Geophysical Survey refers to the systematic gathering of geophysical data related to the physical properties of the Earth (subsurface) by using various geophysical techniques such as magnetic, gravity, electrical resistivity.
It also includes analyzing and interpretation of geophysical data in order to resolve the geophysical problem on hand.
The Physical properties can be gravitational Field, Magnetic Field, Electrical conductivity, but what is exactly measured is their physical parameters such as density contrast, magnetic susceptibility, resistivity, respectively.
Types of geophysical surveys
This is on how the geophysical method being deployed can either be one of the following.
Airborne Survey
This survey is deployed at a height above the Earth’s surface, such that it involve measuring of geophysical properties from the air. Normally it applies either an aircraft or helicopter to carry appropriate survey sensors.
Recently based on level of technology the magnetic, radiometric and Electromagnetic, EM surveys can be conducted from air, while bearing their titles as Aeromagnetic, Airborne Radiometric, and Airborne EM Surveys respectively.
Ground Surveys
Most of geophysical surveys are done on the ground, such that the survey sensor can be direct in contact to ground or placed at a low height above the ground.
The figure below, shows a ground GPR survey
These include magnetic, gravity, radiometric, Ground Penetrating Radar (GPR) , seismic and electrical resistivity surveys
Shipborne Surveys
The geophysical property is measured on the surface of water bodies by either a ship vessel, or a boat which is mounted by measuring sensors. Also the research ship may be equipped with an echo sounder for bathymetric seafloor investigation.
The figure below, shows Marine seismic reflection survey
Example include seismic reflection surveys, magnetic surveys.
2. Nature of the Field
The geophysical surveys can be classified into two (2) groups, based on nature of the field being measured
i/ Passive Surveys
These measures the distortion of static natural fields such as gravitational, magnetic , radiometric fields and some electrical fields such as Spontaneous Potential.
Examples include gravity, magnetic , radiometric surveys and some electrical surveys such as Spontaneous Potential (SP) survey
ii/ Active Surveys
Artificial signals are sent into the ground became modified and the returning signals are measured at suitable points.
Example seismic surveys, Electromagnetic (EM) surveys, electrical resistivity surveys
How are geophysical surveys conducted?
Since there is no general guide that will fit to all geophysical surveys. The following are the basic operations to be considered in any Geophysical Survey so that to ensure it is successful.
1. Conduct Reconnaissance Survey
This is normally short termed as "Recy". It involves physical visiting to the survey site prior to the start conducting the given geophysical survey.
Since you actually observe the site (field area), you will have an idea on what would be the best geophysical method to apply in order to achieve best results.
It also gives you an idea about the size of the site, so that you know how to narrow down the survey so as to detect expected geophysical targets.
If the size of the site is too large (regional scale) it is advisable to apply Airborne techniques, to collect preliminary information about geology.
You have to consider the physical parameters that need to be measured and make a decision on what would be an appropriate method to apply.
Recy is applicable in planning of the geophysical survey.
2. Set up a control Network
Here you have to locate the exact geophysical stations at which measurements will be done.
Since some geophysical devices such as Magnetometer have been integrated with GPS to collect location data such as (x, y, z) coordinates. Also you have to measure the distance between these points accurately.
For example if you want to run ground based magnetic profiling make sure you choose a suitable station interval along the profile line such as 2m, 5m, 10m depending on site coverage.
If it is electrical sounding (VES), make sure electrodes spacing is well measured and electrodes are well positioned.
If it is a seismic survey, make sure that seismic receivers (geophones) are fixed at the correct offset distance from the seismic source depending on your geophysical survey design.
This can be useful when you transform the measurements from ground to Map by using a suitable scale.
3. Ensure Adequate checks
All survey measurements should be checked at the time of recording. This is to ensure quality control, that data gathered are within the given specification.
Example Seismic surveys conducted a pilot survey before commencement of an actual survey to test and checking for data quality.
Also you can can achieve this either by using techniques such as the reverse direction to ensure intervals are well defined or average value (redundancy) to reduce errors
Also you can adapt Automatic methods, For example some devices such as Scintrex CG - 3M gravity meter have been developed with quality control systems that improve field data checks.
4. Choose appropriate geophysical survey method
Here after you have completed the previous operations, you have to make a decision on a more suitable geophysical survey method to use in order to collect quality data so that you can increase the chance of success in your geophysical survey project.
Since you have already did Recy and have an idea on the area on hand , now you can choose a good geophysical method to achieve the survey.
Remember accurate and Modern (recent) geophysical techniques can be prioritized over older techniques. For example using electronic (digital) seismic techniques are highly recommended so that to smooth the seismic data processing.
Also remember that geophysical methods are sensitive to appropriate physical parameters.
Make sure also you consider other factors such as,
Budget
Check for Budget on the table that will be used to complete the given geophysical survey and then utilized it honestly.
For example there is no need to use costful equipments which are less accurate.
It will make sense even if you punchase expensive equipments that are faster and accurate in your Logistic survey budget to complete your work easily, rather than rely on cheap, slow and less accurate techniques.
Time to complete the survey
Consider time on your geophysical survey so that you can deliver your complete work done to your client before the deadline. Time management is a must factor in any successful geophysical survey project.
Other factors like the environment can also be considered. For example it is advised to conduct seismic surveys at night rather than at day, so as to reduce the effect of noise.
5. Record geophysical data carefully
All survey measurements should be written down accurately in a geophysical Log book or appropriate geophysical survey Forms . If it is an equipment (computer) based recording system make sure you save a file containing field survey data by using a well defined header and format so that it is easier to access later when required.
For example you can store seismic survey field data in SEG - D format, while after several processing you have to store in SEG - Y format so that you can transfer them easily.
Remember there is a saying that " If it wasn't written down it did not happen".
You have to be careful regarding what you are recording. Always record what you observe and not what you know, For example If a resistivity meter displays 50.74 ohms, it will make no sense if you record 50 ohms while dropping 0.74 ohm. All estimates, reduction, approximations and other calculations will be done either in the end of the day of survey field or at desk studies provided that specified data quality limits have been taken into consideration.
Geophysical survey is the general term that can be defined and conducted depend on the objective and geophysical method on hand. However the guides described above can be taken as a general guide for any geophysical method and techniques.
To your success!
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