At SEP we understand that a well designed and executed geophysical survey can be one of the most effective solutions for identifying, mapping and understanding the subsurface.
We offer a wide range of geophysical services, including designing, acquiring, processing, and interpreting data to deliver high-quality results that meet your specific application. From identifying archaeological remains to detecting mineralised zones, our geophysical expertise can provide you with the information needed to better understand your site.
We have successfully undertaken a wide range of geophysical surveys for clients in different industry sectors, including oil and gas, mining, utilities and telecommunications. Whatever your project requirements, we have the ability to gather fast, accurate and cost-effective geophysical data.
What is a geophysical survey?
A geophysical survey is a non-invasive method of imaging the subsurface, locating features without damage to the landscape or environment. Geophysical surveys can be conducted using electromagnetic (EM) technology, seismic, gravity and magnetic techniques. Often more than one method is used in the same survey to maximise the information yielded.
Geophysical methods can locate mine workings or voids, identify archaeological remains, support mineral exploration, map features under concrete or masonry, find faults, folds and mineralisations and search for cavities in dams and bridges. Geophysical surveys are used in many industries including mining, environmental services, geotechnical engineering and archaeology.
Geophysical Survey Techniques
SEP is adept in the use of a number of different techniques for conducting a geophysical survey. Our ability to choose an appropriate method is one of our strengths: we know that no technique will work in every situation and we recognise that each technique has its own strengths and weaknesses.
We use an integrated approach when necessary, combining the results of several geophysical techniques to provide accurate results to identify buried features. As well as increasing our ability to find objects, an integrated approach can help reduce costs by allowing us to conduct the survey more efficiently.
At SEP Geophysical we are experienced in conducting a wide range of geophysical techniques including:
Ground Penetrating Radar (GPR)
GPR is a non-intrusive and versatile geophysical technique, which can offer vast quantities of information about the shallow subsurface (~10m). Transmitted electromagnetic waves are reflected back to the surface when encountering objects/materials with differing dielectric properties to the surrounding ground. The reflected waves can then be analysed to produce a pseudo cross section of the ground beneath.
Utilising GPR technology that is at the forefront of its field, we are often requested (and recommend) that GPR is used as a reconnaissance technique to provide a broad screening of the underlaying ground conditions as well as on a more targeted approach to identify specific features of interest
- Mapping of areas of shallow voiding.
- Detection of Buried Utilities.
- Mapping lateral extents and depth of buried obstructions/foundations.
- Delineating Buried Tanks (USTs).
- Ground instability: Mapping of backfilled/poorly consolidated material.
- Structural Investigations such reinforcement mapping and slab construction.
- Bridge Deck Surveys
- Geological Investigations
- Pavement Layer Analysis
GPR is already a well-known technique within the construction industry and has wide ranging applications across the market. Through meticulous planning and careful post-processing, GPR is a powerful and useful tool for identifying features and hazards at a wide range of sites.
UXO Risk mitigation
SEP Geophysical personnel have a proven history of working in UXO risk mitigation. Having worked on projects ranging from small housing development sites through to large scale windfarm construction projects dealing with both land and marine environments.
SEP Geophysical are able to assist with any steps along the process such as:
- Organising and interpreting a desk study
- Non-intrusive survey and data collection and interpretation
- UXO Borehole Clearance (Down-hole magnetometer survey)
- And if required we can liaise with appropriate authorities and expedite the safe removal of identified items of UXO.
Always utilising the highest specification equipment such as the industry standard caesium vapour magnetometers and proprietary UXO data analysis software, SEP Geophysical are highly experienced and ready to assist with and UXO mitigation requirements.
Electrical Resistivity Tomography (ERT)
In geophysical investigations the electrical resistivity method is one of the longest established geophysical techniques. The basis of the method is that when current is applied by conduction into the ground through electrodes, any sub-surface variation in conductivity alters the current flow within the earth and this in turn affects the distribution of electric potential. The degree to which the potential at the surface is affected depends upon the size, location, shape and conductivity of the material within the ground. It is therefore possible to obtain information about the sub-surface distribution of this material from measurements of the electric potential made at the surface. Variations in electrical resistivity (or conductivity) typically correlate with variations in lithology, water saturation, fluid conductivity, porosity and permeability, which may be used to map stratigraphic units, geological structure, sinkholes, fractures and groundwater.
Resistivity imaging involves measuring a series of readings along a profile with the electrodes at a set spacing. The data can be collected using a variety of electrode configurations, the choice being selected by the geophysicist on site to meet the objectives of the survey. The selected electrode spread is interfaced to a computer controlled multi-core cable system which comprises four cables connected to a resistivity imaging meter.
Electrical resistivity imaging provides a complete data set from very close to the surface to considerable depth. The depth of investigation and the resolution achieved are dependent on the electrode spacing and electrode configuration used. Longer spreads will achieve a greater depth, trading resolution to do so.
Vertical Electrical Sounding (VES)
Vertical Electrical Sounding, commonly known as VES, is an electrical method which involves the rapid measurement of variations of the ground resistance with depth. By injecting a current into the ground, at a set probe spacing, allows for the measurement of the potential and hence the calculation of the resistance. This can then be used to calculate the resistivity at the centre of the deployed array, whereby varying the electrode spacings, builds up measurements to produce a 1D resistivity with depth sounding.
- Earthing Matt Systems
- Pipeline Routes
- Landfill Mapping
- Groundwater Investigation
- Soil lithology
VES is widely used within geotechnical investigations and has many applications across the construction industry. Due to its rapid acquisition, VES is viewed as an excellent reconnaissance technique for clients to gain a quick and accurate picture of the soil electrical properties to feed into their site designs.
Thermal Resistivity Testing
SEP Geophysical can conduct in-situ thermal resistivity testing, the TEMpos Thermal resistivity meter used by SEP Geophysical is ASTM 5334- and IEEE 442-compliant, and is engineered using ISO 2008 standards. It takes accurate readings of thermal conductivity, thermal resistivity, thermal diffusivity, and specific heat.
The thermal resistivity and conductivity of soils are critical in the design of underground power transmission systems. The purpose of the thermal resistivity testing is to provide the thermal conductivity (in W/m oK) or thermal resistivity (in oK cm/W) of soils at a selected depth. The tests, performed in-situ, are performed using the transient heat method and are conducted in accordance to IEEE Standard 442-1996.
Thermal resistivity tests are typically performed as part of the testing requirements prior to the installation of high voltage power lines or heated pipelines.
P & S Wave Seismic Refraction
Near-surface seismic refraction is the analysis of first arrival times of seismic waves as they are refracted from interfaces with varying acoustic/elastic properties. Through their examination it is possible to determine subsurface velocity and layer structures.
SEP Geophysical specialise in deploying P and S Wave seismic refraction in varying geometries in order to image and resolve the required targets.
Seismic refraction data is used to produce tomographic velocity models and interpreted cross sections.
Seismic refraction is a key method for understanding lithological variations. As one of the longest understood geophysical techniques, it is common to use the collected datasets to link boreholes information within a geotechnical investigation. This allows for comprehensive geophysical geological models to be constructed supporting greater acracy in the larger design processes of infrastructure projects.
- Depth to bedrock/rockhead
- Water Table Mapping
- Geological Interpretations
- Landfill Delineation
- Cavity and fracture detection
- Near-surface faulting
Another technique in the SEP Geophysical arsenal of survey methods is microgravity surveying. This mapping tool measures localised variations within the Earth’s gravitational field to determine areas of contrasting density. Data is acquired non-intrusively using the latest high accuracy gravimeters which can be deployed across linear profiles and/or equally spaced grids at survey stations in varying increments dependent on the depth of investigation.
SEP Geophysical are highly experienced with microgravity surveys. We understand that precise on-site acquisition, constant data quality monitoring and careful processing; where all corrections and modelling is meticulously carried out, are essential in obtaining results with such a sensitive technique.
- Mapping of variations in subsurface density.
- Voiding, cavities and solution features.
- Cave, Dene and Swallow Hole Detection
- Historic mine-workings.
- Landfill delineation
- Geological Mapping including near-surface fault detection
At SEP Geophysical we can provide a wide range of in-situ geophysical borehole testing methods such as;
- cross-hole and downhole seismic testing,
- parallel seismic testing and,
- downhole magnetometry.
Parallel Seismic Testing (PST)
As the industry looks to re-use or develop existing structures, information is often required regarding the installed depth of a buried pile or sheet pile wall. To determine pile depth a hydrophone array is lowered into a borehole placed close to the pile under investigation. The top of the pile is struck with a hammer and the prorogation of seismic waves through the structure are then measured. The point where there is a change in seismic velocity, where the seismic energy has passed from the pile into the underlying ground, indicating the depth at which the pile ends.
Downhole magnetometry is another good technique for pile determination or any structure that contains steel reinforcement. Measuring variations in the magnetic field at incremental depths down a borehole in close proximity to the structure under investigation. It is possible to identify the ‘toe’ or bottom edge of a steel or steel reinforced structure.
SEP geophysical would always recommend downhole magnetometry is deployed in conjunction with PST as a complementary technique allowing for a more robust analysis of pile /structure depth.
Cross-Hole and Downhole Testing
Cross-hole testing is conducted utilising parallel source and receiver boreholes in relatively close proximity. SEP Geophysical can conduct to a high standard, compressional and shear (P & S Wave respectively) cross hole testing, in accordance with ASTM D4428/D4428M-14, which provides high resolution one-way seismic data for such applications as to determine engineering properties. These quantities are vital for the design process for infrastructure projects.
- Determination of Rock Velocities
- Calculation of dynamic elastic moduli e.g. Young’s Modulus
- Buried Pile Depth
- Hydrological Mapping
- Void/Fracture Location
Similarly SEP Geophysical can also conduct downhole seismic testing in accordance with ASTM D7400/D7400M-19. This methodology differs from the cross-hole testing whereby only one borehole is required and the source(s) are positioned at the surface. Analysis of the first arrivals at various increments down the borehole can be used to calculate the P and S wave velocities as a function of depth.
What can a geophysical survey find?
At SEP Geophysical, our team of experienced geophysicists have the knowledge to guide and advise on which geophysical services techniques are most appropriate for your situation. Used in everything from civil engineering projects, mineral exploration to archaeological sites, geophysics is an important tool for gaining an understanding of the subsurface.
We consistently find answers for clients with difficult challenges. Some of the more common features we are asked to find include:
- Voiding, sinkholes, subsidence, karst and solution features
- Mine shafts, Adits or other mine workings
- Disused quarries /Borrow pits
- Rockhead profiling/engineering properties
- Faults and Fractures
- Underground storage tanks (UST’s)
- Unexploded Ordnance (UXO’s)
- Archaeological features and buried archaeological remains
What data is provided by a geophysical survey?
The type of information you can expect to receive varies between types of surveys and the data processing required. This information allows for an accurate interpretation and provides valuable insights into what lies beneath the surface making it easier to plan mitigation or site development strategies.
Benefits of geophysical surveys
There are numerous benefits to geophysical surveying. Geophysical surveys are an excellent way to identify subsurface features and physical properties especially in areas where traditional intrusive methods, such as digging or drilling, may prove hazardous; for example, if the ground is made up of fragile material or there is a risk of disturbing unexploded ordnance (UXO).
Geophysical surveys are non-intrusive. This means that there is minimal impact on the survey area. Subsurface features can be located without damage to either the features themselves or to the landscape. For this reason, geophysical surveys are often used as the first step in heritage assessments, site development and archaeological digs.
Geophysical surveys can be performed quickly. Geophysical surveys are commonly faster and obtain a more thorough area coverage when compared to intrusive techniques.
Geophysical surveys are cost-effective. Rapidly and comprehensively covering an area of interest allows for a more targeted approach in the next step of your project; reducing overall project costs.
Enhanced site management and mitigation planning. The results of geophysical surveys can help you understand what lies beneath the surface in your project area, giving you the information you need to make informed decisions about site management strategies.
What is involved with geophysical surveys?
Planning and executing a geophysical survey typically involves several key stages:
- Project Scope - Understanding the site, the survey objectives and timescale.
- Survey design - Determining which geophysical survey methods will work best for your project.
- Survey preparation - Detailing and preparing the equipment and people required to undertake the surveys.
- Site investigation - Mobilising to site and performing the geophysical data acquisition.
- Geophysical data processing - Assessing data and interpreting results or outcomes.
- Project Deliverables - Delivering an interpretive report based on your findings, alongside digital data deliverables.
How much do geophysical survey services cost?
The cost of performing geophysical surveys varies considerably based on the required survey equipment, the location and magnitude of the site, the survey design and approach, any specialist reports required and how long the surveys are scheduled to take. For this reason, it is important that you contact us for an individualised quote based on your own project requirements.
Geophysical survey costs can be broken down as follows: equipment hire, local labour for conducting surveys and processing data from those surveys, post-processing software costs and final report writing costs. Depending on your project requirements, this may vary from short-duration surveys to long-term campaigns.
Get a quote for geophysical surveys
To receive an individualised quote for your geophysical survey project or to discuss your geophysical surveying needs, please contact us.