Critical Zone Sensor


Alexandra Arènes

Let’s try to embody an observatory physically. What is it made of? If we were to describe an observatory, we would have to spend time at each sensor, moving through the landscape, a forest for example. Then we can finally be attuned to the “tidings of the earth”[1] 1 - De Gruyter (2015) Tidings of the Earth: Towards a History of Romantic Erdkunde. The Science of Literature: Essays on an Incalculable Difference, Berlin, München, Boston. pp. 47-68.. The instruments are anchor points: they allow us to see or hear the invisible elements hidden in landscapes: the depths, the ages of water, the symphony of chemicals in a river, etc. From and through the Earth, they give us the opportunity to listen to its whispers. From and through the Earth, they make us discover the many entities, parts, variations, heterogeneity, that make up the critical zone. We are inside of it, part of the critical zone in some way, we move through its hidden parts.


Let’s better understand the science and technique of the critical zone, by going to an observatory, a forest in the Vosges, in eastern France, threatened by acid rain and drought (lack of water, parasites, decrease in forest cover). From station to station, the earth takes shape. Below is a description of the CZO map. A video of this map can be seen here (title CZO map – SOC).

Geoseismic station

The geophones are an instrument that sounds the ground at depth by sending vibrations through it and records the sensitivity of the soil to these variations in order to reconstruct rocks porosity down to -150 meters. Geophysics use lines, transects at several locations, here 7 lines of about 100 meters each. Vibrations are triggered on the ground and their propagation is followed. This instrument reconstitutes one dimension of the earth, making us feel the soil depth.

Weather station

Several devices record the variations of the lower atmosphere and the direction of all elements carried by the winds. The sulphur emissions from Asian industries that cause acid rain can cross the Vosges forest in less than 20 days.

Tree Station

Trees are sentinels of the environment. Large areas of the forest are monitored to understand whether or not there is resilience after storms, acid rain, pests and the impact of the forest industry. Large rectangular gutters are placed under the trees and collect rainwater to analyze its pollutants, including sulphur, which acidifies the soil and is responsible for the death of trees.

Gravimetric station

This machine records the signal from the water table at depth. But it is so sensitive that it also records other signals such as the force of gravity of the tides breaking on the sea coast hundreds of kilometers away, like echoes propagating to the continental forest. We go even deeper into what is going on in the soil

Piezometers and coring

The core samples extract soil from deep underground, at different places across the watershed, exposing previously unsuspected pockets of water at a depth of almost 120 meters. The water triggers chemical reactions in the rocks, possibly even allowing the presence of organisms. The boundaries of the CZ extend intensively into the depths.

Riverlab station

The Riverlab is a tiny laboratory set up directly in the field. Inside, the machine records the chemical variations of the river by letting the river water pass through its circuits in real time. More than 10 chemical components are measured every 20 minutes. Scientists show that they behave differently depending on the day and night, the season or the flood. It is a kind of microscope for geochemistry that decomposes the river into as many particles as there are inside a drop of water. In this view, there is not only a river of water but there is a river of sulphur, a river of phosphorus, a river of calcium, a river of magnesium. It changes the way we understand the elements of a landscape.


These instruments, these sensors, offer different, unprecedented views of environmental phenomena and therefore enable us to rediscover the landscape, which is no longer a passive setting, but is crisscrossed by a multitude of phenomena, with lots of entities, which are invisible to the naked eye, and which can only be apprehended thanks to scientific sensors. Space is no longer seen from the sky, from above, but there is another relationship to space, from the ground, even from the micro. And it is the whole of the sensors that ultimately makes it possible to build this new understanding of the Earth.

[1] De Gruyter (2015) Tidings of the Earth: Towards a History of Romantic Erdkunde. The Science of Literature: Essays on an Incalculable Difference, Berlin, München, Boston. pp. 47-68.



TU Delft / Faculty of Architecture