Microscale geophysics used in conceptual modelling

Background
A large part of the northern hemisphere has quarternary deposits consisting of glacial clay till. The till often has a complex hydrogeological structure consisting of networks of fractures, sand stringers and sand lenses each contributing to a transport network where water, free phase and dissolved contaminants are transported from the surface and downwards.

Understanding the nature and structure of these networks are crucial in developing conceptual models (CSMs) of the spreading of contaminants. The toolbox used for characterization of the geological and hydrogeological structures typically consists of different kinds of soil borings, direct imaging tools like EC and MIHPT and different hydraulic/pneumatic tests. On larger scale, different geophysical methods has been used intensively for many years for different mapping activities with success, but most superficial methods have fallen short using it at a local scale.  Due to that, research recently has been directed toward using these methods in different configurations and with more sophisticated data acquisition and processing trying to improve and optimize data collection.

Aim & Method
In 2015 the Capital Region of Denmark initiated a small program where the objective was to identify potential methods that could track/delineate sand stringers/lenses of a thickness of 10 cm in a 10 m by 10 m by 10 m clay till volume. Late summer of 2015 it was decided to test three different cross hole geophysical methods, which had the potential to succeed followed up by an excavation of the 150 m² test area for visual inspection of the volume.

The test was carried out in November/December 2015. The overall objective was to make a proof of concept for the methods, comparing the results and interpretations with the actual geology at the test site. Based on the results further testing of cross hole ground penetrating radar tomography (GPRT) took place October 2016.     

Conclusions
The test field was located near the digging front in a gravel pit west of Copenhagen. In the pit the geology comprises in general of a till clay with sand layers/stringers embedded down to 5-10 mbg. underlain by vast layers of sand and gravel. At the actual test location, the clay was 6-8 m thick. Three cross hole techniques was tested: Ground Penetrating Radar Tomography (GPRT), Seismic Waves Tomography and DCIP Tomography (DCIPT) on a 5-15 m scale.After geophysical testing the pit front was digged in a layer/staircase manner in order to get measures of the actual configuration of the embedded sand layers (size & orientation) giving a more complete picture than well logs alone.

GPRT and DCIPT measurements clearly correlated with the observations from the excavation and borings, the seismic methods could not differentiate between the different layers. The GPRT had the shortest distance between wells in the first setup and was further tested at a larger scale on another site. GPRT has the advantage of ease and economics compared to DCIPT. GPRT on the other hand, with the setup used, had shorter range than DCIPT and the amount and detail of info obtained also seems to be bigger with DCIPT.

Project type: Investigation

Project target: Soil