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Cross-well EM devices are an extension of the resistivity logging tools. These tools measure the conductivity of the formation, where its reciprocals give the resistivity. The measurements are presented by a resistivity log, which is a plot of resistivity versus depth. The resistivity log reflects the nature of hydrocarbon presence in the penetrated formation. The basis behind the measurements is the conductivity and insulation concepts. Conductive materials pass electrical current easily compared to insulators. In the formation, water is an electrolyte. An electrolyte is an electrically conducting solution that contains dissolved cations and anions. These ions help pass electrical currents through the formation. In contrast, completely dry rocks are insulators, which give incredibly large resistivity.

Cross-well EM devices are an extension of the resistivity logging tools. These tools measure the conductivity of the formation, where its reciprocals give the resistivity. The measurements are presented by a resistivity log, which is a plot of resistivity versus depth. The resistivity log reflects the nature of hydrocarbon presence in the penetrated formation. The basis behind the measurements is the conductivity and insulation concepts. Conductive materials pass electrical current easily compared to insulators. In the formation, water is an electrolyte. An electrolyte is an electrically conducting solution that contains dissolved cations and anions. These ions help pass electrical currents through the formation. In contrast, completely dry rocks are insulators, which give incredibly large resistivity.

[[file:AlsaudKatterbauerFigure2.jpg|thumb|300px|{{figure number|2}}Cross-well EM current direction between the wells and to the targeted zone.<ref name=Schlumberger_2021>S. O. Glossary, "Crosswell electromagnetic tomography," Schlumberger, 2021. [Online]. Available: https://glossary.oilfield.slb.com/en/Terms/c/crosswell_electromagnetic_tomography.aspx. Accessed 2021.</ref>]]  

[[file:AlsaudKatterbauerFigure2.jpg|thumb|300px|{{figure number|2}}Cross-well EM current direction between the wells and to the targeted zone.<ref name=Schlumberger_2021>S. O. Glossary, 2021, [https://glossary.oilfield.slb.com/en/Terms/c/crosswell_electromagnetic_tomography.aspx Crosswell electromagnetic tomography]</ref>]]  

Resistivity devices are divided into two primary types, electrode (galvanic) devices, and induction devices. Electrode devices measure the electrical potential (voltage) of the passing electrical current. For the measurement to take place, the electrodes should be within a water-based mud, specifically salty water. The induction devices use an alternating current to generate an alternating primary magnetic field that induces a current into the formation. Then, the induced current generates a secondary magnetic field that is measured by the coils. The induction device's advantage over the electrode devices is that it is used in an oil-based mud or a relatively low conductive mud, like freshwater-based mud.<ref name=Schon_2015>J. Schön, Basic Well Logging and Formation Evaluation, 1st ed., London: Bookboon, p. 51 and 56.</ref>   

Resistivity devices are divided into two primary types, electrode (galvanic) devices, and induction devices. Electrode devices measure the electrical potential (voltage) of the passing electrical current. For the measurement to take place, the electrodes should be within a water-based mud, specifically salty water. The induction devices use an alternating current to generate an alternating primary magnetic field that induces a current into the formation. Then, the induced current generates a secondary magnetic field that is measured by the coils. The induction device's advantage over the electrode devices is that it is used in an oil-based mud or a relatively low conductive mud, like freshwater-based mud.<ref name=Schon_2015>J. Schön, Basic Well Logging and Formation Evaluation, 1st ed., London: Bookboon, p. 51 and 56.</ref>