Absolute gravimetry - for monitoring climate change and geodynamics in Greenland

Abstract

Over the recent decades, there has been an increasing interest in studying climatic changes. The reason for this interest is a wish to gain an understanding of the processes behind these climatic changes, and to be able to predict the consequences of, for example, sea level change. To say something about sea level change requires a knowledge of global ice masses, in this case the Greenland Ice Sheet. Global ice masses are estimated to contain 80% of the global fresh water resources, and the Greenland Ice Sheet holds enough water for an approximated 7m of sea level rise. For these reasons, there is a large interest in studying the mass changes of the Greenland Ice Sheet. There are many geophysical methods which can be used to study climatic changes. However, the interpretation of these changes is complicated due to the presence of many different signals - not all of which are related to present-day climate change. Different geophysical methods have the ability to detect different signals. In many cases, the best results are obtained through incorporating different methods. The use of different methods for identifying certain signals has been the motivation for this PhD project. The signals investigated come from geodynamical processes originating from ice mass changes. Two of the Earth’s properties give rise to two different geophysical signals. One of these is caused by the asthenosphere’s viscoelastic properties, while the other one is due to the lithosphere’s elastic properties. The viscoelastic signal is a “slow” signal. A good example of this is the uplift detectable in Scandinavia today, due to the ice which covered this area during the last ice age 10,000 years ago. The elastic signal is a “fast” signal. An example for this is an annual displacement of the Earth’s surface which can be detected in Greenland, resulting from the loading and unloading of ice during winter and summer. Besides this kind of elastic signal, there will also be a more general trend due to the development of the ice. As already indicated, the two geophysical signals under investigation give rise to a vertical displacement of the Earth’s surface. GPS receivers are used for detecting this movement. In a project which has been ongoing since 2007, the GNET (Greenland Network) project, 53 permanent GPS receivers are deployed on rock at the edges of the ice sheet. The latest results from the GNET project show that all GPS receivers are detecting an uplift of the Earth’s surface. The problem is that the GPS receivers do not provide any information on whether the signal is of viscoelastic or elastic origin. The objective of this project has been to use a different method for detecting the two signals. Together with the GPS data, it is possible to separate the different signals. The method used in this study is absolute gravimetry. An absolute gravimeter of the A10 type has been purchased by DTU Space for this purpose. This instrument can measure gravity changes as small as 6µGal (= 60nm=s2), which provides the unique possibility of geodynamical studies in Greenland. A part of the work in this project has been to maintain the instrument and conduct field work in Greenland and Denmark. This has been beneficial in gaining a deeper understand- ing of the instrument, its possibilities and limitations. During the studies of the instrument it is found that it performs better than the manufacture specifications. The presence of a noise signal in the data, which originates from the instrument itself, has been the motivation to investigate different processing schemes. This noise is called the system response. The time allocated for a PhD project is not sufficient to gather enough data for an elaborated analysis of the different signals which can be detected in Greenland. However, as will be presented in this thesis, the preliminary results indicate interesting possibilities for the use of absolute gravimetry. To get an idea of the expected size of the geodynamical signals in Greenland, they are modelled and the results are compared to the preliminary measurements. These results show that the direct attraction from the ice masses is the largest signal, and care must be taken when modelling it. An example of this is the gravity change detected at Helheim, 54µGal . This is mostly a signal of direct attraction from the ice. Additionally, a signal of direct attraction from the ocean can be present at coastal sites. This is investigated for the THU3 GNET site where tide gauge data are available. The result is that the direct signal from the ocean is in accordance with the measured gravity change. Furthermore, in the thesis it will be presented how the absolute gravimeter is constructed, how the data are processed, and which corrections are needed to obtain the best possible data. Especially is the data processing investigated with different processing methods. These preliminary results of the gravity measurements in Greenland are interesting new data which suggest that as more measurements become available it will be possible, along with the GPS data, to separate the different geodynamical processes and thereby give a better estimate of the geodynamics occurring on Greenland. This in turn allows better mass balance of the Greenland ice sheet.