First International Conference on Saltwater Intrusion and Coastal Aquifers—Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001

Identification of Saltwater Intrusions and Coastal AquifersUsing the BGR Helicopter-borne Geophysical System

B. Siemon 1, K.-P. Sengpiel 2, H.-J. Rehli 2, B. Röttger 2 and D. Eberle 2

1 Institute for Joint Geoscientific Research, Hannover, Germany2 Federal Institute for Geosciences and Natural Resources, Hannover, Germany

ABSTRACT

As part of a research project of the Institute for Joint Geoscientific Research [Kesselset al., 2000, 2001], the Federal Institute for Geosciences and Natural Resources(BGR) conducted an airborne survey in the Coastal Aquifer Test Field (CAT Field)between Bremerhaven and Cuxhaven / NW Germany covering more than 500 km².The objective was to delineate saltwater intrusions from the North Sea into coastalaquifers by determining depths to the groundwater table or to the freshwater/saltwaterinterface, respectively. Only two weeks were required to complete the survey inwhich 2500 line km were flown with a nominal flight-line spacing of 250 m.

The BGR airborne system permits simultaneous electromagnetic (AEM), magnetic,and natural gamma-ray surveying. The AEM system is mounted in a bird towed about45 m below the helicopter and is kept 30 to 40 m above ground (Figure 1). The arrayof the transmitter and receiver coils is horizontal-coplanar. Each coil has a rectangularshape increasing its dipole momentand improving the signal/noiseratio. The transmitter coils are op-erated at frequencies of 383, 1.830,8.600, 41.000 and 192.000 Hz.Coil separations are about 6.7 m.Maximum depths of investigationare in the order of 150 m depend-ing on the mean resistivity of theunderground. The secondary mag-netic fields induced in the conduc-tive underground are picked up bythe receiving coils for each fre-quency, sampled every tenth of asecond and split into their in-phase(R) and out-of-phase (Q) compo-nents. An atomic absorption (Cs-)magnetometer, a laser altimeterand a GPS/GLONASS system areintegrated into the bird. The dis-tance between consecutive valuesis in the order of three meters as-suming an average flight velocityof 150 km/h during survey. Rec-ords are both digital and analogue.

Figure 1: Principle of the BGR helicopter-borne geophysical survey system.

First International Conference on Saltwater Intrusion and Coastal Aquifers—Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001

A 256-channel gamma-ray spectrometer provided with a 16.8 L downward-lookingcrystal pack and mounted at the bottom of the helicopter was used to collect naturalradiation data. A radar altimeter and a PICODAS navigation system are compulsoryto maintain differences between nominal and actual flight parameters as small as pos-sible.

The R and Q components of the secondary field measured in ppm of the primary fieldwere inverted to ground resistivities and depths of burial. New or modified algorithmswere used to calculate both homogeneous and layered half-space parameters [Seng-piel and Siemon, 2000; Siemon, 2001]. Although these inversion algorithms are basedon models of laterally homogenous ground they proved to be highly useful for arbi-trarily structured ground if lateral resistivity contrasts are not too large [Sengpiel andSiemon, 1998].

Due to the five-frequency configuration of the EM data acquisition system informa-tion from different depths was obtained and subsequently inverted into five-layermodels. The three-dimensional resistivity distribution in the survey area was dis-played by apparent resistivity maps for each frequency (Figure 2) and vertical sectionsreflecting the resistivity-depth relation for selected flight lines (Figure 3).

Figure 2: Apparent resistivity map of the frequency 1830 Hz.

First International Conference on Saltwater Intrusion and Coastal Aquifers—Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001

In terms of resistivity, the survey area can be subdivided into three major units (cf.Figure 2). The central part is depicted by increased apparent resistivities neatly corre-lating with the sandy ‘Geest’ ridge. Low apparent resistivity units are predominant inthe south-west and north-east of the survey area where marshlands are widespread.

The resistivity lows in the south-west (around Dorum) and in the north-east (aroundAltenbruch) which are consistent with depth are interpreted as saltwater intrusions.The low east of Altenwalde, however, decreases in size with increasing depth. This isa strong indication of a near-to-surface clay lense embedded in sediments mainlycomposed by sands.

Some mostly SSW/NNE striking conductive features which are considered the signa-ture of the lid clays deposited on top of the quaternary glacial meltwater channels (e.g.Bremerhaven-Cuxhaven channel and Oxstedt channel [Besenecker et al., 1981]) areincised into the central resistivity high.

Surprisingly, increased apparent resistivity values were mapped off-shore, above all,close to the north-western tip of the mainland. A freshwater aquifer extending sea-wards across the shore line is supposed to be the geological source of this resistivityhigh.

Figure 3: Inversion results from flight line 58.1. Top: Vertical section of apparentresistivity/depth ρa(z

*p)-models obtained from inversion of measured R- and

Q-data. Bottom: Final five-layer resistivity/depth models individually ob-tained at each measuring site. The iterative Marquardt inversion procedurewhich uses starting models based on ρa(z

*p)-models.

First International Conference on Saltwater Intrusion and Coastal Aquifers—Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001

The vertical resistivity distribution is depicted better by vertical sections reflecting thedepth to the groundwater table, the freshwater/saltwater interface down to about 80 mdepth. Furthermore, saltwater intrusions up to 10 km inland, off-shore freshwateroutlets, sedimentary characteristics, and several glacial meltwater channels wereclearly identified. An example is shown in Figure 3 showing a WNW-ESE line of thecentral survey area. Further resistivity-depth sections and maps will be shown [seealso Eberle and Siemon, 2001].

Compared with ground electrical methods, AEM is a very fast and cost effectivemethod to provide a valuable data base for monitoring, modeling, and management ofsaltwater intrusions. Particularly, it is planned to interpret the resistivities in terms ofsalinity [Willert et al., 2001] to serve as a sound basis for building a 3-D voxel modelof the CAT-Field area.

References

Besenecker, H., v.Daniels, C.H., Hofmann, W., Höhndorf, A., Knabe, W. and Kuster,H., “Horizontbeständige Schwermineralanreicherungen in pliozänen Sandendes niedersächsischen Küstenraums,“ Geologisches Jahrbuch, Reihe D, 49,23 p., Hannover, Germany, 1981.

Eberle, D. and Siemon, B., “Identification of saltwater intrusions and coastal aquifersusing the BGR helicopter-borne geophysical system,“ In: Proc. SAGEEP2001, 12 p., EEGS, Denver, CO, 2001.

Kessels, W., Dörhöfer, G., Fritz, J. and Fulda, C., “Das Forschungsprojekt Bremer-havener-Cuxhaven Rinne zur Beurteilung von Grundwasservorkommen inRinnesystemen,” In: Arbeitshefte Wasser, Heft 2000/1, Hannover, 189-203,2000.

Kessels, W., Fulda, C., Binot, F., Dörhöfer, G. and Fritz, J., “Monitoring and model-ing in the Coastal Aquifer Test Field (CAT-Field) between Bremerhaven andCuxhaven in the northern part of Germany,” In: Proc. SWICA-M³, Essaouira,Morocco, 2001.

Sengpiel, K.-P. and Siemon, B., “Examples of 1-D inversion of multifrequency HEMdata from 3-D resistivity distributions,“ Exploration Geophysics, 29, 133-141,1998.

Sengpiel, K.-P. and Siemon, B., “Advanced inversion methods for airborne electro-magnetics,“ Geophysics, 65, 1983-1992, 2000.

Siemon, B., “Improved and new resistivity-depth profiles for helicopter electromag-netic data,” J.Appl.Geophys., 46, 65-76, 2001.

Willert, T., Behain, D., Fulda, C., Worzyk, P. and Kessels, W., “Regional saltwaterdistribution in the Coastal Aquifer Test Field (CAT-Field) between Bremer-haven and Cuxhaven, Germany, by DC-geoelectric measurements,” In: Proc.SWICA-M³, Essaouira, Morocco, 2001.

Keywords: Helicopter geophysics, airborne electromagnetics, electrical resistivity

Corresponding author Dr. Bernhard Siemon, Institute for Joint Scientific Research,Stilleweg 2, D-30655 Hannover, Germany. Email: b.siemon@gga-hannover.de