Guralp Systems Limited
MAN-H30-0006 - Orientation sensor - Operator's Guide

Chapter 3. Using the orientation sensor

3.1 Installing the sensor

The orientation sensor should be placed as near as possible to the head of the borehole. The sensor should be placed in an environment where the external interference from other sources will be small and where there is no direct sunlight. The sensor should be covered with a polystyrene box to reduce wind or convection noise sources.

3.2 Installations with surface digitisers

3.2.1 DM24 mk3

Borehole installations using surface 3- or 6-channel DM24 mk3 digitisers are easily connected to the orientation kit with no additional hardware. These digitisers have an extra full-rate input channel which is exposed on pins on the AUXILIARY connector.

A Hand-held Control Unit (HCU) or Break-out Box (BoB) is required in order to unlock and centre the masses of the reference sensor.

The cabling between the Hand-held Control Unit and the AUXILIARY input of the digitiser is provided. This cable is labelled.

Because the two sensors are both connected to the same digitiser, they automatically share a common time base, so a GPS receiver is not strictly necessary.

DM24-based digitisers with more than 6 channels normally contain more than one internal digitiser module. For best results, you must ensure that the orientation sensor is connected to the same internal module as the borehole sensor.

To confirm the best configuration for your equipment, please contact Güralp Systems.

In this example, a PC or laptop is used to perform the orientation calculations on site. If this is impractical, the digitiser can be configured to store the experimental data and calculations can be performed later.

3.2.2 Affinity or DM24SxEAM

The procedure for using an Affinity or DM24SxEAM is essentially the same as that for the MD24 mk3, with the Auxiliary connector being used to provide an input for the fourth channel. Ethernet can be used instead of RS232, however.

3.2.3 Minimus

The auxiliary channel of a Minimus is exposed on the main analogue connector so, in this case, a 'Y'-cable is required so that the signals from both the borehole instrument and the reference sensor can be combined into a single cable.

The auxiliary channel of the Minimus appears in Scream as stream 0AUXX0.

3.3 Using a separate digitiser

The orientation calculation method described in this manual works best when the borehole sensor is digitised using the same time base as the reference sensor.

This is easily accomplished in the set-up above, because signals from the reference and borehole instruments are processed by a single unit. If you have the option, we recommend that you use a single digitiser in this way wherever possible.

However, it is also possible to perform an orientation experiment using a separate digitiser, relying on GPS to keep the time series synchronised.

Of course, if your borehole installation includes a down-hole digitiser, you will need to use a separate digitiser.

To do this:

3.4 Performing the analysis

The Blacknest orientation method generally provides a reliable indication of the sensor's orientation. In most cases, the greatest source of error is in the installation of the reference sensor.

Note: If you have particular difficulty in deriving a stable value, additional information is contained in a third output window, which shows a waterfall plot of Coherence vs Frequency vs Angle. The frequency range used in the calculations is indicated with a black outline.

This plot can be used to select a more advantageous frequency range where the coherence curve is smoother and easier to interpret. In the example above, the frequency range 0.23 to 0.6 Hertz looks particularly promising: the surface within this range forms a smooth, symmetrical arch. Ideally, the peak should be very close to unity and the lowest points, at the margins, should be close to zero. The chosen frequency range can then be entered into the "Between X and Y Hz" boxes at the top of the "Coherence vs Angle" window before clicking again; the data will be filtered accordingly before the coherence is recalculated, increasing the accuracy of the result.

3.5 Applying data rotations

Once you have determined the correct angle, you can program your digitiser to apply real-time mathematical rotations to the raw data. The procedure to do this is different for DM24 digitisers and Minimus digitisers. The DM24 procedure is described in the next section; the Affinity procedure is in section 3.5.2 on page 21 and the Minimus procedure is in section 3.5.3 on page 22.

3.5.1 DM24 digitisers

You can configure a DM24 mk3 digitiser to apply a rotation to the digitised data. It can then produce output streams representing ground motion on true North/South and East/West axes.

This is done within the DSP to minimize the reduction in data quality.

To set up the rotation:

3.5.2 Affinity digitisers

The affinity digitiser can apply real-time rotation to its digitised data. It can then produce output streams representing ground motion on true North/South and East/West axes.

To configure this, visit the "Data acquisition" page of the web interface:

Enter the required angle - the sign-reversed output of the orientation calculations - into the "Azimuth rotation" field and then click . (This feature assumes that the first three primary ADC channels are digitising the vertical, North/South and East/West components, respectively, of a triaxial digitiser.)

Once configured, collect some more data with the transformation active, and carry out another orientation calculation. The data from the down-hole instrument should now have a maximum coherence with the reference sensor at 0°. Check in particular that the sign of the rotation you have applied is correct.

3.5.3 Minimus digitisers

You can configure a Minimus digitiser to apply a real-time rotation to the digitised data. It can then produce output streams representing ground motion on true North/South and East/West axes.

The Minimus is capable of applying arbitrary three-dimensional rotations but we shall only use a single rotation about a vertical axis in this case. In the data-transform subsystem of the Minimus, rotations are specified using unit quaternions. This is a very flexible abstraction which avoids the problems associated with the more usual Euler angles, yaw, pitch and roll. Although four independent variables are required to specify a quaternion, a simple rotation about a vertical axis can be specified using just two variables, derived from the desired angle using simple trigonometric functions.

To configure the rotation:

Field

Value

Scalar

cos(θ/2)

X

0

Y

0

Z

sin(θ/2)

Field

Value

Scalar

0.703

X

0

Y

0

Z

0.711