Marigraph

Principle

The principle of the marigraph is simple: located in a precisely identified site, the marigraph records the sea level during time. The complication of the marigraph is thus not the principle but concerned technology, for the calibration of measurements, the recording…

Types of marigraphs

There exist various types of marigraph. It is possible to separate them in two classes: marigraphs of surface (standard 1) and marigraphs plunger (type 2).

1-Scales type and float marigraphs.

To take measurements of the sea level it seems logical to be placed on the level of surface and to record the evolutions of this surface in time. Two types of marigraphs function according to this principle. On the one hand the scales, still visible in many ports and near the Bridge S enjambeant the rivers, their principle is simplistic: it amounts placing a reference graduated starting from the bottom and making a direct measurement of the level of surface. This system of measurement is not interesting that by its simplicity because it requires the presence of an observer and does not allow a perennial recording of measurement. And in addition, the float marigraphs have a principle almost as simple as the scales. Plunged in a well of tranquillization which places them safe from the Houle and the clapots, the floats oscillate according to the tides and by a scientist play of Poulie S allow a measurement on a paper roller unrolling according to time. The precision of measurements of these two types of marigraphs is dependant on the observer.

coastal numerical 1-Marigraphs type.

Another type of marigraph of surface, the coastal numerical marigraphs function by a principle of send-receive of acoustic waves (40-50 Khz or Radar (

>

1 GHz)). A Transducteur is placed at water surface, it emits an impulse and collects the considered signal. The time passed between the emission and the reception of the signal is translated into height of the water column. The Transducteur takes several measurements periodically then measurements are realized in order to limit the effects of the Houle or the clapot. The network of observation of the French coasts is mainly made up of this type of marigraph.

2-Marigraphs type of basic pressure.

The second category of marigraphs makes up of pressure pick-up marigraphs. Placed on the ocean floor, they measure the surrounding Pression. This pressure is characteristic the height of the water column and the Atmospheric pressure on the surface. The knowledge of the atmopheric pressure and the Densité of water is essential to obtain the variations of the sea level by the equation:

$P_ {m} = G \ int_ {- H} ^ {\ eta} \ rho dz + P_ {atmosphere}$

with:

$P$ : the pressure
$\ rho$ : the Density
$g$ : the Constant gravitational
$-H$ : depth compared to a reference
$\ eta$ : the rise in surface

The marigraph of basic pressure consists of a quartz whose frequency varies by piezoelectric effect according to the pressure. This frequency is recorded then integrated (realized) on a Period in order to free itself from the effects of the Houle. The Eigen frequency of the quartz is dependant on the temperature, it is thus necessary simultaneously to take a temperature measurement to the measurement of pressure. This type of apparatus has a measuring accuracy around 2 to 3 cm, according to the precision of measurements of Température and Atmospheric pressure.

Very autonomous, these apparatuses provide data in areas located off the coasts, they are of thus very great interest. Their implementation in deep seas of the Océan S is an main issue and in limit thus the deployment. It is moreover, frequent to lose the apparatus by flight or loss of the Balisage of surface or because of a accidental Chalutage. Lastly, because of the difficulties of maintenance, the duration of measurements is often limited.

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