Presently five GWR Compact Tidal Superconducting Gravimeters are operating worldwide. The relatively small size and simple mounting requirements of this superconducting gravimeter (SG) have allowed its installation at an existing seismic station and a volcano observatory. This was difficult, if not impossible, with previous SG models that required construction of an elaborate support pier. In contrast, the Compact Tidal SG rests on three small granite blocks which can be cemented to any stable surface. In addition, the efficiency of the helium Dewar which cools the gravimeter has more than doubled. This makes operation more feasible at sites where the cryogen is expensive or difficult to obtain.

A list of Compact Tidal SGs presently in operation is shown below. The following brief descriptions of these sites illustrates the wide variety of conditions under which the these instruments can operate.

Serial #	Site Location	Operator
GWR C021	Membach, Belgium	Observatoire Royal de Belgique, Brussels, Belgium
GWR C022	Mt. Aso, Japan	Aso Volcanological Observatory, Kumamoto, Japan
GWR C023	Table Mountain, Colorado, USA	National Oceanic and Atmospheric Administration (NOAA) Table Mountain, Colorado, USA
GWR C024	Table Mountain Colorado, USA	Institut fur Angewandte and Geodasie (IfAG) Frankfurt, Germany
GWR C025	Vienna, Austria	Zentralanstalt fur Meteorologie und Geodynamic Vienna, Austria

The Royal Observatory of Belgium, located in Brussels, received delivery of GWR C021 in August of 1994. The instrument was initially run at the observatory along side GWR T002, an older model TT-30. Data from GWR T002 has been collected since May 1983, the longest uninterrupted observation with an SG.

After comparison with GWR T002, GWR C021 was moved to a seismic station located in Membach, Belgium. This site is situated in the countryside removed from most human activity. Perturbations from trains and automobiles which could contaminate seismic data are therefore minimized. The gravimeter is installed in a separate room at the end of a 100 m long observation tunnel. During installation the granite support blocks were cemented directly to the tunnel floor. Since it is close to long period seismometers, care was taken to minimize vibrations produced by the cryogenic refrigeration system. The refrigeration compressor and its water cooler are installed in a shed outside the entrance of the tunnel. Extension hoses 100 meters in length connect the compressor with the coldhead at the gravimeter.

The site characteristics are excellent for seismic and gravity studies. Operation inside the tunnel minimizes surface ground noise and provides an extremely stable ambient temperature. Therefore, temperature induced signals from the electronics, and thermal expansion of the floor or Dewar are minimized. At the time of installation, humidity inside the tunnel was very high. Water was dripping off the ceiling and collecting on the floor of the gravity room. A low power heating element was installed inside the closed gravity room to dry it out without significantly affecting the tunnel temperature.

In December 1994, GWR C022 was installed in an observation tunnel in the side of Mt. Aso. This active volcano is located in Japan's largest caldera near Kumamoto. During installation, the gravimeter was hand carried from a nearby road to the entrance shaft (see below). An electric winch was used to lower the instrument 30 m down the shaft to the tunnel. There it was installed on the existing tunnel floor. No problems were observed operating in this high humidity environment. Self heating from the electronics keeps them dry and a small heating band placed around the top of the Dewar keeps the connectors dry. The compressor and water chiller operate at ground level in the entrance building to the shaft and are connected to the coldhead using copper extension tubes.

The observation area is outfitted with an array of instrumentation including extensometers, water tube tilt meters and long period seismographs. These studies are of importance both for increasing the fundamental understanding of volcanoes, as well as for assessing the likelihood of pending volcanic activity.

Two Compact SGs have been operating at Table Mountain Observatory in Boulder, Colorado since March 1995. This study is a cooperative effort between the U.S. National Oceanic and Atmospheric Administration (NOAA) and the Institut fhr Angewandte and Geod@sie (IfAG) in Frankfurt Germany. The observatory building is a single story semi-permanent building with approximate dimensions of 6 m x 12 m which rests directly on surface gravel. Isolated, concrete pads were poured about 1 m deep into the ground for supporting and testing gravimeters. The two instruments are presently running side by side for comparison and calibration. Eventually they will be moved to other locations.

In the photo below, GWR C024 is shown at right with a calibration device temporarily mounted in place of the automatic levelling units. This acceleration device was developed at IfAG specifically for calibrating the SG. The calibrator can be installed with minimal interruption and can be moved from site to site for calibrating SGs and other gravimeters.

C025 is operated by the Central Institute for Meteorology and Geodynamics in Vienna, Austria. The instrument was installed in the basement of an observatory building in July 1995. At this site the granite support blocks were cemented onto an existing gravimeter pier approximately 2 m wide by 4 m long, . Eventually the gravimeter will be moved to the Conrad Observatory, a Geodynamic observation site planned for construction in southern Austria. The new observatory will house an array of instrumentation in an underground tunnel constructed specifically for this purpose.

In this case, the installation of the gravimeter at an interim location allows the users to become familiar with the operation and maintenance of the instrument, at a convenient local site. This will prepare them for extended periods of operation without interruption, as well as aid in detailed planning of the final installation site.