16 research outputs found
Step-on versus step-off signals in time-domain controlled source electromagnetic methods using a grounded electric dipole
The timeâdomain controlled source electromagnetic method is a geophysical prospecting tool applied to image the subsurface resistivity distribution on land and in the marine environment. In its most general setup, a squareâwave current is fed into a grounded horizontal electric dipole, and several electric and magnetic field receivers at defined offsets to the imposed current measure the electromagnetic response of the Earth. In the marine environment, the application often uses only inline electric field receivers that, for a 50% dutyâcycle current waveform, include both stepâon and stepâoff signals. Here, forward and inverse 1D modelling is used to demonstrate limited sensitivity towards shallow resistive layers in the stepâoff electric field when transmitter and receivers are surrounded by conductive seawater. This observation is explained by a masking effect of the direct current signal that flows through the seawater and primarily affects stepâoff data. During a stepâoff measurement, this direct current is orders of magnitude larger than the inductive response at early and intermediate times, limiting the stepâoff sensitivity towards shallow resistive layers in the seafloor. Stepâon data measure the resistive layer at times preceding the arrival of the direct current signal leading to higher sensitivity compared to stepâoff data. Such dichotomous behaviour between stepâon and stepâoff data is less obvious in onshore experiments due to the lack of a strong overlying conductive zone and corresponding masking effect from direct current flow. Supported by synthetic 1D inversion studies, we conclude that timeâdomain controlled source electromagnetic measurements on land should apply both stepâon and stepâoff data in a combined inversion approach to maximise signalâtoânoise ratios and utilise the sensitivity characteristics of each signal. In an isotropic marine environment, stepâoff electric fields have inferior sensitivity towards shallow resistive layers compared to stepâon data, resulting in an increase of nonâuniqueness when interpreting stepâoff data in a single or combined inversion
On the exploration of a marine aquifer offshore Israel by longâoffset transient electromagnetics
Demonstrationsmessungen mit dem Transient-Elektromagnetischen Tiefensondierungsverfahren in Problemgebieten der Kohlenwasserstoffexploration in Indien und China Schlussbericht
SIGLEAvailable from TIB Hannover: DtF QN1(45,10) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman