Abstract
Over the last decade, active seismic data have been increasingly used to image the subsurface architecture of the Critical Zone (CZ), map the water table and monitor moisture changes at depth. Yet, the heavy weight of conventional equipment required to collect seismic data (>150 kg for a 48-trace setup) and the time needed to lay out each profile (up to several hours) seriously limit the implementation of spatially extensive surveys or repeated timelapse measurements. This is all the more true in catchments with rugged terrain (i.e. with steep and/or rocky slopes, thick vegetation cover or limited road access) such as those instrumented within the French CZ observatories network OZCAR. Seismic nodal acquisition systems (or nodes) developed within the last fifteen years were first introduced to the deep exploration community, and are now reaching the CZ near-surface geophysics community. Here we show examples of data collected with such nodes at several sites within OZCAR and beyond. These nodes offer a cable-free alternative to conventional instruments which is both lighter (<50 kg for a 48-trace setup) and easier to set up (no cables between the geophones, the sources and the central processing system). It is also more versatile in terms of acquisition geometries, as the spacing between sensors is not limited by cables anymore. Each node is autonomous, with its own Li-ion battery, internal memory and GPS chip (ensuring time synchronization between nodes). A smartphone or tablet is used to assign line and station number to each node, and to check their operating status. Once activated, each node can record seismic data continuously for several hours or days (depending on the chosen sampling frequency), thus offering the possibility to combine passive (e.g. recording during quiet periods such as lunch break) and active seismic data (e.g. conventional hammer shots) without additional effort. Active shooting times can be accurately recorded with a simple GPS logger connected to the source. Once back at the lab, seismic data is uploaded separately from each node and stored in a single database. Shooting times can then be used to extract seismic data from all nodes in the active shooting time window and generate conventional shot gathers. Alternatively, passive data can be extracted by selecting a quiet time window without major active events. A major drawback of this approach is the absence of on-site data visualization, although upcoming developpements should at least provide single wiggle display. In the light of the use we have been able to make of them so far, their use and implementation in the field is highly advantageous, and reduces acquisition time enormously.