Joining geotechnical investigations with geophysical results

Introduction

The Swedish consulting company, Bjerking AB, has a long tradition of utilizing geophysical investigations in infrastructure projects. To achieve the most comprehensive results, Bjerking has developed an efficient method for jointly reporting geotechnical investigations (geotechnical soundings) with geophysical results. This ensures high quality data used for planning of all types of infrastructure projects.

In this case story, three examples are presented where both geophysical and geotechnical methods have been applied, and all results have been presented together. The examples showcase three geophysical methods: resistivity, GPR, and seismic, used in conjunction with geotechnical soundings.

Example 1: Resistivity for transmission line development

An extensive resistivity measurement, in total approximately 4.5 km, was conducted along a planned transmission line north of Stockholm, Sweden. The transmission line is going to be used for wastewater and the depth to bedrock was needed for the design. In Figure 1, a section of about 300 meter is shown together with the results from geotechnical soundings.

The resistivity investigation was made with an ABEM Terrameter LS2 with an electrode separation of 5 meters, using a Gradient protocol (Figure 2). For processing the Res2Dinv software was used.

Figure 1. Resistivity profile displayed together with geotechnical soundings. The black line represents the interpretation of the bedrock level.

Figure 2. In example 1, the Terrameter LS2 was used in the project to map soil sequences and the depth to bedrock.

Example 2: GPR investigations to calculate the soil volume

In order to calculate the mass balance of soils above the bedrock surface a GPR survey with a total length of approximately 1 km was conducted along a planned road in an area nearby Märsta, north of Stockholm, Sweden. A part of the profile, 130 meters, is shown in Figure 4.

The measurements were made with the MALÅ GX450 GPR system (Figure 3). Data was trigged by a wheel with a trace interval of 10 cm and a time window of 200 ns. For processing the ReflexWin software was used.

Figure 4. GPR measurements and geotechnical drillings at one and the same place from example 2.

Figure 3. Left: The GPR profile is displayed together with geotechnical soundings. The black line represents the interpretation of the bedrock level. Together with the radargram, three geotechnical soundings are shown. The purple structures in the radargram below the bedrock surface indicates zones of more fractured bedrock. Right: Interpretation of the bedrock level, if should only geotechnical soundings be used.

Example 3: Refraction seismics for ground investigations

Refraction seismic tomography was performed along several profiles with a combined length of approximately 2 km within an industrial area located in Upplands Väsby, north of Stockholm, Sweden. The area consists of a landfill, and the purpose of the investigation was to map the depth to bedrock and to better understand the geotechnical conditions of the area before planning constructions within it. Figure 5 shows a seismic tomography profile with a length of about 500 meters, combined with the associated geotechnical investigations and soundings.

The measurements were made with the ABEM Terraloc Pro 2, which has 48 channels. The geophones have a frequency of 4.5 Hz and were placed every 5 meters. A 40-kg accelerated weight drop mounted on an ATV was used to generate seismic energy at every 10 meters, providing high-resolution seismic images (Figure 6). To skip triggering cables and make seismic acquisition more flexible, a radio trigger system provided by ABEM was used. ReflexWin software was used for processing.

Figure 5. Results from a seismic refraction investigation. The black line represents the interpretation of the bedrock level. Note that the overburden consists of landfilled material and exhibits low seismic velocities (500-1400 m/s).

Figure 6. Seismic refraction tomography was acquired using the Terraloc Pro2 and an accelerated weight-drop in example 3.

Acknowledgement

We would like to thank Bjerking AB (www.bjerking.se) for sharing the information above.

Bjerking is an employee-owned technology consulting company with 430 employees that has been active in the industry since 1943. The company offers services in various business areas, including community development, buildings and real estate, life sciences, environment and sustainability, and digitalization. Bjerking is today established in Uppsala, Stockholm, Luleå, and Gothenburg, in Sweden.

In addition to other services, Bjerking has been offering geophysical services for about 25 years, since they acquired their first multi-electrode resistivity imaging equipment, the SAS4000 from ABEM. Today, Bjerking possesses specialized expertise and resources to perform several geophysical measurement methods, including resistivity, ground-penetrating radar, refraction, and reflection seismics, and geophysical borehole measurements.