The over-all structure for the seismic application is given in Figure 5. The data start to be sensed by the sensors in the field. Locally, the analog data is converted to digital signals before they are sent over the LOFAR network to a central unit, in this case the Blue Gene of IBM at Groningen. There, the data are collected, (automatically) analyzed and pre-processed if necessary. Then, based on the desired type of data, the data is sent via standard internet to TNO and/or KNMI for storage, further analysis and interpretation. These two institutes then allow these data to be retrieved via standard internet by other users.
Next to the data stream, also a control system is implemented. The control of the data and meta-data can be done locally on a sensor field, globally via the central server at Groningen or remotely via any place on the internet, although not all functionality is available at all locations.
The total system is divided into the following subsystems:
local (sensor) fields
central LOFAR network
central system (Computercenter Groningen, with Blue Gene and TNO facilities)
data-storage and retrieval sites (TNO, KNMI)
data access for users (e.g., TU Delft)
Figure 5. Schematic overview of seismic system LOFAR
Local sensor fields
Two seismic geometries are implemented
A province-scale geometry focusing on the lower frequencies (< 20 Hz) and sparsely sampling the wavefield (see Figure 6). This geometry can be used for inferring information of the deeper subsurface, and can be used for low-frequency interferometry.
A kilometer-scale geometry focusing on the higher frequency bands, up to 100Hz, and adequately sampling the wavefield (see Figure 7 on top of a producing gas-field. This geometry is most suitable for research on time-lapse seismic and the passive seismic interferometry.
Figure 6. Configuration of sparse-sensor (3C) fields for province-scale network (3 fields shown here)
Figure 7. Configuration of many-sensor (4C) field for local/km-scale network near Annerveen
Total software system for seismic application
In Figure 8 the total system for the seismic application is given. Locally on a sensor filed, there is a local control system (LCS) and local data storage (LDS). Via the LOFAR network, these systems connect to the central systems at the Rekencentrum of Groningen. There, a data-storage (CDS) and control system (GCS) are active, but now on a higher level. At Groningen, processing is done such as the correlation for the seismic interferometry. , a global data service (GDS) is running which deals with the dissemination of the data. The tool that has been developed for the purpose of getting data from the LOFAR network is the GeoLOFAR-web site. The URL of the web site is: http://geolofar.nitg.tno.nl.
Figure 8. Overview of data flow and control flow for seismic data, centralized at Groningen.