Professor Stefan Buske explaining to field work participants how the seismic measurement system works.

Exploring the bedrock in the Sodankylä area

Text: Timo Hämäläinen

GTK and its partners surveyed the bedrock in the Sodankylä area using seismic sounding. Reflection waves provided researchers with data on the bedrock structure up to a depth of three kilometres.

The seismic soundings, conducted as a part of the XSoDEx project (Experiment of Sodankylä Deep Exploration), a joint project between GTK, Freiberg University of Mining and Technology, and the University of Oulu, began in early July. Over a period of two months during summer 2017, the researchers collected seismic data along tens of kilometers of gravel roads.

– The survey mapped bedrock in a region where Finland’s most promising ore deposits are located, says GTK’s research professor, Raimo Lahtinen.

On the northern side of Sodankylä, at the eastern end of the measurement line Boliden Kevitsa mine extracts nickel, copper and platinum-group elements (Ni-Cu-PGE) from an ore body. South of the ore body is the Sakatti Ni- Cu-PGE deposit. The mining company Anglo American describes it as one of the most significant European ore discoveries for more than a generation.

The measurement line continues towards the west, through the Petäjäselkä gold deposit to the Suurikuusikko area in Kittilä, where a company called Agnico Eagle Mines excavates gold ore. It is the largest gold mine in Europe.

– Local seismic soundings had already been conducted in the Kevitsa, Petäjäselkä, and Suurikuusikko areas before this. The measurement line of the FIRE4 sounding project, carried out in the early 2000s, also crosses Kittilä. These old surveys serve as comparison material for the new measurements, says GTK’s senior scientist, Suvi Heinonen.

Making the material available for everyone

The new vibroseis truck of the Freiberg University of Mining and Technology (TUBAF) was used for seismic sounding. The seismic waves generated by the vibrator reflect off structures when rock types change and the density or seismic velocity in the rock changes. The reflections are measured with small geophones that are inserted into the ground.

The University of Oulu contributed to the XSoDEx project by investigating the speed changes of seismic waves in the bedrock. TUBAF’s vibroseis truck was used as the seismic source, but the wireless geophones were placed at a distance three times as far from each other as in the main measurement line. These surveys complement the data on the structure of the bedrock.

– Seismic soundings do not necessarily produce data about the location of ore deposits. Instead, we are primarily interested in the structure of the bedrock that contains ore, in the locations of the ore deposits, and in the shapes of the ore veins in the bedrock, Heinonen explains.

The vibroseis truck has a shaker unit which has a base plate that is pressed against the ground. The whole weight of the truck, 32 tons, is lifted on top of this plate before the shaker unit produces a upsweep from 10 to 170 Hz.
In the Kevitsa mine area, the ore deposit is located in a mafic intrusion, whereas the gold deposits of Kittilä and Suurikuusikko are hosted by bedrock shear zones. The purpose of the surveys is to reveal the special characteristics of these geological bedrock types and to reveal anomalous zones such as faults or fracture zones.

In addition to seismic sounding, the project studied changes in the electric conductivity with audio-magnetotelluric AMT measurements and changes in density using gravity surveys. Densities, seismic wave velocity and electric conductivity is measured in a petrophysic laboratory from samples that are collected from the Earth’s surface, and from drill core samples.

Raimo Lahtinen notes that the reflection sounding was the most important part of the XSoDEx project, and the other geophysical measurements complemented it. One of the goals of the project is to develop the parameters used in seismic sounding to improve the accuracy of the method in specifying what the reflections measured from crystalline bedrock indicate.

– The sounding must be conducted carefully, and the quality of the data produced must be as high as possible so that it can be interpreted and refined afterwards, Lahtinen says.

Seismic sounding produces a two-reflection cross-sectional image of the bedrock structure. Later, researchers make a 3D model out of it. All materials, data and observations collected using different methods are going to be used to create the model.

– Interpreting seismic data takes time. When the work is finished, the material will be made available to everyone after an embargo period, Heinonen says.

Measurement progresses at walking speed

The measurement group consisted of approximately 20 people. Eight of them were from TUBAF, three were from the University of Oulu, and the rest were from GTK. GTK was responsible for organising everything and for many practical issues. GTK’s researchers also participated in the measurement work.

When conducting measurements, the vibroseis truck lowers a metal base plate to the ground so that the truck’s mass of 32 tonnes is supported by the base plate. After that, the machinery installed on the car shakes the base plate at a specific frequency range. In the Sodankylä project, the length of the measurement line was almost four kilometres, and the distance between each geophone was ten metres. After measuring 12 shot points with 40 meter interval, 48 geophones and their cables had to be moved from the back of the measurement line to the front of the line.

– The measurement team worked in two shifts, six days a week. We encountered unfortunate breaking of measurement devices in a sudden thunderstorm after which measurement slowed down significantly. However, our team worked fast and precisely and we were able to measure almost 80 km of reflection seismic profile despite the problems, Heinonen says.

Seismic sounding is sensitive to different kinds of interference. Even wind, rain or a person walking by cause noise. These errors can be eliminated because the same points are measured multiple times as the truck moves forward along the measurement line. Noise was minimized by stopping traffic along the line during the measurement and during shooting, people did not move in the vicinity of the geophones.

Reindeers were sometimes causing noise to the measurement by walking along the measurement profile.

The measurement equipment moved mostly on forest roads, and the entire personnel participated in a short training session on Finnish traffic regulations. GTK was responsible for traffic control.

– The residents in the area were informed about the project with an announcement in a newspaper and at a briefing session. Local papers also wrote about the measurements, because people are interested in this kind of operation, that brings tens of people in their reflective clothing to the forest roads, Heinonen says.

1. XSoDEx project used Geodes (yellow box) to make the measurements. Each Geode is connected to battery (orange box) for power supply. For each Geode, 24 geophones are connected to a geophone cable (black plug-in) and Geodes are connected to each other and finally to computer with yellow interface cable. 2. Seismic measurement required prompt book keeping not only by the operator in the recording van but also by vibroseis truck driver Michael Lay.

Expertise in seismic acquisition

Freiberg University of Mining and Technology (TUBAF) was responsible for the technical aspects of the seismic data acquisition. TUBAF provided the equipment and also contributes to processing the data afterwards to obtain a high-resolution subsurface image.

– XSoDEx is an important project for us in terms of establishing and strengthening cooperation with a leading geological survey in Scandinavia, and also a good opportunity to acquire interesting seismic data in the frame of mineral exploration, says Professor Stefan Buske from TUBAF.

– This was also a good opportunity to train our students in such field experiments. A total of ten TUBAF students were trained during the two months, Buske adds.

TUBAF’s expertise is the acquisition and processing of seismic data from different scales from very shallow investigations at around 100 m depth down to very deep investigations at around 100 km depth. Its main speciality is the development of seismic imaging methods and software to generate high-resolution images of the subsurface.

TUBAF has been doing seismic reflection acquisition for around a decade in various places on Earth, recently for example in central Sweden, in the Atacama desert in Chile, and on the South Island of New Zealand.

Measurement technology used in the Sodankylä area

  • Vibroseis truck generating seismic waves
  • Truck weight: 32 tonnes
  • Seismic waves frequency: 10–170 Hz
  • Exploration depth: up to 3 km
  • 3,600 m long geophone spread
  • 360 geophones
  • Distance between geophones: 10 m
  • Distance between shot points 20 or 40 m