Search This Blog

Monday, 31 January 2011

Scanning Details of Rotomahana Lake Bed

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)

Saturday, 29 January 2011

Rotomahana lake floor comes into focus

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.

Friday, 28 January 2011

Underwater with an AUV

Follow the AUVs into Rotomahana's depths:

Sensing Remote Rotomahana

The submersibles, like two faithful and intelligent mechanised mammals, conitnued their diving and tracking progress today, closely monitored by their attentive minders. Seeking them at the end of a mission when they automatically float to the surface, is very reminiscent of looking for whales or dolphins, with only their 'dorsal fins' visible above water.

Most of today I spent with Vaughen Stagpoole and Fabio Caratori-Tontini from GNS Science, who were continuing their gravity and magnetic surveys. The instruments used allow extremely subtle variations of these forces to be measured. In contrast to the lake water and lake bed bathymetric measurements that are made by the AUVs, gravity and magnetism are influenced by the layout and distirbution of rocks deep in the crust.

The second photo shows Vaughan taking notes whilst Fabio is taking magnetic readings

We were ferried to the different locations by Brad Scott, a volcanologist also from GNS Science, who is very knowledgable about New Zealand's volcanoes.

As we progressed along the shore, we passed some underwater hydrothermal vents where streams of bubbles welled up from below the water surface.Brad reminded us that recent measurments of the CO2 gas released from the bed of Rotomahana showed that the total flow is twice as great as that from Ruapehu Crater.

 As we travelled around the edges of the lake, we climbed ashore at places that would be totally inaccessible other than by boat.

Because Rotomahana has restricted access, on a calm windless day like this it is a very peaceful place. As we moved in stages towards the looming presence of Tarawera the atmosphere of the place seemd very powerful. At one point we landed at a small jetty, from where a 2 km long track through the bush led us to Lake Tarawera.

On the way we passed a memorial to the victims of the 1886 eruption.

Tuesday, 25 January 2011

Lake Rotomahana starts to show her hidden side

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.

Sunday, 23 January 2011

Yellow Submarines

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.

Marae welcome and Rotomahana blessing

The Rotomahana Project is of great interest to the Te Arawa people who regard the whole area around Mount Tarawera to be a sacred site. It is 125 years since that eruption of Mount Tarawera which took over 100 lives, destroyed several villages along with the Pink and White Terraces, and devasted the surviving Maori community who had been living in the area.

This history is still very much alive in the minds of locals around Rotorua, some of whom are direct descendants of those who died. There is therefore a strong personal interest in many people to learn more about the details of the eruption aftermath that may be revealed by the Rotomahana project.

At Hinemihi Marae, Ngapuna, all members of the project team were invited yesterday to a formal welcome ceremony, officiated by several senior elders of Te Arawa. The first photo shows the team assembling outside the Marae, awaiting the invitation to enter. It was a great privelege to be hosted with this welcome, and to be offered words of greeting, waiata (songs) recounting the history of the eruption and prayers for the success of the project. To round of the event there was a feast shared by all participants.

This morning we awoke to a widespread downpour of rain. This created a sombre atmosphere for a blessing ceremony held on the shores of Lake Rotomahana. It was again a beautiful and powerful event that expressed the deep feelings and respect associated with the history of this lake.

Friday, 21 January 2011

Rotorua kids show their inspiration about Lake Rotomahana

The Rotorua Public Library has a display of about 70 posters created by Rotorua school children who have taken up the challenge of the GNS Science Poster Competition. Their challenge was to represent their prediction of what the Rotomahana Project scientists will discover in the next few days in their survey of the lake.

There is a wonderful range of images and ideas of what may lie beneath the surface of Lake Rotomahana. I'm sure you will agree that it is a colourful and diverse display

If you live somewhere near Rotorua or are passing through in the next two or three weeks, drop in for a look at the entries. The overall standard and impact of the posters is very high and shows that the children have really thought about what the scientists may find with the AUV survey being undertaken over the next week.It is going to be a tough job to choose some prizewinners! Scientists will make their choice based on visual impact and scientific accuracy in comparison with their survey results, and the verdict will be announced in mid February

Meanwhile, the team is gathering in Rotorua, including scientists from GNS Science, Columbia, NOAA and Woods Hole Oceanographic Institution in the US, the University of Waikato, Auckland University and Bayreuth University in Germany. There is also a film crew and a couple of TV news teams coming along for some or all of the time.Tomorrow our first engagement will be a formal welcome of scientists and visitors on the local Marae.

Wednesday, 19 January 2011

The Deep Fault Drilling Project

The Deep Fault Drilling Project begins its first phase of drilling in the next few days. In this video Rupert Sutherland tells why scientists want to drill into New Zealand's huge Alpine Fault.

Friday, 14 January 2011

New Zealand's Alpine Fault

 For the latest on the Alpine fault drilling visit Rupert's Blog

This NASA photo of the South Island of New Zealand shows the green of lowland vegetation contrasting clearly with snow in the mountains of the Southern Alps. The straight edge of the mountains is the line of the famous Alpine Fault. This fault is the boundary between the Pacific and Australian tectonic plates which are sliding past each other in this region at an average rate of about 40 mm per year. The Alpine Fault is a globally significant feature and similar in character to the San Andreas Fault in America or the North Anatolian Fault in Turkey.

Every 200 - 400 years the plate movement is accomodated by a violent earthquake of about magnitude 8, and dislocation along a segment of the fault of roughly 8 metres. The last big earthquake rupture occurred in 1717 AD.

Most of the fault movement is sideways, but a portion of it is vertical, and has uplifted the mountains on the eastern side to reveal exposures of the rocks along the fault that have been buried and altered over millions of years. The second image shows a slice of this metamorphic rock with white streaks of quartz, black mica and a red garnet crystal that is about 5mm across.
Rupert Sutherland at GNS Science is one of the leaders of the Deep Fault Drilling Project (DFDP) which is about to drill into the Alpine Fault. Many other research organisations are involved in this very large, multi year project. To listen to a short radio interview of Rupert talking about the project go here.

From next week, the  DFDP project will start by drilling two shallow (150m) boreholes through the fault near Whataroa on the West Coast. Rocks will be sampled and analysed and instruments will be left in the ground as part of a long term monitoring programme.  In future years this reasearch will be extended by drilling down several kilometres.

In the photo of Rupert he is holding a specimen of fault breccia - a bit of rock that has been fragmented by rupturing of the Alpine Fault. In the close up image you can see how the rock is broken up. The dark patches are where some of the rock was melted and then solidified again in the spaces between the fragments.