Geophysical mapping for structural geology, prospecting and environmentprotection purposes

Andrzej Gajewski*, Stanis³aw Szczypa*, Adam Wójcicki*

Abstract . Applications of geophysical mapping to structural geology,prospecting and environment protection purposes are presented. Both clas-sic and more advanced geophysical methods can be employed to enhanceeither regional, semi-detailed or detailed mapping of geological structures.In the case of regional and semi-detailed studies, gravity andmagnetotelluric methods are supplementary to seismic, especially wheresignificant screening and attenuating of seismic waves occur. Detailedinvestigations of environmental protection issues might include a widerange of geophysical methods, but in the case of investigation of groundwa-ter pollution usually resistivity methods are applied. Examples of geophysi-cal works performed by the PBG Geophysical Exploration Company

illustrate these applications.

Key words: geophysical mapping, structural recognition, geophysical prospecting, soil and groundwater pollution detecting andmonitoring

The use of geophysical methods to solve tasks of struc-tural geology dates back almost to the beginning of the geo-logy as science. Usually it is assumed (e.g., Telford et al.,1990) that the beginning of geophysics is marked by Newtonand Gilbert works at the dawn of the modern era and thereforethe gravity and magnetic methods are the eldest — the “clas-sic” ones. Seismic methods, which are usually acknowledgedas the best tool for mapping and imaging of geological met-hod, were developed during the twentieth century, as were thegeoelectric and radioactivity methods, etc.

It is difficult to characterise the whole scope of geophysi-cal investigations because dozens of measurement techniquesare applied to surface and borehole studies (Telford et al.,1990) and there are also airborne and land geophysical met-hods. This applies as well to structural and prospecting issuesas to environmental protection studies (Vogelsang, 1995).

In general, working principles of geophysical techniqu-es are based on measurements of physical parameters ofgeological medium (or actually changes in these parame-ters). These studies provide us with information regardingnature and extent of geological structures and tectonics.Well logging techniques provide us with valuable informa-tion in boreholes, decreasing the need of expensive drillcore sampling (usually only 10% of the well total depth issampled now). Integrated surface geophysical investiga-tions (e.g., seismic and gravity and/or magnetotellurics,together with well logging) allow us to obtain a structu-ral-parametric model of the geological medium, verified byborehole information. So, geophysical investigations andtheir results belong to principal tools utilised by geologists.

Though the term “geophysical mapping” is not sowidely applied, there is no doubt that geophysical investi-gations were used in geophysical mapping for many deca-des worldwide, in Europe and in Poland.

Let us consider applied research priorities of the Fra-mework Programmes of European Commission (CORDISwebsite — www.cordis.lu). There are no traces of “geo-physical mapping” term in the relevant work programmesbut geophysics is present in the projects supported by EC,for example, among issues on structural recognition ofgeothermal systems, characterization of geological stora-

ges and, last but not the least, characterization and monito-ring of geo-hazards to sediment-groundwater-soil systems.

In Poland, a long-term policy of the Ministry of Envi-ronment on geological mapping (Ber & Jezierski 2004)includes a priority on establishing of (a digital & GIS) Inte-grated System of Geological Mapping (ISGM — referringto all geological structures, structural surfaces and relatedgeoscience information). This includes a component ofgeophysical mapping as one of the subsystems.

We can conclude that most of geophysical studies con-ducted in Poland by the PBG during the previous decadeshave actually resulted in geophysical mapping, so we canpropose a great deal of information to be included into theIntegrated System of Geological Mapping.

Geophysical data resources

The territory of Poland is covered by basic regional,semi-detailed (mostly) and detailed ground geophysicalsurveys, like seismic, gravity and magnetics. Both gravityand magnetic surveys include over a million of stations.Hundreds of thousands of geoelectric (resistivity) measure-ments were carried out. Well logging measurements havebeen conducted in hundreds of wells as well as laboratoryanalyses on millions of drill core samples. A great deal ofthese geophysical surveys and studies was completed bythe PBG Geophysical Exploration Company. Hundreds ofprojects involving each of the above methods were comple-ted, for various purposes. Databases in a digital form inclu-de most of that information, easily accessible for producinggeophysical maps. Surface data collected in the 1970s andearlier require converting from the “Borowa Góra” co-or-dinate system to any other, currently used co-ordinate sys-tem (and this is either time-consuming or inaccurate).

A range of geophysical data applications

The following application examples of geophysicalsurveys conducted by the PBG for various purposes can bementioned:

� detecting heavy metal traces in soil air,� detecting voids and caverns in post-industrial areas

of the Upper Silesia,

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*Geophysical Exploration Company, Jagielloñska 76,03-301 Warszawa, Poland

A. Gajewski A. WójcickiS. Szczypa

� evaluation of technical state of flood banks,� prospecting for drinking water,� monitoring of contamination around flotation rese-

rvoirs,� lignite prospecting,� geophysical surveys for pre-design studies on locali-

zation of nuclear power stations (8 projects in the 1980s),� salt dome structural recognition,� geophysical surveys and studies regarding hydrocar-

bon prospecting in Poland,� recognition of deep basement of the Carpathians

with magnetotelluric method,� aeromagnetic surveys in the Carpathians and the

Sudetes, etc.It is clear that utilization of archive and new, more deta-

iled geophysical data significantly supports solvingvarious tasks of geological cartography. For example, awide range of recently developed geophysical methods(DC resistivity, electromagnetic, microgravity and micro-magnetic methods, etc.) can be applied in environmentalcartography (Vogelsang, 1995).

Examples of application of geophysical mapping

Gravity surveys provided basic information for reco-gnition of regional geological structures of Poland. A good

example of application of gravimetry to geological map-ping is recognition of the Miocene basement features,which differ significantly in physical parameters (velocityor density), especially if at the top of the basement earlyPaleozoic to Precambrian formations appear or evaporatesare present (Karnkowski, 1993) below Miocene clastic for-mations. Generally, gravity residual anomalies (transforms)attributed to a depth range of (sub)-Miocene basement andoverlying Miocene formations are an excellent mapping toolto trace anticlinal and synclinal structures within the top ofbasement (Fig. 1), supporting seismic mapping where cove-rage of seismic surveys is incomplete or seismic data are ofinsufficient quality (e.g., Szczypa & Oniszk, 2001).

Residual anomalies reflect the depth range of the roof ofMiocene basement (0.5–2.5 km). Residual anomaly highs(yellow, orange and red) reflect elevated structures within theroof of Miocene basement (Wójcicki, 2003) and synclinalstructures are reflected by residual anomaly lows (green).

Another example is also related to structural featuresbut in a more detailed scale, on mapping of features withinthe top of Meso-Paleozoic basement of Carpathians (Fig.2). Seismic structural map was extrapolated, where no seis-mic profiles exist or seismic survey results are of poorquality, with the use of magnetotelluric data, acquired forthe whole area. On the basis of geological-geophysicalinterpretation (magnetotelluric, seismic, gravity, well-log-

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ging and geological data and concepts) a number of knownhydrocarbon structures was analyzed and then localizationof perspective structures has been proposed.

The third example refers to environmental geologymapping. Resistivity survey was employed to perform gro-undwater pollution monitoring around the flotation rese-rvoir of a copper mine. The pollutant agents in this case areheavy metals. Polluted areas are marked out as low resi-stivity zones around the reservoir (Fig. 3). So, changes ofthe extent of low-resistivity zones around the reservoir areattributed to changes of the extent of contamination.

Low resistivity (dark and light blue) zones are highlypolluted and high resistivity (red/orange) are unpolluted.Whether the zones of intermediate resistivity are pollutedor unpolluted it is resolved on the base of repeated measu-rements (monitoring of pollution changes). The workshave been performed by the PBG (Farbisz, 1986, 1997) inco-operation with the AGH University of Science andTechnology, Kraków for the Lubin Copper Mine.

Conclusions — geophysics within the IntegratedSystem of Geological Mapping (ISGM)

Integration of old and new data, including data conver-sions, with other geoscience data, will be possible withinthe frames of the long-term priority task of the Polish Mini-stry of Environment — the ISGM. Surely, the system willbe based on database components developed in the PolishGeological Institute (PGI website — pgi.gov.pl/pgi en).The project will start from elaborating a metadata compo-nent (information on all relevant geophysical data availa-ble). A geoelectric (actually DC resistivity) metadatabase,elaborated by the PBG and maintained by the PGI as acomponent of a geological database is an example of suchinformation. It is proposed to enable metadata presentationboth in text (catalog) and in graphic form for selected areas,defined by co-oordinates (rectangles), geological units,map sheets, survey areas, etc. Metadata attributes (GIStables) should include relevant survey information (met-hod, equipment, survey parameters, contractor, etc.). Thengeophysical data necessary for mapping applications shouldbe collected, merged (this include unification of co-ordinateand geodetic systems, data standards and formats wherenecessary) and stored in previously designed geophysicaldatabase components. Data can be either in digital form(enabling various ways of their presentations as plots, maps,sections) or raster format (selected categories of archivedata, which can be eventually digized or transformed intoraster formats like seismic SEG-Y/SEG-B data). It is impor-

tant to include all public geoscience informationinto the ISGM. This applies also to a great deal ofarchive information produced by all Polish explo-ration companies using public funds.

Finally, a subsystem of geophysical mappingis proposed enabling spatial analysis of geophysi-cal data (grids, sections, cubes, etc.) and theirtransforms in required scales (depending on theproblem). The end users of the whole systemshould be research institutions, prospecting com-panies, local authorities and citizens.

References

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Fig. 3. Resistivity Map reflecting a range of ground water pollution around theflotation reservoir in the copper field a mining area (resistivity soundings)

Cities and towns inthous. inhabit.

100 to 49920 to 995 to 19

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Wells where integrated well-logginginterpretation was conductedOther wells (selected)

Seismic profiles(selected)

0 2 4 6 8 10km

Main faults in the basement

Gas deposits(selected)

Oil deposits(selected)

Perspective structuresafter magnetotelluric, gravityand seismic interpretation

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Fig. 2. The roof of the basement of the Carpathians according toseismic (northern and central part) and magnetotelluric (southernpart) works, with indicated known hydrocarbon and perspectivestructures (after Wójcicki & Stefaniuk, 2002)

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