35 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 mapping seafloor mineral deposits with central loop transient electromagnetics
Electromagnetic methods are commonly employed in exploration for land-based mineral deposits. A suite of airborne, land, and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, although the commercial value of marine mineral deposits has been recognized for decades, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. One particularly interesting electromagnetic method, which could be used to image a mineral deposit on the ocean floor, is the central loop configuration. Central loop systems consist of concentric transmitting and receiving loops of wire. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, because the transmitter and receiver are concentric, and (2) simplified operation because only one compact unit must be deployed. We produced layered seafloor type curves for two particular types of central loop methods: the in-loop and coincident loop configurations. In particular, we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick. Modeling and resolution analyses showed that, using a 50 m(2) transmitting loop with 20 A of current, these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness. In the most extreme case, absolute voltage errors on the order of 10 nV are required to resolve the base of a 30 m thick ore deposit. Whether such noise floors can be achieved in real marine environments remains to be seen
Minimum Wages, Wage Inflation, and the Relative Wage Structure
This paper reports an analysis of the effect of changes in legislated minimum wages on negotiated wage increases, using micro data on Canadian labor contracts between 1966 and 1975. The analysis departs from previous studies in that it divides the sample data base into high- and low-wage sectors. The results indicate that union negotiated wage settlements reflect at least partially legislated minimum wage changes over the past contract period. However, this minimum wage effect is restricted to the low-wage sector. In addition, the sectoral results suggest that the relative wage structure should tend to narrow in the long run.