Some of the most significant variables affecting the technical feasibility and cost of constructing a directionally drilled crossing are generally related to the ground conditions and spatial constraints above and below surface at the crossing site. This is especially true for river crossings, where unfortunately for logistical reasons, it may be difficult to put down over water boreholes between the two river banks.

Over-water geophysical techniques can provide a potential solution to this problem, although from a geotechnical perspective, the best results are always obtained when geophysical methods are combined with the use of boreholes. A typical suite of over water surveys completed for an investigation includes precision bathymetry, side scan sonar and seismic reflection. Other methods such as electrical resistivity may also be employed depending on the investigation objectives.

Precision Bathymetry Precision bathymetry produces a detailed image of the variation of the river bed based on highly accurate measurements of the water depth. The depth measurements are acquired with a survey-grade bathymetric system which provides a display in real-time and acquires the data digitally. Multiple depth measurements are typically acquired per second and each measurement is tagged with a position.

Sidescan Sonar

Sidescan sonar is used to map surficial features and sediment types including mud, sand and rock on the river bed. This system produces a plan view image of the river bed to the left and right of the survey trackline. Discrete targets; cables, boulders, debris, geomorphologic features; scouring and bedding ripples or variations in sediment; silt, sand, gravel or rock produce varying shades, or intensity of print, on the sonogram record. High intensity reflections represent coarse-grained material, such as cobble or boulders, bedrock exposures, cultural artefacts; cables and sunken vessels and miscellaneous debris. Low intensity reflections of light grey to white represent fine-grained sediment such as silt and sand. Seismic Reflection

Seismic reflection profiling provides a continuous subsurface image of the river bed, the underlying stratigraphy and structural geology. The subsurface acoustic image is produced in real-time on a graphic recorder which displays the data as a profile or cross-sectional view along the transect. Subsurface reflection data is typically acquired every 1 to 2 m and the location of each data point is determined with a global position satellite system. Preliminary interpretation of the data can be done in the field without the need for additional processing.

The survey will typically be conducted using a low frequency seismic reflection system to obtain maximum subsurface penetration in fined-grained to coarse-grained sediment. The acoustic energy source is mounted on the side of the survey boat, with a hydrophone towed a set distance behind the acoustic transducer. The advantages of water surveys

Once mobilised to site, the surveys can typically be completed in a few days. The post processing of the data, digital map production and reporting usually take an additional couple of weeks. Also because the methods are non-intrusive, permitting for the investigation is easier when working in environmentally sensitive areas. All of these advantages and the insights the data provides, makes over water geophysical methods worth considering when planning any significant river crossing.