Quantitative resistivity methods for marine and fluvial site investigations

Peteralv Brabers Demco NV, Wintershoven, Belgium

Keywords: aquares, resistivity survey, geoelectrical, pipeline route, dredging, exploration

ABSTRACT: Geophysical methods are often being used in offshore site investigation programs as an explo-ration tool to define the general geological structure of the subsurface and its vertical and horizontal variabil-ity. Geophysical results are used either to define in a more economically and geologically justified way geo-technical sampling locations or to get more continuous information between widely spaced borehole and CPTlocations. Although earlier resistivity methods, as originally developed for land applications, have been usedin offshore applications for more than 15 years now, they often tended to produce less quantitative informa-tion as compared to the classical seismic methods and as a consequence the offshore site investigation mar-kets had lost their interest in them. The Aquares resistivity or geoelectrical method was developed specifically for offshore applications. The lastfew years this technique has been applied successfully on various marine and fluvial site investigation pro-jects and is more and more being used on port design & engineering projects, dredging reconnaissance pro-jects, pipeline routes and gravel and sand exploration projects. The resistivity data is acquired using a multichannel resistivity cable trailing behind the survey vessel on theseafloor. While the vessel is sailing electrical soundings are obtained every 3 seconds. During the fieldsurveyqualitative results are monitored on computer screen and allow already on site to define a number of subse-quent CPT or borehole locations in an economically justified manner. After processing the resistivity results are presented as colorcoded horizontal and vertical sections derivedfrom a digital 3D model of the geological subsurface. Quantitative marine resistivity methods can effectively be applied in conditions where gas-masking, multi-ple reflections in shallow water, diffractions on coarse gravel, cap rock tend to reduce the effectiveness ofclassical seismic reflection methods. A number of case studies are presented.

1 INTRODUCTION

As geotechnical investigations tend to be relatively expensive in offshore environments, offshore site investigators generally use geophysical methods to obtain a more detailed knowledge of the subsurface geology. Geophysical results were used either to de-fine in a more economically and geologically justi-fied way geotechnical sampling locations or to get more continuous information between the widely spaced borehole and CPT locations. For this purpose resistivity or geoelectrical methods, as originally de-veloped for land applications, have been used in off-shore applications for more than 15 years. As they had a reputation of producing less quantitative in-formation compared to the more classical seismic methods offshore site investigation markets had lost their interest in them. At this moment these older

methods are still doing useful work supplying for-mation factors and average resistivity values on ma-rine cable routes with less importance attached to more quantitative information on sediment thick-nesses and depths.

The Aquares resistivity method was recently de-veloped specifically for quantitative offshore appli-cations involving depths and thicknesses of geologi-cal structures. During the last several years this method has been successfully applied on various projects and is gradually again drawing the attention of port design engineers, dredgers, pipeline route de-signers and sand- and gravel markets.

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Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.) 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

2 PRINCIPLES

2.1 Land based applications

An electrical current is injected into the subsurface by means of two currrent electrodes. The voltage gradient associated with the electrical field of this current is measured between two voltage electrodes placed in between the current electrodes (see fig. 1a). Based on the measured values of current and voltage the average resistivity of the subsurface is calculated for a subsurface volume from the seafloor surface down to a certain penetration depth. The penetration depth depends on the distance between the current electrodes. Larger electrode distances are associated with increasing penetration depths.

If the measurements are repeated with progres-sively increasing current electrode distances infor-mation is obtained from progressively deeper geo-logical structures (fig. 1a). As such, a fieldcurve is obtained showing the resistivity as a function of the horizontal distance between current electrodes. After computermodelling this fieldcurve is transformed into a geophysical subsurface section showing the resistivity as a function of depth. Various algorithms exist to carry out the resistivity curve inversion (M.H. Loke and R.D. Barker 1996, L.R. Lines and S. Treitel 1984, Zohdy A.A.R. 1989, Koefoed O. 1970).

Figure 1a: Principles of Vertical Electrical Soundings On land

The resistivity of a geological structure depends on its porosity, water saturation and the water resistiv-ity. Gravel usually has a lower porosity than sand and its resistivity thus is higher. Clay with generally very high porosities shows very low resistivities. Solid rock, on the other hand, has a low porosity and shows very high resistivities. Weathered rock tends to show relatively lower resistivities compared to solid rock. Every geological structure thus has its own specific resistivity value.

2.2 Marine/fluvial applications

For water based applications the electrodes are placed on a multichannel cable trailing behind the survey vessel (fig. 1b).

During the fieldsurvey qualitative results are al-ready shown on computer screen. This allows on site

for a quick and well justified choice of subsequent CPT and borehole locations.

2.3 Data Processing

A complicated sequence of mathematical operations has to be followed before any interpretable results can be obtained.

First, the resistivity field data are edited and fil-tered to improve the signal/noise ratio. Geometrical corrections are applied to correct for the fact that the resistivity cable may show more or less significant curvature. Measurements made with a strongly curved cable are rejected. Other corrections are made to account for current losses into the water- column.

Figure 1b: Principles of Vertical Electrical Soundings On water

After interpolation of the resistivity information into a regular grid a digital 3D model of the subsur-face is obtained. The results are visualised in color on vertical and horizontal cross sections showing the different geological structures in function of depth and geographical position.

The processing procedure described above is an interactive process. In order to extract a maximum of information out of the raw survey data the process-ing sequence has to be repeated several times to find the optimum processing parameters.

3 ADVANTAGES

At a speed of 1 sounding every 3 seconds the Aquares resistivity method is very fast compared to mechanical drilling or CPTs. It also has the advan-tage that a much larger volume is being sampled by a single sounding. While the subsurface volume sampled by a mechanically drilled borehole corre-sponds exactly with the borehole diameter (a few centimeters), the volume sampled by an electrical sounding may in some cases exceed 5 or 10 m in di-ameter. As a consequence, resistivity methods are quite suitable for determining various degrees of fracturing and weathering in rock.

Classical acoustical geophysical methods often have difficulties coping with geological situations limiting their effectiveness:

428 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

The presence of gravel in the subsurface tends to obscure the information due to the appearance of diffractions.

The application in shallow water causes multiple reflections to obscure the data.

The application in areas with sediments rich in organic matter may cause gas-masking prob-lems.

The presence of cap rock on the seafloor tends to hide all geological structures underneath by reflecting most acoustical energy.

Thanks to the nature of geoelectrical methods, the abovementioned effects generally are not bother-some to resistivity methods. Furthermore, the results of a quantitative resistivity survey not only provide depths and thicknesses, it also results in a resistivity value for each geological structure which is an indi-cation of its porosity.

Thanks to the use of 3000 fold stacks, high elec-trical currents, signal enhancing electrode configura-tions, noise free electrode design, newly developed statistical techniques and appropriate processing al-gorithms signal/noise ratios have been improved significantly.

Rather than give an in-depth insight into the tech-nical aspects of the method, this papers merely aims to present a number of case studies showing the pos-sibilities of quantitative marine resistivity methods. For practical reasons the original colorcoded resis-tivity sections have been rendered in black-and-white by various ways of hatching.

4 CORRIB PIPELINE LANDFALL

In 2001 a resistivity survey was carried out at the landfall area of the Corrib gas pipeline along the West-Coast of Ireland. The proposed burial burial depth of the pipeline was 3m below seabed level. The geology is known to consist of sand, clay and gravel overlying a very hard metamorphic basement rock. About 10 survey lines were sailed parallel to the proposed piperoute, a 3D model was worked out including a number of horizontal and vertical sec-tions to represent the geological subsurface. A hori-zontal section parallel to the seafloor at the proposed burial depth of 3 m is shown in figure 2 as well as a vertical resistivity section along the proposed pipe route.

Figure 2: Corrib pipeline landfall resistivity survey

High resistivities (> 4 Ohmm) and intermediate resistivities (2-4 Ohmm) are hatched following the resistivity legend. High resistivity basement rocks reaching pipeline burial levels are visible at KP500. Other rockheads are located at deeper levels at KP850 and south of the route at KP1250. Intermedi-ate resistivities correspond to gravel as confirmed by drilling. The lower resistivities (< 2 Ohmm) corre-spond to sand and clay.

Based on these results trenching costs were esti-mated and pipe burial operations were planned.

5 WEST-INDIA PORT DEVELOPMENT

In 2000 a resistivity survey was carried out along the West-Coast of India in view of the development of a new port. The exact location is not disclosed for commercial reasons. The geology along the West Coast of India is generally known to consist of mudand clay on basalt rock. Seismic exploration meth-ods often fail because of multiple reflections in shal-low water and effects of gas-masking in organicrich muds.

A large number of resistivity lines were sailed in various directions following the accessibility of the

4-30 Ohm

survey area, a 3D model was worked out including horizontal and vertical resistivity sections as the one shown in figure 3.

The upper part of figure 3 shows a horizontal sec-tion at 11 m below Chart Datum while the lower part

shows a vertical section along profile line P3 marked on the horizontal section. The vertical scale is exag-gerated 100 times.

Figure 3: West-India port development resistivity survey

The vertical resistivity sections shows a low resis-tivity top structure of soft mud (< 0.5 Ohmm) and more consolidated marine clay (0.5-0.7 Ohmm) on top of a substratum consisting of high resistivity solid basalt (> 3 Ohmm) and weathered basalt with slightly lower resistivities (2-3 Ohmm) Intermediate resistivities (1-2 Ohmm) correspond to detritic sediments of continental origin deposited in valleys cut into the basaltic basement.

Based on above resistivity results, a drilling cam-paign was organised involving a limited number of (expensive) boreholes to confirm the geotechnical nature of each of above described structures. Based on the combined geophysical and geotechnical in-formation turning basins, access channels and rec-lamation areas were designed in a most cost-effective way.

6 INN RIVER RESISTIVITY SURVEY

In 2003 a site investigation program was carried out on the river Inn in Austria and Germany along a 4 km long river section located downstream of a high dam. The geology in the survey area is known to consist of two gravel units separated by a clay inter-calation. As the lower gravel unit is an important aquifer contributing to the regions drinking water supply local authorities were worried about man-induced effects of river erosion cutting through the impervious clay layer and exposing the lower aqui-fer to contaminated surface water.

In order to obtain more detailed information on the geological structure of the river bed a resistivity survey was carried out on the river before starting the drilling campaign. Seven lines were sailed at about 10 m interspacing, a 3D model was con-

0,5 0,7 Ohmm

Resistivity Scale

> 3 Ohmm

2 3 Ohmm

1 2 Ohmm

< 0,5 Ohmm

P3

P3

-12

-8

-4

0

1 km

weatheredbasalt

Solidbasalt

Soft mudmarine clay

Continental sediments

-11 m

Solid basalt

Marine clay

Continental sediments

430 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

structed and presented as various horizontal and ver-tical resistivity sections. Figure 4 shows some ex-amples of such horizontal and vertical sections lo-cated between KP207.5 (downstream) to KP209.5 (upstream). The upper horizontal section is situated

at NN458 (458 m above German Chart Datum) in the upper gravel unit while the lower horizontal sec-tion at NN454 is cutting through the lower gravel unit.

Figure 4: Inn river resistivity survey

Subsequent borehole information made clear that high resistivities (>150 Ohmm) shown in Figure 4 correspond to sandy gravel while lower resistivities (80-120 Ohmm) correlate with finer sediments such as sand and silt deposited as channel-fill sediments. Very low resistivities (< 80 Ohmm) are associated with the clay intercalation mentioned above. Very low resistivities are also associated with clay and silt deposits found at KP209.3 at the confluence of a small tributary on the right bank of the river. The vertical section below in Figure 4 shows this silt and clay deposit to be very thick. Apparently the vortex associated with the confluence of both the Inn and its tributary must have been cutting through both gravel units as well as the clay intercalation. The idea that upper and lower gravel formations were two hydrologically separate units thus has to be abandoned. Even between KP208.5 and KP209.1 the low resistivity clay intercalation seems to be missing and mostly replaced by gravel.

7 MIDDLE EAST SAND SEARCH SURVEY

A resistivity survey was carried out on a location in the Persian Gulf with the aim of finding sand for reclamation purposes. The geology consisted of sand and silt sediments on top of a basement rock. A large surface area was overgrown with coral fields. Previ-ous seismic and geotechnical surveys failed to find sand deposits in this area.

Figure 5: Sand search in caprock covered area

The western reaches of the resistivity section shown in Figure 5 show a high resistivity caprock layer (0.8-3.0 Ohmm) on top of a 4 to 5 m thick sand deposit ( 150 Ohmm

NN 485 m

NN 454 m

Resistivity Scale

Km 208

km 209

450

454

458

462

Km 208 Km 209

Sandy gravel Silty clay

Upper gravel unit Lower gravel unit

Clay intercalation

Sand and silt

< 80 Ohmm

Silty clay

Tributary

confluenceSand and silt

0,8-3,0 Ohmm

0,7-1,5 Ohmm

< 0,7 Ohmm

W E

1000 2000 3000

-25

-20

-15

Cap rock sandrock

431Proceedings ISC2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

tative resistivity methods offer the possibility to de-velop digital 3D models of the geological subsurface which can be visualised as colored horizontal and vertical sections.

REFERENCES

Koefoed, O. 1970. A fast method for determining the Layer Distribution from the Raised Kernel Function in Geoelec-trical Sounding. Geoph. Prosp. 18, 564-569.

Lines, L.R. and Treitel S. 1984. Tutorial. A review of least-squares inversion and its application to Geophysical Prob-lems. Geoph. Prosp. 32, 159-186.

Loke, M.H. and Barker, R.D. 1996. Rapid Least-Squares In-version of Apparent Resistivity Pseudosections by a quasi-Newtonian Method. Geoph. Prosp. 44, 131-152.

Zohdy, A.A.R. 1989. A new method for the automated inter-pretation of Schlumberger and Wenner sounding curves. Geoph. 54, 245-253.

432 2004 Millpress, Rotterdam, ISBN 90 5966 009 9