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Workreport 1998-13

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Name:

Hästholmenin tutkimusalueen vuoden 1997 sähkömagneettisten Iuotausten tulkinta

Writer:

Markku Paananen; Matti Oksama; Heikki Soininen

Language:

Finnish

Page count:

193

Summary:

Working report: INTERPRETATION OF ELECTROMAGNETIC SOUNDINGS CARRIED OUT
IN 1997 AT HÄSTHOLMEN STUDY SITE


ABSTRACT

This report presents the interpretations of electromagnetic soundings at Hästholmen,
carried out by Geological Survey of Finland in 1997. The aim of the study was to obtain
information of saline groundwater and fracture zones within the bedrock. The total
amount of sounding points was 143 with coil separations of 200 - 800 m. The
interpretation included layer model and profile interpretations and compilation of three
resistivity sections. Moreover, 3-dimensional effects were studied with the aid of
numerical modelling. Also, sensitivity of the measuring system, as well as equivalence
and accuracy of the layer interpretations were examined. From the most prominent
indications, resistivity and total salinity (TDS) of groundwater was calculated by
Archie’s law using an apparent porosity value of 2 %. This value was obtained by fitting
the sounding results with chemical analyses of the water samples taken from boreholes.

From the point of view of electromagnetic methods, Hästholmen is a problematic study
site due to sivilization and the surrounding sea. The most reliable results were obtained
from the central and southern part of Hästholmen island with the coil separation of 200
m and also from the Källa area, northwest of Hästholmen. In these places, the effects of
sivilization and sea water have obviously been most insignificant. Some interesting
deeper indications of possible saline groudwater at the depth of c. 300 - 400 m were also
obtained with coil separations of 300 - 525 m. A typical result of interpretation was the
upper surface of brackish or saline groundwater located at maximum at the depth of c.
200 m.

TDS-values, calculated from the sounding results, varied between 1070 - c. 56000 mg/l.
From the total of 52 results, 22 can be classified as saline water (TDS > 10000 mg/l),
the rest being brackish (TDS 1000 - 10000 mg/l).

Key words: Geophysical, resistivity, electromagnetic sounding, saline groundwater


INTERPRETATION OF ELECTROMAGNETIC SOUNDINGS CARRIED OUT
IN 1997 AT HÄSTHOLMEN STUDY SITE


ABSTRACT

This report presents the interpretations of electromagnetic soundings at Hästholmen,
carried out by Geological Survey of Finland in 1997. The aim of the study was to obtain
information of saline groundwater and fracture zones within the bedrock. The total
amount of sounding points was 143 with coil separations of 200 - 800 m. The
interpretation included layer model and profile interpretations and compilation of three
resistivity sections. Moreover, 3-dimensional effects were studied with the aid of
numerical modelling. Also, sensitivity of the measuring system, as well as equivalence
and accuracy of the layer interpretations were examined. From the most prominent
indications, resistivity and total salinity (TDS) of groundwater was calculated by
Archie’s law using an apparent porosity value of 2 %. This value was obtained by fitting
the sounding results with chemical analyses of the water samples taken from boreholes.

From the point of view of electromagnetic methods, Hästholmen is a problematic study
site due to sivilization and the surrounding sea. The most reliable results were obtained
from the central and southern part of Hästholmen island with the coil separation of 200
m and also from the Källa area, northwest of Hästholmen. In these places, the effects of
sivilization and sea water have obviously been most insignificant. Some interesting
deeper indications of possible saline groudwater at the depth of c. 300 - 400 m were also
obtained with coil separations of 300 - 525 m. A typical result of interpretation was the
upper surface of brackish or saline groundwater located at maximum at the depth of c.
200 m.

TDS-values, calculated from the sounding results, varied between 1070 - c. 56000 mg/l.
From the total of 52 results, 22 can be classified as saline water (TDS > 10000 mg/l),
the rest being brackish (TDS 1000 - 10000 mg/l).

Key words: Geophysical, resistivity, electromagnetic sounding, saline groundwater


Keywords:

Geofysikaalinen; ominaisvastus; sähkömagneettinen luotaus; suolainen pohjavesi

File(s):

Hästholmenin tutkimusalueen vuoden 1997 sähkömagneettisten Iuotausten tulkinta (pdf) (5.7 MB)


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