Case Study • Borehole Investigation In A Seawater Tidal Area • Robertson Geo Logo

Case Study • Borehole Investigation In A Seawater Tidal Area • Robertson Geo

Snapshot
Project
Port Development
Service
Provide Constraining Parameters for Modelling the Development and Planning of a Proposed Sea Wall and Port
Role
Service Provider
Location
Anglesey, United Kingdom
Environment
Port/Harbour
Region
Europe
Details
The customer required information to assist with constraining parameters for modelling the development and planning of a proposed sea wall and port.

This required logging a framework to investigate boreholes in a shallow water tidal area where the proposed sea wall and marine offloading facility would be constructed off the north coast of Anglesey.

Borehole depth was 10-80m - a suite of probes was used to identify fractures, fault patterns and ground stability, including Optical Televiewer, High Resolution Acoustic Televiewer, Full Waveform Sonic and Borehole Geometry with inbuilt X and Y calliper. Surface equipment was a 2,000m winch with 3/16” core cable, connected to a Micrologger system.

Logging of 26 holes was successfully completed over a period of two months. The Robertson Geo team supported two jack-up rigs, with Engineers on 24 hours availability over the duration of the project. The jack-up rigs were accessed from a small rib boat that docked at nearby Cemaes Bay.
 
Equipment
Sensors & Instruments
Robertson Geo's High Resolution Optical Televiewer (Hi-OPTV) provides a continuous very high resolution oriented image of the borehole walls using a conventional light source. A unique optical system based on a fisheye lens allows the probe to survey 360 degrees simultaneously. This information is processed in real time to produce a complete 'unwrapped' ... Moreimage of the borehole oriented to magnetic north. The probe offers superior resolution to the High Resolution Acoustic Televiewer (HRAT) and produces images in real colour. While, unlike the HRAT, it can operate in air-filled boreholes, it is unsuitable for boreholes containing mud or cloudy fluids.

WellCad™ Image-Processing Software
WellCad™ is a Windows-based package for processing, interpreting and displaying acoustic and optical televiewer image logs. Standard log presentations include tadpole and stick plots, stereographic projections of poles to planes and azimuth frequency diagrams. The synthetic core display allows convenient comparison of log and field data for orientation of fractured or incomplete core sections.
Robertson Geo's High Resolution Acoustic Televiewer (HRAT) provides a continuous high-resolution, oriented ultrasound image of the borehole wall. The probe uses a fixed acoustic transducer and a rotating acoustic mirror to scan the borehole walls with a focussed ultrasound beam. The amplitude and travel time of the reflected acoustic signal are recorded ... Moreas separate image logs.

Features such as fractures reduce the reflected amplitude and appear as dark sinusoidal traces on the log. The travel time log is equivalent to a 360°-arm caliper and shows diameter changes within open fractures and 'break-outs'.

Directional information is also recorded and used to orient the images in real time.
Robertson Geo's PS Logger Probe is a low-frequency acoustic sonde designed to measure compressional (P) and shear-wave (S) velocities in soils and soft rock formations. It operates using indirect excitation rather than mode conversion as in a conventional sonic. It is capable of acquiring high resolution P and S wave data in borehole depths of up to ... More600m in water.

The PS Logger probe contains a unique design of powerful hammer source and two receivers, separated by acoustic damping tubes. To acquire data, the probe is stopped at the required depth and the source is fired under surface command. Firing causes a solenoid-operated shuttle aligned across the borehole axis to strike plates on opposite sides of the probe in turn, setting up a pressure doublet in the surrounding fluid. The resultant fluid motion produces a tube wave at the borehole wall with velocity close to the shear velocity of the formation together with a compressional wave.

As the waves propagate parallel to the borehole axis, they set up corresponding fluid movements that are detected by the two neutral buoyancy 3D hydrophone receivers and geophones, allowing the wave velocity to be determined. The facility to stack multiple shots and filter the data as in normal seismic data acquisition is included in the operating software.
Related Case Studies
The project was to investigate the small strain moduli of the sea bed down to 45m using the Robertson Geo PS Logger. The acquired data would then be used to calculate the parameters for constructing the Vesterhav Offshore Wind Farm off the North West coast of Denmark. The geologic formations were typically sands, clays and mudstone.... More

Logging was conducted from the support vessel Freja, which has a compensated drilling platform. The PS Logger provided high resolution, shear wave and compressional velocity data in rock and soils at depths up to 500m, from measurements within a single borehole. Often it proves technically superior and more cost effective than a downhole or cross-hole seismic survey.

Thirteen boreholes were logged over a period of 10 weeks to obtain the P and S wave velocities of the sea bed down to approximately 45m at 1m intervals. These velocities were then applied to the small strain moduli equations with the addition of density, to give Shear, Bulk and Youngs modulus.

The logging probe was successfully deployed with 100% reliability; boreholes were logged from approximately 45m up to the overburden casing. Following logging operations, the operator would quickly process the data acquired. Robertson Geo Engineers were on-board the Freja throughout the project and available for logging through 24 hours each day.
Robertson Geo service engineers are highly trained and fully certified for offshore work. Logging on offshore locations demands versatility of both crews and equipment to get the job done. Here, the 2,000m marine winch has been suspended in the gantry to deploy geophysical probes for offshore subsurface investigation near New Bedford, Massachusetts, ... MoreUSA.

If you look carefully you can just about see the Robertson Geo logging crew behind the marine winch in this image of the set up prior to logging.

Working experience by Robertson Geo offshore logging crews has led to the introduction of the 2,000m marine winch, designed to resist corrosive and saline conditions. Precision engineered for reliable use in the most challenging offshore applications, the marine winch is fully compatible with the Micrologger2 surface data acquisition system and the full range of probes to depths of 2,000m.
Keywords
Geotechnical ReportingInstrumentationSite InvestigationBorehole Investigation
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