Frequency Domain Electromagnetic prospection registers the secondary EM field resulting from induction currents propagation, allowing a continuous recording of electrical resistivity on surface as well as in wells to quickly investigate the subsoil at low cost.
Thermal resistivity describes the subsoil capability to oppose to thermal flow. Thermal resistivity measurements are performed using a needle-shapes thermal sensor introduced into subsoil and registering the temperature changes produced by heating it.
The Ground Probing Radar (GPR) is a high-resolution tool recording high frequency subsurface EM waves reflection, allowing a precise estimation of depth for the detected objects, as well as of their nature and origin. It is a non-invasive method which permits investigations without excavations and that is normally required for environmental, engineering, archeology and other shallow studies.
Both MASW (Multichannel Acquisition Surface Waves) and REMI (Refraction Microtremor Arrays) consist in the characterization of surface seismic waves and specifically for Vs (shear wave velocity). MASW uses an active seismic source and registers high frequencies signal while REMI uses passive sources and registers medium-low frequencies. Together they allow obtaining the total velocity dispersion over the complete frequency range to obtain Vs.
Seismic Tomography is a geophysical method for subsoil exploration through waves transmission. It consists in generating seismic waves by hitting the ground (manually or using mechanical systems such as portable Propelled Energy Generators and Accelerated Weight Drop Systems or thumper trucks) and detecting them in different locations through geophones equally spaced along a profile. This technique allows recording the subsoil compressional velocity Vp.
The Vertical Electrical Sounding (VES) allows to measure a subsoil resistivity vertical profile (1D), using a four electrodes array. Two current electrodes are installed with a specific spacing on the ground and are used to transmit the electrical current to the subsoil. The other two electrodes, also installed on the ground, are used to measure the voltage difference. The investigation depth is directly related to the current electrodes spacing.
The Electrical Resistivity Tomography (ERT) allows obtaining 2D sections and 3D models for the subsoil resistivity distribution. The difference with the related and simpler VES technique is the much higher measurements density and the 2D/3D algorithms to model and interpret data. ERT is a contact method which uses a set of several electrodes installed on the surface along a profile (2D) or according a regular mesh (3D). Direct current injection into subsoil through pairs of electrodes allows voltage measurements between other electrodes pairs. A switching device, configured according specific protocols, automatically performs all the possible combination, of injection and voltage electrodes pairs, resulting in high density data laterally as well as with depth. This data set allows a detailed reconnaissance of the subsoil electrical resistivity distribution.
Well wall in-situ density continuous recording is performed with a formation-density sensor, which provides data to identify low density as well as high density stratus related, for instance, to coal presence and basic metal mineralogy, respectively.
The mechanic caliper well profiling represents the average well diameter, as detected by the tool mobile fingers that are adjusted through springs. The caliper anomalies could represent fractures, stratification plans, fluid transmission cavities and, usually, show the distance between the fractures that cross with wells.
Dowhole and Crosshole are seismic techniques to measure wave’s propagation velocity in wells, with higher precision and resolution compared to surface recordings. The Downhole registry consists in a tri-axial geophone lowered into a well while seismic waves are generated on surface. At specific depths, the geophone records directly the waves transmission times. The Crosshole utilizes two wells, one to generate the seismic waves and the other to register the wave’s transmission times from one to the other.
Porosity well log based on slowing down and capture of neutrons by the hydrogen contained in the subsoil formations. This technique allows registering the correct porosity for specific formations, (limestone, sandstone, dolomite), assuming that formation pores are fluid-filled and in units related to formation type (vol/vol or u.p.). The measurement is statistical and based on thermal neutron detection, so precisions is higher for high counting speed, i.e. low porosity.
The technique for a sonic or acoustic velocity well profile uses acoustic transducers to transmit ultrasonic waves through the well fluid into the surrounding rock. This technique allows to determine in-situ compressional and shear waves to calculate the rock mechanical properties. Also, these registries can detect fractured zones and estimate permeability.
Wellfield Services introduced the Borehole Televiewer technique in Chile in 1996, developing a wide experience in its application. The high resolution acoustic Televiewer provides well wall pseudo-images to investigate the subsoil structure features through computer assisted images reconnaissance. It consists of a transductor operating at high frequency in pulse-eco mode. The important amount of obtained information, combined with a classification scheme for structural features and with the lithological, geological, chronology and tectonic information, all together, are ideal for structural statistical analysis. The technique is applicable to large scale structure, up to various meters size, as well as to microstructures.
If the well is dry or filled with clean transparent fluid, it is possible to obtain a well wall high resolution true colors optical image. This also provides information about the fractures infilling material, besides of all the other information. The best performance is for diamond drilled smooth and even well walls, intercepting colored rocks.