Earth Science

Vicki Ferrini from Columbia University in the US, is a specialist at processing data from the side scanning sonar. In the photo taken from the shelter on the lake shore, she is showing her american colleagues some of the images that she has produced shortly after downloading data from the last AUV mission. Sharon Walker (left), Dan Fornari (just behind Vicki) and Amy Kukulya (right). The image that Vicki has produced is an oblique side view of the lake floor, with a vertical scale of about 50 metres (the water depth). The amazing detail of the scan is visible in this image. You can see lots of little pock mark craters on the lake floor with streams of bubbles rising vertically above them. Obviously this area is hydrothermically very active. A lot of the emitted gas gets dissolved in the lake water before it reaches the surface, especially when it originates from vents at relatively deep levels in the water. As the project reaches its final stages, the results of all the different surveys are being combined to fill out a fascinating picture of the Rotomahana hydrothermal system and present day bathymetry (lake floor topography). As project leader Cornel de Ronde keeps an overview and guides the activity of the different teams. (This photo Sharon Walker)

There are three more days left before the scientists have to end the survey, and all systems are operating well. A boat from the University of Waikato has been used for several different purposes, including a magnetometer survey and also CTD (Conductivity Temperature and Depth) measurements. Data from the different surveys are processed each day and combined to give an overview map of the lake floor and the distribution of its features.The imagery is starting to show the big picture, as well as some interesting details.   In this second image, Cornel de Ronde is discussing the magnetic anomalies map that Fabio has devised from a grid survey of the lake. The boat tows the magnetometer behind it as it travels up and down in a grid pattern. This was aligned mostly at right angles to the volcanic rift, with a few lines along it to ‘tie’ the readings together across the gaps. You can see the grid pattern as dark lines in the third picture. They more or less cover the lake area, and are about 100 metres apart. The colours on the map represent the intensity of the magnetic field as measured from the surface. They cover an area wider that the lake itself as readings were taken from the surrounding land as well. Blue and green show a low level, whilst red and pink indicate more intense magnetism. Volcanic rocks like basalt are typically very magnetic, but hydrothermal activity can alter the rocks and  reduce the magnetism. This means that the measurements tell us something about the underlying geology below the lake as well as the distribution of hydrothermal activity. The positions of hydrothermal vents have also been roughly plotted, but at this stage the locations are approximate. Sharon Walker processes the data from the CTD device that is lowered from the boat as it travels up and down. Different measurements can be combined to clarify the information about hydrothermal activity that affects the water above the lake floor. The screen shows some readings from yesterday. On the left hand side, the blue line represents temperature, which is about 22 degrees C at the surface, but falls to roughly 14 degrees at 20 metres depth, and gets slowly cooler further down. The green line is a measure of the amount of light  scattering caused by particles in the water. Near the surface the reading is high, due to the presence of mud washed in by rain, as well as algae growing in the sunlight. Down near the bed of the lake there are some spikes that are caused by hydrothermal activity stirring up particles and introducing gas bubbles.

Follow the AUVs into Rotomahana’s depths:

Today was a very active day for our research below and around Lake Rotomahana The different teams were working on or around the lake included: A gravity survey of the back country to the East of the Lake Magnetic survey of the lake floor Rock sampling survey of volcanic deposits from points around the lake shore The Automatous Underwater Vehicle survey I was with the gravity survey yesterday. This involves taking a very precise gravity meter to different locations and making measurements through a small lens in the meter. The force of gravity varies very slightly over the surface of the Earth, depending on the density of rocks in the crust as well as the presence of mountains and valleys which cause a slight sideways pull. By measuring these variations and taking precise readings of the measurement locations using a GPS, a lot of information can be found out about the make up of the crust. In the photo, Vaughan Stagpoole of GNS Science is taking a gravity reading. Although I spent most of today with the AUV team, I did get to see a fascinating cliff section that the rock samplers had had a look at. The pale grey upper section of the cliff is a sequence of fragments of pumice known as Rotomahana Mud, that was erupted on June 10th 1886. The black line in the photo is the soil horizon that was the ground surface on June 9th. The incredible power of the Tarawera eruption is captured in this single location – it shows how much the landscape was buried by the volcanic debris in the space of perhaps four or five hours. The AUVs operated over the north eastern section of the lake, including over the area of the old Lake Rotomahana, and Pink and White Terraces. I went out with the boat to deploy and retrieve the AUVs as they were tested some more and launched on their missions. They are set to travel at a specific altitude above the lake floor – one at 10 metres, the other at 15, and so they rise and sink lower as they travel. However, at one point one of them collided with an underwater pinnacle which must have reared up vertically infront of it – allowing no warning to the sub. Although we had to check it over, the AUV was undamaged and was set back to work again. In the photo, Amy is letting an AUV off on its next mission. It is amazing to watch these robotic machines set off from the side of the boat, steer towards a prescribed direction and then suddenly vanish from the surface into the depths. They are then tracked on the land based computers which are informed of every aspect of progress, including position, speed, direction, depth, battery power etc. Once they are finished with a mission, or if there is a malfunction, they float to the surface and are easily located by GPS to be hauled onto the boat and back to the shore. They are then hooked up to the computer to download all the data, which can be immediately processed to produce 3D images, plots and maps. Of interest today was the discovery of numerous pock mark holes on the floor of the lake, with streams of gas bubbles above them. Down there also was even a terrace like formation, about 30 metres across – but don’t be too excited, it was well away from the location of the former Pink and White Terraces. It is not possible so soon to know exactly what this feature is, or if it is one of the many other features known around the lake before the 1886 eruption.

Over the last two days quite some time was spent testing the Automated Underwater Vehicles. They are transported in the back of a van – in the photo you can see Rob Littlefield from the Woods Hole Oceanographic Institution preparing one of them for a trial run. The two AUVs each carry a different type of scanner that measures characteristics of the lake floor:  A multibeam scanner – this creates a detailed 3D topographical (bathymetric) map of the lake bed. A sidescanner – this technology uses acoustic (sound) signals to gather information about the hardness or softness of the lake floor sediments. Lava, mud or coarse boulders will give different signals, and so some idea of the geology of the lake floor can be gleaned. The second photo shows Amy Kukulya, from WHOI making adjustments to one of the AUVs on Rotomahana lake shore. Different sensors on the AUVs take other readings. If you think about it you will soon see how they all combine help to detect mineral rich, hot, hydrothermal vents: Temperature pH (acidity or alkalinity)  Eh – this is the oxygen reduction potential of the water, where a high reading means that it has been introduced into the lake recently. Turbidity – this uses a strobe light and measures the amount of reflection, indicating the density of particles in the water. Conductivity – this depends on the amount of dissolved salt, which is also a feature of hydrothermal fluids. Other sensors on the AUVs are there to help with navigation. This is a highly technical subject in itself, and makes it possible for the controller to have real-time information and remote control of the speed, direction and depth of the AUV. Amy deals with all this remotely from her computer in the back of the van. All the information she needs is at her fingertips… The photos show that the weather has been pretty wet over the last two days, but some testing was done on the lake yesterday… and also in the hotel today: Some of the guests were a bit surprised to see a torpedo like vessel in the swimming pool. One of the scanners wasn’t quite behaving, until some of the settings were checked. This adds up to a very expensive way of finding out that the water depth was about 2 metres. Everything is now set for work to begin.