INTRODUCTION

Very often, geophysical measurements may lead to ambiguous results. In most cases they can be overcome by using or combining more than one method in order to measure different physical quantities. Today the more or less standard method for conducting a pipeline survey is employing a sub-bottom profiler, an echosounder, at times a magnetometer and an accurate positioning system, which nowadays, in most cases, is a DGPS system.

As one of the aims of subsea pipeline construction is to carry natural gas or oil towards some distant shore and in doing so, this same pipeline at one stage has to cross a coastal area, a few problems will arise. Due to tidal movements, estuaries discharging massive amounts of sediment into the nearby sea, heavy storms stirring up shallow waters, the original ‘buried’ depth of a particular pipeline is certainly not a constant factor. Due to the above factors, or varying temperatures of their contents, pipelines have even been known to rise from the seabed and surface altogether, or get buried deeper than ever was intended.

In other words: it would be advisable to keep track of the ‘hidden’ pipeline. Thus it is understandable that mining and environmental authorities have come up with a set of rules to regularly carry out inspections of the buried metal, check its position and depth and keep a record of its movements. In the case of a pipeline being buried deeper than ever was intended another problem arises.
How do we find it back?What if it is buried too deep and the standard equipment yields no results? Then indeed we have a problem!

CT Systems has a long field experience with subjects related to the geophysical industry and the general aspect of offshore surveys and have encountered that very same problem numerous times. Derived from the above problem and also from the high costs of employing a sub-bottom profiler, the two above companies have spent considerable time developing a highly sophisticated magnetometer that not only can pinpoint a disturbance in the earth’s magnetic field, but can also establish the ‘buried’ depth, both with a very high degree of accuracy.

The nT Window can be switched off, showing only the Topography and Bathymetry.

A water depth of 20 meters has been no exception. Relatively small objects have been detected buried as much as 10 meters.

The survey is normally carried out with light-weight easy to mobilize equipment consisting of:

• a positioning device like DGPS
• the GTC Gradiometer/Magnetometer
• a Gyro, or a GPS antenna mounted on the magnetometer
• PC - an Echosounder- - the GEOS software package.

In most cases there will be a standard DGPS, Echosounder and GYRO mounted already on a particular survey vessel which will not only make the installation easier and quicker, but also lowers the costs of a survey. A survey can easily be carried out by one surveyor/geophysicist and one technical assistant.

 

DGPS POSITIONING

As mentioned above, normally an already installed standard DGPS receiver is used unless the specification of a particular client will not apply to that instrument. In that case, at extra cost another instrument will have be to be acquired. The GEOS software can interface to a variety of different positioning devices.

 

SEDIMENT ECHO SOUNDING

As with the above positioning devices, the same applies here. Normally a device already in use and installed by the customer is employed. Again, the GEOS software can handle most input formats. On the command line a number can be passed on to the program defining the size of the window where the nT’s (nanoTeslas) are displayed. The Magnetometer sensor is connected to a PC via an Analogue to Digital converter.

The other sensors are:

• DGPS. receiver with an NMEA 183 output
• Gyro or Fluxgate compass
• Dual frequency echosounder

The software has been optimized for speed. and as many as 30 Magnetometer readings per second have been achieved, using a standard Pentium 120mHz P.C.

What exactly is a MAGNETOMETER? 
Ferromagnetic objects, disturbing the magnetic field of the earth, can be detected by using a differential arrangement of detectors, or for that matter a single magnetometer. The magnetometer detector used in combination with the GEOS package, does exactly that. When only one detector is used then due to the innovative design of the detector, the directional sensitivity will concentrate on the direction it is ‘pointed’ at. I.e. heavy objects which normally would disturb the magnetic field from an unwanted direction have hardly any influence.

Thus, in a marine environment, passing vessels will not have the disturbing effects which are encountered by using a PROTON magnetometer. This passive system has accuracies and sensitivities ranging from just a few nanoTeslas to 20000 nT’s. See our bulletin (MAGNETOMETER - A BRIEF HISTORY) which goes deeper into the theory and history of the subject.

Together with a German University a special advanced algorithm has been developed that will analyse the recorded data and from it calculate the depth of burial from for example a pipeline. The results so far have been amazingly accurate. Naturally, to be able to achieve these results, an accurate echosounder has to be used in combination with the magnetometer and Gyro, to be able to take into account the effect of either sea or fresh water and also various positioning offsets. Pipelines, buried as deep as 8 meters, with a water-depth of 10 meters have been detected without any problems.

SOFTWARE 
The software consists of two sections. The GEOS acquisition program and the GEOS processing program. All the above illustrations are from the GEOS post-processing program. The display consists of up to three windows.

• nT window
• Topography window
• Bathymetry window

As shown in the various examples, the nT window can be enlarged or made smaller, even made full screen, or take it away altogether. In the case of the nT window filling the whole screen of course the topography and echosounder data are no longer visible. Recorded data can be played back and analysed. It is fully mouse controlled and data can be replayed backwards and forwards or stopped at a certain position in the data file. The chart window can be zoomed in and out or panned across the screen. The recorded track can be show in colours depending on the intensity of the nT readings. Also, if a dual frequency echo sounder has been used, it can display the mud layers.

The GEOS acquisition program has just one window to optimise it for speed and is extremely easy to use. The only action of the operator is to increase or decrease the nT magnification, or to centre the display. Recorded DATA is appended to the data file, preventing accidental, annoting loss of data.

The data file (LatLong) looks as follows:
14:56:23 -38.155 0 52.562546 5.7893521 4.25 4.28
14:56:23 -38.157 0 52.562544 5.7893311 4.26 4.29

A processed final (UTM) file looks as follows:
17:24:00 33.405 0 658876.62 5895223.16 -5.90 -5.90
17:24:00 33.458 0 658876.62 5895223.16 -5.90 -5.90
17:24:00 33.342 0 658876.62 5895223.16 -5.90 -5.90

 

The ACQUISITION SOFTWARE

Magnetometer-Gradiometer

 

Magnetometer hard- and software

A screen layout, showing the nT display,topography and echosounder window.

Parts of the track showing Magnetometer Survey on the river Maas.

The object of the exercise was to find back two pipelines, a 24 inch and a 36 inch, and using a specially developed algorithm, calculate from the accumulated data the depth of burial. The two straight lines, both under an angle of 286 degrees, are the theoretical positions of the pipelines.

The parameters needed are:

• Water depth
• strength of magnetic deviation (nano Tesla)
• position
• direction of the pipelines relative to the magnetic North
• angle of crossing of the vessel relative to the pipeline
• consistency of material
• which material
 

In the above 'screen-dump' and the following.

Some of the shortcomings were:

• irregular speed, due to testing two different kind of instruments
• irregular heading due to the same above mentioned reason
• lack of synchronised positioning data

However, the first two crossing have been analysed and immediately showed an approximate on-line depth of burial of 4-6 meters. 

The compressed MAGNETO screen

The above screen shows 4 crossings marked 1 to 4. The vertical tick-marks are in this case an interval of 10 meters. In the MAGNFILE.EXE program, of which the above screen-dump is a sample.

The recorded data can be manipulated, i.e.

• use a magnification factor
• stretch or compress the data
• set vertical tick-marks
• write/create different data files

In the above case all the required parameters were present to predict/calculate not only the horizontal position, but also the vertical depth of burial.

 

Crossing ONE and TWO

In the following examples crossing ONE and TWO have been enlarged/stretched out and all the other relevant items required to do the calculation have been marked on the screen.

The large deflection, showing a well-organised curve is the moment of crossing the 36 inch pipeline. The two smaller ones are those of the 24 inch pipe. Since both crossing were almost at the same location, only two of them have been analysed.

CROSSING -1- A 36 INCH BURIAL 9.60 meter from water level to pipe center
5.70 meter from river bottom to top pipe

CROSSING -2- A 24 INCH Pipeline
All the parameters have been marked and all that remains now is to calculate the depth.

• Water depth 2.77 meter (local)
• Position
• Eastings 207936.8 RD
• Northings 380084.4 RD
• Pipeline heading 286 degrees
• Angle to pipeline 074 degrees

BURIAL 7.10 meter from water level to pipe center
4.03 meter from river-bottom to top pipe

compressed track

crossing -1-

crossing -2-

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