Influence of water pressure dynamics and fluid flow on the streaming-potential response for unsaturated conditions

International audienceA B S T R A C T Streaming-potentials are produced by electrokinetic effects in relation to fluid flow and are used for geophysical prospecting. The aim of this study is to model streaming potential measurements for unsaturated conditions using an empirical approach. A conceptual model is applied to streaming potential measurements obtained from two drainage experiments in sand. The streaming potential data presented here show a non-monotonous behaviour with increasing water saturation, following a pattern that cannot be predicted by existing models. A model involving quasi-static and dynamic components is proposed to reproduce the streaming potential measurements. The dynamic component is based on the first time derivative of the driving pore pressure. The influence of this component is investigated with respect to fluid velocity, which is very different between the two experiments. The results demonstrate that the dynamic component is predominant at the onset of drainage in experiments with the slowest water flow. On the other hand, its influence appears to vanish with increasing drainage velocity. Our results suggest that fluid flow and water distribution at the pore scale have an important influence on the streaming potential response for unsat-urated conditions. We propose to explain this specific streaming potential response in terms of the behaviour of both rock/water interface and water/air interfaces created during desaturation processes. The water/air interfaces are negatively charged, as also observed in the case of water/rock interfaces. Both the surface area and the flow velocity across these interfaces are thought to contribute to the non-monotonous behaviour of the streaming potential coefficient as well as the variations in its amplitude. The non-monotonous behaviour of air/water interfaces created during the flow was highlighted as it was measured and modelled by studies published in the literature. The streaming potential coefficient can increase to about 10 to 40 when water saturation decreases. Such an increase is possible if the amount of water/air interfaces is increased in sufficient amount, which can be the case

A Closed-Form Alternative for the Frequency Distribution of Stably Distributed Random Components in Daily-Mean Self-Potential (SP) Time Series: Example from Urla Station, Turkey

The author of this paper has recently shown that daily-mean self-potential (SP) time series recorded at Urla station, Turkey, exhibit a unit-root nonstationary behavior with weakly correlated stationary differences driven by stable noise of tail index around 1.12–1.15. This work mainly aims to assess whether the Exponentiated-Exponential Cauchy (EEC) family distributions could be utilized as an alternative to the stable distributions. The EEC family distributions have many advantages as they have closed-form representations. Those goodness-of-fit statistics, relative strengths and predictive abilities of the three EEC family distributions with two- (Cauchy), three- (EEC-3) and four (EEC-4) parameters were compared with that of the stable one. The Akaike Information Criterion (AIC)-based relative strength for the EEC-3 candidate indicated that it is approximately 1.85 times more likely to be the best approximating model than the stable one. Additionally, those considerably smaller relative mean absolute errors (RMAE) for the EEC-3 model indicated that the asymptotic tail properties of the empirical distribution could be better represented in comparison with the stable model. The results of this work revealed, on the example of Urla, that an EEC-3 distribution would plausibly the stable one. We hope the findings of this paper would provide many advantages and facilitate the numerical computations in solving problems arising from the use of stable laws

Radyal Taban Fonksiyonlu Yapay Sinir Ağları (RTFA) ve Levenberg-Marquardt(LM) Ters Çözüm Yöntemleriyle Küre Şekilli Yapıların Doğal Uçlaşma Anomalilerinin Değerlendirilmesi

Doğal uçlaşama (DU) uygulamalarında kaynak yapı özelliklerini belirlemek yöntemin temel amacıdır. Çeşitli yöntemler bu özelliklerin saptanmasında kullanlmaktadır. Bu çalışmada Radyal Taban Fonksiyonlu Yapay Sinir Ağları (RTFA) ve geleneksel Levenberg-Marquardt (LM) ters çözüm yöntemleri DU verilerine uygulanmıştır. Çalışma iki aşamadan oluşmaktadır. İlk aşamada, gürültüsüz ve gürültülü küre şekilli kuramsal modelin DU anomalisinin her iki yöntemle ters çözümleri yapılarak model parametreleri saptanmıştır. İkinci aşamada ise yöntemler Seferihisar(İzmir) alanından toplanmış olan DU verilerine uygulanmıştır. Elde edilen sonuçlar karşılaştırıldığında, RTFA’nın LM ters çözüm sonuçlarına göre nispeten daha küçük hata değeriyle model parametrelerini saptadığı görülmüştür. Sonuç olarak, bu çalışma, RTFA yöntemi kullanılarak DU küre modeli ters çözümünün güvenilir bir şekilde yapılabileceğini ortaya koymuştur.The main purpose of the method is to determine the source structure properties in self-potential (SP) applications. Various methods are used to determine these properties. In this study, Radial Basis Function Neural Network (RBFNN) and traditional Levenberg-Marquardt (LM) inversion methods were applied to SP data. The study consists of two stages. In the first stage, the model parameters were determined by performing inverse solutions of the SP anomaly of the noise-free and noisy spherical polarized synthetic model with both methods. In the second stage, the methods were applied to the SP data collected from the Seferihisar (İzmir) field. When the results were compared, it was seen that RBFNN determined the model parameters with a relatively smaller error value than the LM inversion results. In conclusion, this study revealed that the SP sphere model inversion can be reliably performed using the RBFNN method