The facilitation economical dredging using Stema Systems Silas sub-bottom profiling for the seismic investigation of sand aggregates in the Maas river

The facilitation economical dredging using Stema Systems Silas sub-bottom profiling for the seismic investigation of sand aggregates in the Maas river

Introduction

Problem description

In order to enable the most cost efficient dredging it was decided to execute a seismic site investigation to gauge the occurrence and distribution of sand aggregates in the Maas river, between Arcen and Venlo. Thus allowing the dredging party to best plan and execute according to the findings. The major purpose of this investigation was to investigate the location of claydeposits as this information was used for volume calculations resulting in the economical dredgeable sand and gravel deposits.

Site description

Environment The investigated area is located in the Maas river between Venlo and Arcen
in the Netherlands, at a water depth of about 4 to 5 meter.

Geology At the top of the river bed Holocene river deposits occur with clay and sands. In investigated area often 1-2 m thick clay lenses occurs, which could prevent economical dredging of underlying sand and gravel deposits.

At many locations underneath the top of the river bed older Pleistocene glacial meltwater deposits are found. These deposits consist of clay, peat, sand, gravel and loam and have partly been reworked by river transport. These layers can have a large inclination and show large scale foreset or folding structures. The overall setting is that of a very irregular depositional relief with merely sand and gravel and intermixed clay lenses.

Methodology

In order to perform the most accurate investigating and analysis of the results of the riverbed Stema Systems decided to use its own developed Silas EBP-10 transceiver system incorporating a high resolution seismics with a 5 kHz frequency source array. The full signal of EBP – 10 array was recorded using the Silas software package and positioning

The investigation was complemented by superimposing the results of numerous boreholes made possible by employing the Silas Site software thus allowing both seismic and borehole datasets to be combined. The full signal was recorded using the afore mentioned software package and positioning was carried out using Stema Systems 960 RTK and Qinsy survey software.

The 5 kHz was decided to be the best frequency which would be able to penetrate several meters of sand and gravel while providing accurate resolution to map all clay lenses. In order to obtain accurate riverbed elevation data it was decided to carry out Multibeam measurements simultaneously. The survey was carried out using a small vessel.

Results

After completion of the seismic survey the seismic data was combined in the Silas Site processing program with all available borehole data (fig. 5) resulting in a highly accurate mapping of the clay lenses (fig 6),

The Stema Systems EBP-10 system being accurate enough to further detect the other sediment types present in the area in an highly accurate manner. The system not only detected clay layers, but also the distribution and thickness of sand layers (see fig. 7a, 7b)

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