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Aquares Resistivity Surveys

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OEMG global has partnered with Demco NV to provide this innovative system in the Australasian region. The Aquares Resistivity System is a high resolution, advanced geophysical method,  utilising enhanced acquisition and processing techniques that provide both quantitative (i.e. depths and thicknesses) and qualitative (i.e. quality or resistivity) data about the structural geology. Aquares has brought resistivity to the forefront of fluvial and marine sub-bottom investigations and, together with the Integrated Digital Ground Model (IDGM) is the superior choice for geological investigations.

What is the Aquares Resistivity System?

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 unconsolidated 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 consolidated units as well as intra-layer variability). High resolution qualitative data is extremely useful for planning construction activities including, dredging, trenching, under bores, or piling. Utilising Aquares results, project planners and engineers have a complete picture of the existing sub-bottom environment. This allows designs to take into account sub-bottom structures such as paleo-channels, weathered and un-weathered rock outcrops to considerably reduced dredging and construction costs. Marine deployment methods have also been refined to allow 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 variants:

  • On land
  • Shallow water (0.1m-35m)
  • Deep water (15m 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.

Left to right 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

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).

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.

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

Principles of Vertical Electrical Sounding – On water

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.

Typical marine setup, including generator, bang box and tow package mobilised on a small vessel.

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 900 miliseconds. At a boat speed of 2 m/s this corresponds to a horizontal resolution of 1 sounding every 2 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.

Vertical and Horizontal sections from Panama Canal expansion

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.

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 complement 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 results of the Eden port pre-construction survey allowed engineers to visualise the sub-bottom structures and take advantage of the the existing geological structure to reduce dredge costs and environmental impacts.

Aquares Resistivity System 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.

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 centimeters), while the volume sampled by each electrical sounding may in some cases exceed 5m or 10m 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. However, as the boreholes are targeted, they contribute to a robust analysis of the ground and provide associated opportunities or upside risk.

Acoustic Sub-Bottom Profilers

Refraction (top) shot along the same line as Aquares (bottom). The Aquares data is higher resolution and a palaeochannel is seen at chainage 480 that is not seen in the refraction data.

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 (Errey J and Brabres P (2009)b)

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

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. Finally, classical acoustic methods are 2 Dimensional in nature and do not contribute to project opportunities.

For example, reflection (or depth to layer) systems do not provide an indication of quality variation within a layer and is often provides insufficient resolution to distinguish structural layers and deposition layers. Towed refraction systems do not provide reliable height or high resolution quality data. Static refraction systems have the capability to provide high resolution quality data, the deployment of such a system is an expensive, slow process. Opportunities that can be derived from geophysical data are directly related to the resolution and reliability of the data. It is therefore critical to adequately define the needs and user requirements and assessing which technology can address required outcomes, before undertaking a ground study.

 

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.

Off the shelf resistivity systems are readily available (above) but do not acquire high resolution data of the Aquares system (below)

Survey Procedures

Flow diagram of the Aquares resistivity system

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, multi channel cable, positioning and sounding gear and spare parts. The positioning system and echo sounder 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.

References

Brabers P & Errey J (2010) “The Aquares Resistivity System, as an Exploration Tool for Rock Dredging Applications” in Proceedings of the Oceans 2010 IEEE Sydney Conference, Sydney, Australia. 24-17 May 2010. IEEE, Piscataway, NJ. Download the article here.

 

 

Errey J & Brabers P (2009), “Mapping Diamondiferous Gravel: Using the Aquares Resistivity System on a Fluvial Diamond Concession”, Hydro International, Vol 13, No 5, pp12-15. Download the article here.

Errey J & Brabers P (2009)a, “Aquares: A geophysical method for exploration of marine and fluvial sediments.”, Demco NV. Download the article here.

 

 

 

Errey J & Brabers P (2009)b, “A comparison of the Aquares Resistivity System and Seismic Refraction”, Demco NV.
Request this article here.