East. However, it could be very difficult to deploy a horizontal sensor in a well-bore, as most of the sensors have very little tolerance to the vertical angle. That means, that if you install your sensor to to its write angle (vertical or horizontal), but you are off bu a few degrees, you may have nothing useful from that sensor.

Omni-directional geophones can be installed as the directional ones, but you don't have to worry about their final angle, because they will work when you got the wright angle or when you didn't. Additionally, if you are deploying three orthogonal sensors, you can can change their observation direction with a simple matrix multiplication.

Summarizing:

-Geophones should be installed in an assembly of three orthogonal sensors, and it will be a good practice if yow make sure that one will be oriented vertically, one pointing to the North, and the other pointing to the East.

- If your angles are not right, you can correct them by a simple matrix multiplication, provided that you know the final orientation of the sensors.

- If you use directional geophones, you will have to buy one type for the vertical direction, and another type for the horizontal direction. In that case, you have to make sure you get the right inclination with very little tolerance. If you don't get the right orientation, the sensor won't produce useful recordings.

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Dear Vikalp,

the answer from Dr Mejia is correct.

I would only add that if you install 3 Component (orthogonal) sensors in borehole, at the end of the instalaltion you don't know the real orientation of the 3 component, due to the uncontrolled rotation of the tool during the descent. You only know that they are ortogonal. Normally, one component (the original vertical) is oriented as the borehole axis, but the 2 other (original horizontals) can be rotated in any direction. So, in oder to process correctly the data, even with omni-directional sensors which works at any angle of tilt and make your life easier, you need to recover orientation angles to be used for rotating the 3 signals to an absolut reference (for instance Vertical, North, East), as Dr Mejia said.

To do this, you can either have a meter (compass, gyro,...) on the tool, but it is costly, or rely on indirect measure, i.e. you can record same kind of shots or impacts made at the surface at some distance from the borehole, and find the difference between the real direction (you know it) and the one derived by the signals recorded by your sensors. Obviously, the deeper the borehole the further and stronger the shots you need.

Another possibility, you can use always a s cross-check, is to staudy some regional or teleseismic event and to compare its direction with the one derived from your sensors.

Ciao.

In Mining Engineering we install three-dimensional arrays of the Omni-directional type. The first recommended thing is to survey the empty holes (usually diamond drilled holes) very carefully according to the mining grid that you may be using. It does not have to coincide with true North.

Despite the Omni-directional capabilities, we ensure that the tri-axial sonde has a vertically oriented sensor, which can be installed at the angle indicated by the hole survey to ensure verticality. Another sensor is always horizontally pointing to the centre of the blast event (radial). The third sensor is parallel to the blast monitoring event (transverse).

During installation we use PVC pipe, which can be marked with a the top of core orientation line, to ensure the triaxial sonde is properly following the hole. We can install multiple tri-axial sensors within a single hole.

For a 50mm diameter sonde, we recommend an HQ hole (96.1mm diameter hole). We grout with a 0.4 Water/Cement.

Geophone not omnidirectional! Geophone measures the three orthogonal components of the displacement vector. If you specify the location of the geophone in the space can be determined and the direction of the original vector.

See the attached article 8990922.pdf