The Science Behind the Survey

So for those who are interested, here is some technical stuff, for a more in depth description of what I plan to do in the field, and why I am doing this.

The overall aim of this project is to measure the changes in the microscale air and water content stored between ice crystals. We can use geophysics to measure these properties as radar and seismic surveys are each sensitive to any changes. Each technique is differently affect by property changes, therefore for the best result, it is necessary to do both. In particular we want to see how the properties vary as we measure deeper into the ice.

The surveys we planned to complete were cross-borehole profiles (XHP), using two boreholes, one containing an energy source, and the other a receiver, measuring the energy output. These produce accurate measurements for each depth. However, last year we had many issues when we attempted the seismic XHP. I therefore decided to investigate alternative methods, such as the Vertical Seismic Profile (VSP). This uses 1 borehole containing receivers, and the source set up on the surface. Logistically, this is easier, requiring only 1 borehole, however, the energy must travel further and therefore may not be as accurate.

Squinting at the computer screen as I set up the radar. Photo by Yoann Drocourt.
Squinting at the computer screen as I set up the radar. Photo by Yoann Drocourt.

I modelled the results I would get, using synthetic data so I could control the inputs. As expected, I found the speed of seismic energy depends a lot on the air content of the ice, whereas the radar energy is affected by both. The XHP model was more accurate for measurements deeper in the ice. However, the VSPs still produced a good result. The ability to obtain data we need, in addition to the improved logistical ease of collecting data in a VSP survey, lead me to decide that this was the best option to ensure data collection this time in the field.

A more realistic view of the practicalities of seismic data collection. Hoping for better weather next time!
A more realistic view of the practicalities of seismic data collection. Hoping for better weather next time!

We have since hired a hydrophone string to use in the field. This has 24 receivers separated by 1m, which is non-adjustable. The acoustic energy source will be a hammer blow at the surface. The survey will involve multiple shots at the surface, with at least 12 lines of shot locations – like a clock face. Each shot location needs to be repeated several times so we can average out the noise.

All this pre-planning should hopefully lead to better data collection in order to calculate what we want. Improving the calculation of ice properties, like I am attempting to do here, should inevitably lead to the improved production of ice flow model, to see how ice should react to future changes in climate -but that is a LONG way off yet!

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