Aquares Resistivity Surveys

OEMG global has partnered with Demco NV (developer of the Aquares Resistivity System - Aquares) to bring the system to the Australasian region. The Aquares Resistivity System (Aquares) is a high resolution sub-bottom profiler modified from the proven principles and methods of land based geoelectrical survey. The Aquares includes enhanced acquisition and processing techniques that provide quantitative (i.e. depths and thicknesses) and qualitative (i.e. quality or resistivity) data for sub-bottom strata. It has brought resistivity to the forefront of fluvial and marine sub-bottom investigations. This page explains why the Aquares is the superior choice for shallow sub-bottom surveys in most circumstances over conventional acoustic systems.

What is the Aquares Resistivity System?

The Aquares is a sub-bottom profiling system that uses resistivity techniques modified from traditional land based systems to provide high resolution quantitative and qualitative data of both consolidated and un-consolidated sub-bottom strata. Quantitative data (commonly acquired from Acoustic reflection systems) is defined as data relating to depth and thicknesses of sub-bottom structures. Qualitative data is defined as data that relates to the quality of the observed structures (e.g. sand, silt or clay in the case of sediments and rock quality, fresh or weathered in the case of rocks). The unique ability of the Aquares to provide qualitative data is extremely useful for dredge and trenching operations. Utilising Aquares results engineers have a complete picture of the existing sub-bottom environment. This allows (e.g. port) designs to take into account existing sub-bottom structures such as paleo-channels as well as weathered and un-weathered rock outcrops too considerably reduced dredging and construction costs. Deployment methods have also been refined to allow the Aquares to be deployed as a standalone system or in concert with other geophysical systems.

Deployment Options

The Aquares Resistivity System is available in three varients:

• On land;
• Shallow water (0-20m); and
• Deep water (15 to 200m).

The land system is deployed using a standard light truck battery, the shallow system utilises a kevlar reinforced umbilical and the deep system a 600m long, 23mm diameter steel jacketed umbilical deployed from a winch. Additionally, the system may be modified to accommodate special circumstances or requirements such as deeper penetrations, however these are discussed on a required basis. The operating theory is common to all variants and is discussed below.

Figure 1 Respectively the Deep water, Shallow water and Land variants of the Aquares system. Note the Shallow water system (orange cable) operating in concert with a Boomer (blue cables on the right) and Side Scan (towed alongside – not visible).

Principles of Operation

Both the acquisition and processing methods of the Aquares have been modified from traditional land based resistivity methods. In traditional land-based resistivity surveys, an electrical current is injected into the sub-surface by means of two current electrodes or a Current Electrode Pair (CEP). The voltage gradient associated with the electric field of the injected current is measured by a Voltage Electrode Pair (VEP) placed between the CEP (Figure 1). Based then on the values of current and voltage measured between the VEP, the average or Calculated Resistivity (CR - measured as Ohmmeter (Ohmm)) for the volume of subsurface between the VEP is determined to the limit of penetration. Penetration is largely determined by the distance between the CEP. Therefore, multiple CEPs are placed at increasing distances about the VEP to cover a range of depths. The result is a field curve (Figure 1) that reflects the changing CR from each CEP. In this example, the CR for CEP 1 (Point 1) has returned a relatively low CR that rises quickly (points 2 to 3, etc). This is typical of a thin layer of soft sediments overlying rock. The above described field curves are then the basis for a qualitative assessment of the sub-surface geology.

Figure 2 Principles of Vertical Electrical Sounding - On land. Left: a typical land base setup for a resistivity survey. The VEPs are placed between the CEP. Current and voltage measured at the VEP is used to derive the Calculated Resistivity (CR). Right: a typical resistivity curve for a given injection of current as a function of CEP (X axis) and CR (Y axis).

The CR of a geological structure depends on its porosity, water saturation and the water resistivity. Gravel usually has a lower porosity than sand and its resistivity thus is higher. Clay with generally very high porosities shows very low resistivities. Conversely, solid rock has a low porosity resulting in very high resistivities. Every geological structure therefore has a unique resistivity.

Fluvial and Marine Applications

For fluvial and marine based applications the CEP's and VEP's are placed in a multichannel cable trailing behind the survey vessel (Figure 2). According to the circumstances the cable may be floating or towed on the seafloor. A floating cable may be more efficient in shallow water or if obstacles on the seafloor hamper the use of a bottom towed cable however, a bottom towed cable provides far better resolution and is therefore preferred. The electrode geometry is chosen such that good quality data may be obtained even for shallower targets.

Figure 3 Principles of Vertical Electrical Sounding - On water

While the survey vessel is underway measurements are carried out and stored automatically without any intervention from the operator. As such, an electrical sounding is be obtained every 2.5 seconds. At a boat speed of 2 m/s this corresponds to a horizontal resolution of 1 sounding every 5 meters. In applications concerning the exploration of alluvial diamonds this resolution may be needed to detect smaller diamond bearing "potholes" and buried channels.

A typical marine setup comprises a generator (3 phase, 400V), acquisition and control laptop, power regulator, switch box, signal controller, navigation control laptop, DGPS unit, single beam echo sounder, Conductivity Temperature and Depth (CTD) sensors, an umbilical and a resistivity cable (approximately 60m in length). The system then collects Resistivity, depth (via a single beam echo sounder) position and conductivity data. Resistivity is collected on the acquisition and control laptop while position and depth is displayed and collected on the navigation laptop. GPS time is delivered to both the navigation and acquisition computers and used to synchronise time system wide. Each single resistivity measurement is then tied to GPS time and later merged with navigation, depth, CTD and tidal data.

During the field survey qualitative results are shown on the acquisition computer. The quality of the field data may thus be monitored on line so the operator can intervene to adjust and optimise the survey parameters.

Data Processing and Interpretation

Many current land and marine resistivity systems simply provide a qualitative assessment. However, the Aquares has unique and proprietary modelling tools that enable the system to enhance qualitative and derive quantitative results from the field curves. Processing is undertaken through a sequence of operations in a two stage process: Field processing and final processing undertaken in the processing office.

Field Processing
First, the resistivity field data are edited and filtered to improve the signal/noise ratio. The bathymetric and positioning data are edited (cleaned) and if required, tides applied. The resistivity, positioning and bathymetric data are then combined.

Geometrical corrections are then applied to remove instances where the sailed line (including the cable) may show significant curvatures. Additional corrections are made to account for the water depth and changes in salinity throughout the water column. This then allows Demco and the client to have confidence in the coverage and data prior to demobilization from the field.

Final Processing
After field corrections have taken place post processing and interpolation of the collected data occurs off site. The resistivity data is interpolated onto a regular grid providing vertical and horizontal sections. Interpolation of the resistivity data is undertaken utilizing proprietary Demco software. Results are visualised in colour on cross sections showing the different geological structures as a function of depth and geographical position (Figure 3). At this stage, if available, the results may be calibrated with information from a limited number of boreholes in order to verify and sample each geological structure.

Figure 4 Horizontal and vertical resistivity sections from a pipe route survey in Ireland.

Reasonable qualitative and quantitative estimates of the sub-bottom strata is achieved in the absence of boreholes. However, when engineering studies are required, particularly for the characterization of rock and sediment types boreholes are considered essential. If a geotechnical survey is to be conducted after an Aquares survey, Demco is able to provide advice on the number and location of Boreholes required to compliment the survey.

If the survey design provides for sufficient density of data, a 3D representation (model) of the subsurface may be constructed. Vertical cross sections and horizontal slices may then be extracted in all possible directions and levels (Figure 3). The processing software also includes a module to calculate volumes of each structure found as a result of the Aquares survey.

The Aquares and Traditional Sub-Bottom Profiling

The Aquares is an invaluable part of any marine construction project. It allows the user to minimise the number of boreholes and tailor designs to utilise existing geological structures (Figure 5 and 6). The Aquares provides superior quantitative and qualitative results to traditional acoustic geophysical systems and is able to operate in geological and environmental conditions that would prevent the operation of acoustic systems.

Figure 5 The results of the JNPT port pre-construction survey allowed engineers to visualise the sub-bottom structures and take advantage of the paleo-channel (light blue) and paleo-delta areas (dark blue) to locate channels and vessel turning areas resulting in substantially reduces dredging costs.

Geotechnical Surveys
The high information density obtained with the Aquares allows for quicker and more accurate mapping of the study area when compared to geotechnical surveys. Compared to drilling, the Aquares method has the advantage that a much larger volume is being sampled by a single sounding. The subsurface volume sampled by a borehole corresponds exactly with the borehole diameter (a few centimetres), while the volume sampled by each electrical sounding may in some cases exceed 5 or 10 m in diameter. As such, the resistivity technique are more suitable for determining horizontal variations of sub-bottom structures as well as various degrees of fracturing and weathering in rock. As discussed, an Aquares survey does benefit from a limited number of boreholes to provide ground truthing, particularly in the case of engineering studies.

Acoustic Sub-Bottom Profilers
Classical acoustical methods for shallow sub-bottom surveys are often limited by prevailing geological and environmental conditions, these include:

• The presence of gravel in the subsurface tending to obscure information due to the appearance of diffractions.
• The occurrence of multiple reflections in shallow water that obscure real data.
• The possible occurrence of "gas masking" occurring in sediments rich in organic matter.
• The presence of strong shallow reflectors such as "cap rock" tending to reflect almost all acoustic energy preventing the exploration of lower geological formations.
• The inability of refraction to provide information about softer layers underlying hard layers (figure 6 and Errey J and Brabres P (2009)b)

Figure 6 A detailed image of resistivity (left) and refraction (right) results run over the same line. Resistivity clearly shows more detail of the sub-bottom.

Due to the nature of geo-electrical methods, the above mentioned effects do not cause problems in an Aquares survey. Furthermore, the results of an Aquares survey not only provide depths and thicknesses, but information about the nature (resistivity) of the geology (sediments and rock) including instances where soft or extremely soft sediments underlie compacted sediments or rock.

Figure 7 Highly fractured rock overlying less fractured rocks as a result of previous blast dredging.

Other Geoelectric Systems
Compared to more traditional marine geo-electrical systems, the Aquares sustains a very high sampling rate (down to 2.5 seconds per sounding). This is combined with the use of 3000 fold stacks, high electrical currents, signal enhancing electrode configurations, noise free electrode design and newly developed statistical techniques provide excellent signal/noise ratios. In addition, proprietary processing algorithms result in a significant increase of the vertical resolution of the shallower geological structures.

Survey Procedures

Based on geological information supplied by the client a noise evaluation is carried out to define the maximum resolution and penetration depth expected. If the conditions appear to be suitable for the geoelectrical survey to be successful the survey project is accepted. A typical Aquares survey involves the following steps:

PREPARATION - Based on the information provided by the client concerning water depth, depth of interest, exploration targets, general geology, geography and specific details concerning the local situation, survey parameters are determined and a multichannel cable is designed and constructed. Each project requires a cable to be designed and constructed to meet the requirements imposed by each specific survey site.

SURVEY - A typical survey crew consists of 2 or 3 geophysicists and the Aquares comprising Laptops, acquisition software, power supply, accessories, multichannel cable, positioning and sounding gear and spare parts. The positioning system and echosounder may be supplied either by Demco or by the client. The vessel with sailing crew is supplied by the client. The vessel should contain a shelter for the equipment and should offer a minimum of work space. The fieldwork of a typical Aquares survey is usually carried out in close cooperation with the client. Good communication with the client during the survey increases the efficiency by offering the flexibility to adjust survey strategies according to local field conditions.

PROCESSING AND INTERPRETATION - After the field survey data is processed following the procedures explained above the data may be calibrated based on additional information (eg bore holes) supplied by the client. In most cases subsequent drilling operations may be planned. Structural geological knowledge obtained from the Aquares survey allows borehole locations to be planned in a justified and systemic way reducing drilling costs to a minimum. Finally a report is prepared for the client.

Conclusions

The Aquares Resistivity System is a unique sub-bottom profiling system that is usually operated in water depth of up to 180m. The system is tailored to each job and will usually work in weather conditions in which the client provided vessel can operate safely. The Aquares system provides the client with a fast, reliable and high resolution sub-bottom data collection system that is not affected by issues that prevent or degrade data collected by traditional acoustic or electrical systems. The Aquares system will even provide data about (thickness and densities) and below high density (eg. cap rock) structures. Further information about the Aquares system or whether the system is right for a particular application, please contact Peteralv Brabers or Jason Errey.

Aquares Resistivity Surveys, Case Studies in Australia

Figure 1. An Aquares survey at Cape York utilising multiple and varied towed sensors.

OEMG is proud to have partnered with Demco NV and showcase the history of the Aquares system in Australia. The Aquares system is set apart from any other resistivity system and is at the forefront of technologies available to profile shallow sub-bottom geology due to proprietary acquisition and processing methods developed by Demco. No other system (reflection, refraction or resistivity based) offers the accuracy, or flexibility of the Aquares system.

Papua New Guinea Pipe Route Survey, NT

Figure 2. Here horizontal slices can be seen from various depths below seabed on the same line. Caprock can be seen from 0m to 1m below the seabed (green to red) and bedrock at 5m below the seabed. Enlarge »
Here horizontal slices can be seen from various depths below seabed on the same line. Caprock can be seen from 0m to 1m below the seabed (green to red) and bedrock at 5m below the seabed. Right click image to open in new window.

A pipe route survey was conducted between Cape York and Papua New Guinea for Exxon Mobil in 2005. The survey comprised Aquares Resistivity, pinger and Side Scan Sonar Surveys, all undertaken simultaneously. Post survey, a geotechnical study was carried out with borehole locations and numbers optimised using the geophysical results, primarily the Aquares results. In general, excellent correlations were noted between the resistivity and the seismic studies. However, in areas where cap rocks (calcarenites) were present and the acoustic systems failed to provide data, the Aquares system continued to function flawlessly.

Flinders Port, SA

Figure 3. Here vertical sections are shown from the resistivity and the refraction results from the same line. From the geotechnical results and the Aquares results, it is clear that there is a (high density) hard capping over softer sediments. However the refraction results incorrectly indicate a low velocity layer over a high velocity base structure. This velocity inversion is common in refraction results where there is a hard layer over soft sediments. Enlarge »
Here vertical sections are shown from the resistivity and the refraction results from the same line. From the geotechnical results and the Aquares results, it is clear that there is a (high density) hard capping over softer sediments. However the refraction results incorrectly indicate a low velocity layer over a high velocity base structure. This velocity inversion is common in refraction results where there is a hard layer over soft sediments. Right click image to open in new window.

A pre dredge study of the Flinders Port, access channel (2005). After geotechnical studies failed to confirm seismic refraction results, the client elected to carry out an Aquares resistivity survey. A thick caprock layer was found covering a significant portion of the access channel seabed. The lateral and vertical extent of this caprock layer was mapped in detail using the resistivity results of survey lines sailed every 20 m. The results were shown as horizontal and vertical sections extracted from a 3D resistivity model of the entire access channel. A comparison of the resistivity and refraction results are shown here. An Aquares line was run along the same line as a refraction line. The refraction results show a low velocity layer on top of a high velocity base structure, while the resistivity results suggest the opposite: a high resistivity structure on top of low resistivity sediments at depth. The resistivity results were confirmed by drilling and sampling results.

Port Dampier, WA

Figure 4. Here a thin veneer of sand/silt is seen overlying a calcarenite capping and a Granophyre basement. Enlarge »
Here a thin veneer of sand/silt is seen overlying a calcarenite capping and a Granophyre basement. Right click image to open in new window.

Port Dampire Port Expansion Mediation Survey for Boskalis and Van Oord marine contractors, 2007. During the Port Dampier port expansion, significant differences were noted by the dredge contractors between the actual seabed conditions and those described as a result of geophysical surveys undertaken by the consultant. In order to minimise further downtime and optimise the dredging, the client requested Demco to complete an Aquares survey of the area. The Aquares survey successfully distinguished between the calcarenite capping, Granophyre Basement and soft sediments. Following the study, the contractors were able to provide the consultant with a realistic estimate of time and cost for dredging operations.

Botany Bay, NSW

Figure 5. Varying grades of soft sediments are clearly distinguished in high resolution. A good correlation is also seen between the Aquares results and the bore log. Enlarge »
Varying grades of soft sediments are clearly distinguished in high resolution. A good correlation is also seen between the Aquares results and the bore log. Right click image to open in new window.

Port Botany expansion pre- tender evaluation survey, 2007. In order to respond to a dredge tender regarding the expansion of Port Botany, Van Oord Marine Contractors decided to carry out an Aquares resistivity survey in the area. A 3D resistivity model of the survey area clearly outlines different geological structures including harder and softer sediments.

Cape Lambert, WA

Figure 4. Here the horizontal distribution of calcarenites are visualised (red). As there are pre-dredge results, the operator is able to plan the most cost effective dredge methods. Enlarge »
Here the horizontal distribution of calcarenites are visualised. As there are pre-dredge results, the operator is able to plan the most cost effective dredge methods. Right click image to open in new window.

Cape Lambert Port expansion, Van Oord, 2010. Van Oord commissioned OEMG to undertake an resistivity survey in the expansion area of the Cape Lambert port utilising the Demco Aquares Resistivity System. Several harder cemented rock types described in bore holes, collectively here called caprock, were visualised horizontally and vertically. The client was provided with a very accurate map of the extents of the caprocks and their respective densities. They are able to appropriately apply dredge methods from "ripping" to "cutting" to cost effectively manage their campaign.

Aquares Resistivity System Projects

Projects undertaken by OEMG staff are listed here, for a full list of Demco projects visit http://www.demco-surveys.com.

Pre-dredge Survey, Cape Lambert
Van Oord commissioned OEMG to undertake a resistivity survey in the expansion area of the Cape Lambert port utilising the Demco Aquares Resistivity System. The Aquares survey was undertaken simultaniously with a multibeam survey at speeds of between 4 and 5 knots. The client now has a three dimential view along with a qualitative and quantative assesment of the calcarenites present and is able to tailor a dredge campain accordingly.

Luderitz, Namibia - Senior Surveyor
Namport has requested CSIR and Demco NV to carry out a combined resistivity, boomer and sidescan survey in the port of Luderitz, Namibia. The boomer and sidescan surveys were carried out by CSIR while the resistivity survey was carried out by Demco using the Aquares Resistivity System . All three systems were run simultaneously. The aim of the survey was to map subsurface geology in view of the planned port expansion.

Pre-dredge Survey, Mussafah Channel Abu Dhabi
This project was to provide information on a calcarinite layer within the dredge area.

Gravel Search, Sankaru River DR Congo
This survey involved the installation of the Aquares resistivity system onto a small boat in remote areas of the DR Congo to search for river gravel in the diamond baring Sankaru river.

Dredging reconnaissance surveys along the UAE and Mussendam coastlines
This project utilised the Aquares resistivity system developed by Demco Surveys, single beam surveys and positioning.

Djibouti Geophysical Assessment
This project involved undertaking a sub bottom survey of the area for the proposed container terminal. The water depth in the survey area varied between 2 and 20m with the client requiring 40m penetration. Despite the difficult conditions, we worked with the client to modify the system producing a dedicated tow cable that achieved good data to 35m below ACD.