OEMG Digital Geophysics Surveys

OEMG Digital Geophysics for marine environments is a high resolution, advanced geophysical method. OEMG Digital Geophysics  utilises 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 upto 40m below the ground. OEMG has brought digital geophysics 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 OEMG Digital Geophysics?

OEMG Digital Geophysics 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 OEMG Digital Geophysical methods to be deployed as a standalone system or in concert with other geophysical systems.

Deployment Options

OEMG Digital Geophysical systems are 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 utilises 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.

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.

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, OEMG Digital Geophysics has unique proprietary modelling tools that enable the system to enhance qualitative and derive quantitative results from the field curves.

Field Processing

OEMG Digital Geophysics 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 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. Data are interpolated onto a regular grid providing vertical and horizontal sections. Interpolation of the resistivity data is undertaken utilizing proprietary 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.

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. The processing software also includes a module to calculate volumes of each structure found as a result of the OEMG Digital Geophysics survey.

OEMG Digital Geophysics and Traditional Sub-Bottom Profiling

OEMG Digital Geophysics 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). OEMG Digital Geophysics 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 OEMG Digital Geophysics allows for quicker and more accurate mapping of the study area when compared to geotechnical surveys. Compared to drilling, OEMG’s Digital Geophysical methods have 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, OEMG’s Digital methods 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 OEMG Digital Geophysical 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 OEMG Digital Geophysics (bottom). The OEMG 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 OEMG Digital Geophysical survey. Furthermore, the results of an OEMG 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.

 

Compared to more traditional marine geo-electrical systems, the OEMG’s Digital Geophysics sustains a very high sampling rate (down to 0.9 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 an OEMG Digital Geophysical survey to be successful, the survey project is accepted. A typical OEMG Digital Geophysical 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 OEMG Digital Geophysical System 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 OEMG or by the client. The vessel with sailing crew can be supplied by the client or OEMG. The vessel should contain a shelter for the equipment and suitable work space. The fieldwork of a typical Digital Geophysical 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.

The Future

OEMG Digital Geophysics is a fully digital sub-bottom acquisition system. Data can be directly inserted into just about any package that facilitates sustainable development, design or data communication.

Conclusions

The OEMG Digital Geophysical 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 OEMG Digital Geophysical 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 OEMG system will even provide data about (thickness and densities) and below high density (eg. cap rock) structures. Further information about OEMG Digital Geophysics or whether the system is right for a particular application, please contact OEMG Global.