Seismic research using RheoTune and Silas for remediation of the Otheense Creek (NL)

Problem description

Environmental research has indicated that the siltation layer at the seabed in the Otheense Creek area has been polluted. In order to obtain more information about thickness and distribution of this layer and to determine the total amount of silt Stema Systems was asked to do a shallow seismic investigation, combined with shallow drilling.

Site description

Environment

The Otheense Creek is an old tidal creek which is forming the eastern boundary of the city of Terneuzen in the Netherlands.

Geology

The seabed consists of a layer of approx. 0.7 m black unconsolidated  watery silt on top of a black more consolidated siltlayer with varying content of peat.

Below the silt layer the following layers can occur:

• Yellow (often clayey) sand
• Black clay
• Reddish peat

Methodology

As an ultra high resolution was required, Stema Systems decided to apply high resolution seismics with an ODOM MKIII transceiver unit and a 24 kHz centerfrequency. The Stema Systems Silas Program is an ultra high solution data acquisition, processing and interpretation tool for sub-bottom profiling and seismic survey. The complete reflected signal was digitally recorded using a laptop with the Silas system while position was recorded using a Trimble RTK 7400 system.

Fig. 2  24 kHz transducer

This Stema Systems methodology enabled several meters of penetration in clay while a centimetric resolution was obtained to perform a rough volume estimate. The resolution of this grid would give sufficient resolution to perform a rough volume estimate.

The location of the base of the contamination, the consolidated silt layer was verified using a suction drill (see fig. 3b)

Fig. 3a Set up scheme

Results

The seismic Silas data accurately detected the base of the contaminated mud  (see red line in figures 4-6), which was positively verified with borehole data.

Moreover this area is situated in a tidal environment, which resulted in the temporal and local occurrence of a salt water wedge (see figure 6)

Fig.4 Example of recording of 24 kHz data with:

-Blue line: top of fluid silt layer: interface between watercolumn and seabed

-Blue coded data underneath the yellow coded fluid silt: top of consolidated silt

 -Red line: base of silt (red line) as verified with boreholes.

Fig. 5

Conclusion

The use of a traditional 24kHz echosounder could only result in 2 depth values without registration of the layers which are located in between. The application of a 24 kHz echsounder transducer in combination with the high resolution seismic system Silas assured a high resolution while the base of the contaminated consolidated  silt could be detected in the entire investigated area. The resulting data enabled an accurate volume calculation.

For illustrations please download the paper